Monocot seeds have one "seed leaf" termed a cotyledon (in fact monocot is a shortening of monocotyledon). Dicots have two cotyledons. Both groups, however, have the same basic architecture of nodes, internodes, etc.

Comparison of monocot (left, oat) and dicot (right, bean) gross anatomy. Image from Purves et al., Life: The Science of Biology, 4th Edition, by Sinauer Associates (www.sinauer.com) and WH Freeman (www.whfreeman.com), used with permission.

Monocot stems have scattered vascular bundles. Dicot stems have their vascular bundles in a ring arrangement. Monocot stems have most of their vascular bundles near the outside edge of the stem. The bundles are surrounded by large parenchyma in the cortex region. There is no pith region in monocots. Dicot stems have bundles in a ring surrounding parenchyma cells in a pith region. Between the bundles and the epidermis are smaller (as compared to the pith) parenchyma cells making up the cortex region.).

Plant Growth

Growth in plants occurs chiefly at meristems where rapid mitosis provides new cells. As these cells differentiate, they provide new plant tissue.

Stem Growth

In stems, mitosis in the apical meristem of the shoot apex (also called the terminal bud) produces cells that enable the stem to grow longer and, periodically, cells that will give rise to leaves. The point on the stem

where leaves develop is called a node. The region between a pair of adjacent nodes is called the internode.

The internodes in the terminal bud are very short so that the developing leaves grow above the apical meristem that produced them and thus protect it.

New meristems, the lateral buds, develop at the nodes, each just above the point where a leaf is attached. When the lateral buds develop, they produce new stem tissue, and thus branches are formed.

Under special circumstances (such as changes in photoperiod), the apical meristem is converted into a flower bud. This develops into a flower. The conversion of the apical meristem to a flower bud "uses up" the meristem so that no further growth of the stem can occur at that point. However, lateral buds behind the flower can develop into branches.

The drawing is of a typical woody dicot, the horse chestnut, as seen during the dormant season. The leaves have dropped off, leaving a leaf scar; the dots inside each leaf scar show where the vascular bundles (xylem and phloem) had entered the petiole of the leaf.

A flower had been produced the season before, so that during the season just ended two branches had grown out on either side of the flower bud scar.

Lenticels are openings that allow oxygen and carbon dioxide to diffuse between the living cells of the stem and the air.

Roots

Structure

The Root Tip

The root tip consists of a

• meristem - a region of rapid mitosis, which produces the new cells for root growth.

• root cap - a sheath of cells tha

• t protects the meristem from abrasion and damage as the root tip grows through the soil.

Because of the frequency of mitosis in the meristem, root tips are often used to demonstrate mitosis in the laboratory the inset is a photo (courtesy of Carolina Biological Supply Co.) of anaphase in the meristem of an onion root tip.

The Region of Elongation

Here the cells produced by mitosis undergo a period of elongation in the direction of the axis of the root. It is at this time that they are sensitive to gravity and respond with gravitropism.

The Region of Differentiation

Here develop the differentiated tissues of the root.

• Epidermis. A single layer of flattened cells at the surface. When first formed, epidermal cells have extensions — the root hairs — which greatly increase the surface area available for the uptake of nutrients from the soil. The photo below shows the root hairs in the region of differentiation of a germinating radish seed.

• Cortex. A band of parenchyma cells that develops beneath the epidermis. It stores food. Its inner surface is bounded by a single layer of cells, the

• Endodermis.

• Stele

  • Pericycle — the outer boundary of the stele. Secondary roots branch from it.

  • Xylem — arranged in bundles in a spoke-like fashion

  • Phloem — alternates with xylem

• Cambium — In older parts of the root, another meristem forms between the xylem and phloem. Mitosis in the cambium produces new "secondary xylem" to the inside and secondary phloem to the outside.

Water Uptake

Water enters the root through the epidermis. Once within the epidermis, water passes through the cortex, mainly traveling between the cells. However, in order to enter the stele, it must pass through the cytoplasm of the cells of the endodermis.

Once within the stele, water is free again to move between cells as well as through them. In young roots, water enters directly into the xylem. In older roots, it may have to pass first through a band of phloem and cambium. It does so by traveling through horizontally-elongated cells, the xylem rays.

Mineral Uptake

One might have expected that minerals would enter the root dissolved in water. But, in fact, minerals enter separately:

• Even when no water is being absorbed, minerals enter freely — mostly through the root hairs.

• Minerals can enter against their concentration gradient; that is, by active transport. For example, plants can take up K⁺ from the soil against a ten-thousand-fold concentration gradient; e.g., from as little as 10 µM in the soil to 100 mM in the cell.

• Anything that interferes with the metabolism of root hairs interferes with mineral absorption.

• The root hairs are also the point of entry of mycorrhizal fungi. These transport minerals — especially phosphorus — to the root hair in exchange for carbohydrates from the plant.

• In legumes, the root hairs are the point of entry of rhizobia that will establish the mutualistic partnership enabling the plant to convert atmospheric nitrogen into protein.

Plants absorb their nutrients in inorganic form.

For examples:

• nitrogen enters as nitrate (NO₃⁻) or ammonium ions (NH₄⁺)

• phosphorus as PO₄³⁻

• potassium as K⁺

• calcium as Ca²⁺

When you hear of the virtues of organic fertilizers, remember that such materials meet no nutritional need of the plant until their constituents have been degraded to inorganic forms. Organic matter does play an important role in making good soil texture, but only to the extent that it can yield inorganic ions can it meet the nutritional needs of the plant.

Once within the epidermis, inorganic ions pass inward from cell to cell, probably through plasmodesmata. The final step from the cytoplasm of the pericycle cells to the xylem is probably accomplished once again by active transport.

Gas Exchange

The older parts of roots are sheathed in layers of dead cork cells impregnated with a waxy, waterproof (and airproof) substance called suberin. This sheath reduces water loss but is as impervious to oxygen and carbon dioxide as it is to water.

However, the cork is perforated by nonsuberized pores called lenticels. These permit the exchange of oxygen and carbon dioxide between the air and the living cells beneath.

Plant Parts - Leaves

Leaves are the food making factories of green plants. Leaves come in many different shapes and sizes. Leaves can be simple. They are made of a single leaf blade connected by a petiole to the stem. An oak leaf or a maple leaf is examples. A compound leaf is a leaf made up of separate leaflets attached by a petiole to the stem like an ash or a locust.

Leaves are made to catch light and have openings to allow water and air to come and go. The outer surface of the leaf has a waxy coating called acuticle which protects the leaf. Veins carry water and nutrients within the leaf.

Leaves are the site of the food making process calledphotosynthesis. In this process, carbon dioxide and water in the presence of chlorophyll (the green pigment) and light energy are changed into glucose (a sugar). This energy rich sugar is the source of food used by most plants.

Photosynthesis is unique to green plants! Photosynthesis supplies food for the plant and oxygen for other forms of life.

A green plant helped make the oxygen you are breathing today.

Plant Parts - Flowers

Flowers not only look pretty but, in fact, are important in making seeds. Flowers have some basic parts. The female part is thepistil. The pistil usually is located in the center of the flower and is made up of three parts: the stigma, style, and ovary. The stigma is the sticky knob at the top of the pistil. It is attached to the long, tubelike structure called the style. The style leads to the ovary that contains the female egg cells calledovules.

The male parts are called stamens and usually surround the pistil. The stamen is made up of two parts: the anther and filament. The anther produces pollen (male reproductive cells). The filament holds the anther up.

During the process of fertilization, pollen lands on the stigma, a tube grows down the style and enters the ovary. Male reproductive cells travel down the tube and join with the ovule, fertilizing it. The fertilized ovule becomes the seed, and the ovary becomes the fruit.

Plant Parts – Fruit

The fruit is the ripened ovary of a plant containing the seeds. After fertilization, the ovary swells and becomes either fleshy or hard and dry to protect the developing seeds. Many fruits help seeds spread (maple seeds). Many things we call vegetables are really fruits such as tomatoes, cucumbers, and beans.

Every seed is a tiny plant (embryo) with leaves, stems, and root parts waiting for the right things to happen to make it germinate and grow. Seeds are protected by a coat. This coat can be thin or thick and hard. Thin coats don't protect the embryo well. But thick coats can let the embryo survive some tough conditions.

The seed also contains a short-term food supply called the endosperm which is formed at fertilization but is not part of the embryo. It is used by the embryo to help its growth. In the bean that is shown, the endosperm is no longer there. It has been used for the growth of the embryo, and most of its nutrients and energy are now in a different form within the tissues of the cotyledon.

Plants with one cotyledon (like corn) are calledmonocots. If they have two cotyledons (like beans), they are called dicots.

Seeds are a plant's way of getting from one area to another by wind, water or animals.

Petals are also important parts of the flower, because they help attract pollinators such as bees, butterflies and bats. You can also see tiny green leaf-like parts called sepals at the base of the flower. They help to protect the developing bud.

LEAF ANATOMY

The leaf is the primary photosynthetic organ of the plant. It consists of a flattened portion, called the blade that is attached to the plant by a structure called the petiole. Sometimes leaves are divided into two or more sections called leaflets. Leaves with a single undivided blade are called simple, those with two or more leaflets are called compound.

The outer surface of the leaf has a thin waxy covering called the cuticle (A), this layer's primary function is to prevent water loss within the leaf. (Plants that leave entirely within water do not have a cuticle). Directly underneath the cuticle is a layer of cells called the epidermis (B). The vascular tissue, xylem and phloem are found within the veins of the leaf. Veins are actually extensions that run from to tips of the roots all the way up to the edges of the leaves. The outer layer of the vein is made of cells called bundle sheath cells (E), and they create a circle around the xylem and the phloem. On the picture, xylem is the upper layer of cells (G) and is shaded a little lighter than the lower layer of cells -phloem (H). Recall that xylem transports water and phloem transports sugar (food).

Within the leaf, there is a layer of cells called the mesophyll. The word mesophyll is greek and means "middle" (meso) "leaf" (phyllon). Mesophyll can then be divided into two layers, the palisade layer (D) and the spongy layer. Palisade cells are more column-like, and lie just under the epidermis, the spongy cells are more loosely packed and lie between the palisade layer and the lower epidermis. The air spaces between the spongy cells allow for gas exchange. Mesophyll cells (both palisade and spongy) are packed with chloroplasts, and this is where photosynthesis actually occurs.

Epidermis also lines the lower area of the leaf (as does the cuticle). The leaf also has tiny holes within the epidermis called stomata. Specialized cells, called guard cells (C) surround the stomata and are shaped like two cupped hands. Changes within water pressure cause the stoma (singular of stomata) to open or close. If the guard cells are full of water, they swell up and bend away from each other which open the stoma. During dry times, the guard cells close.

Parts of the leaf

The leaf consists of the following parts:

Petiole - a thin stalk that connects the blade of the leaf to the plant's stem. These veins are called

Blade - a thin, flat part of the leaf that extends off the end of the petiole. It is green as it contains chlorophyll which is necessary in making the plant's food.

The blade contains veins which help bring in the water necessary for photosynthesis and to transport food out to all parts of the plant. The xylem is responsible for water transportation and the phloem for food.

The blade also has several layers. The top layer is covered with a waterproof coat called the cuticle. Both the upper and lower layer contains paired cells called guard cells. These guard cells manage the stomata, little openings in the leaf that allow for the taking in of carbon dioxide and the emission of oxygen during photosynthesis. Water vapor is also emitted in a process called transpiration.

In between the top and bottom layers of the blade are cells that are full of chlorophyll that is essential for the plant to make its food during photosynthesis.

Leaves come in a variety of shapes, sizes, color, and textures. Leaves can be grouped as simple with one blade or compound with many leaflets. The edges of leaves can be smooth, have jagged edges referred to as toothed or scalloped edges referred to as lobed edges. Leaves can also be divided into how their veins are arranged. The leaves of plants referred to as monocots have leaves that run parallel to each other off one central vein. Those that are referred to as dicots have leaves with veins that branch out into two different ways.Pinnate have one main vein called the midrib, and smaller branching veins. Palmates have several large veins branching from the leaf base into the blade.

Some leaves have special features such as spines and bud scales to protect them. Some are climbing (tendrils). Others catch insects. Some store water. These are called succulents or store food such as bulb scales.

Leaves contain other substances besides chlorophyll that give the leaf color. These substances are called pigments and cause leaves to turn colors in autumn. These leaves are on deciduous trees and fall off and die. New onesgrow back in the spring. Conifers or trees that bear cones can keep their needles for years and grow new ones as soon as theylose them.

Leaf Structure

The leaf is the site of two major processes: gas exchange and light capture, which lead to photosynthesis.

If you’ve ever eaten a piece of lettuce, cabbage, celery or onion, you’ve eaten a leaf or at least part of it. Celery is a petiole, which is the part of the leaf that connects the blade to the stem. Now that you’re acquainted with what leaves look and taste like, let’s take a closer look at what’s on the inside of leaves.

The leaf is arranged like a layered cake. It has a spongy layer, apalisade layer that consists of parenchyma cells and is where photosynthesis happens and an outer layer (the epidermis) with little holes (stomata) that could hold birthday candles. The only problem with our lovely analogy is that the candles would have to go on the bottom of the cake, since most of the stomata are on the bottom of the leaf. We’ll come back to that.

The inside cake layers are made up of parenchyma cells. This tissue is called the mesophyll, meaning "middle leaf," and comes in two flavors: the palisade mesophyll (sometimes called palisade parenchyma) and the spongy mesophyll. Most photosynthesis takes place in the palisade mesophyll, which is conveniently located at the top of the leaf just under the epidermis. Palisade parenchyma cells are long, bunched close together and look like sausages hanging from the ceiling in a butcher’s shop.

The inside of the leaf looks like this:

Spongy mesophyll cells are not packed so tightly together, which allows carbon dioxide and oxygen to reach the palisade cells where they are needed in photosynthesis. Spongy mesophyll cells and guard cells (see below) also get some photosynthetic action.

Carbon dioxide and oxygen can’t just diffuse across the epidermis to get into the leaf. They have to be let in through special doors called stomata. Stomata can be on both sides of the leaf, but are usually concentrated on the bottom of the leaf to limit water loss due to evaporation.

Spongy mesophyll cells are not packed so tightly together, which allows carbon dioxide and oxygen to reach the palisade cells where they are needed in photosynthesis. Spongy mesophyll cells and guard cells (see below) also get some photosynthetic action.

Carbon dioxide and oxygen can’t just diffuse across the epidermis to get into the leaf. They have to be let in through special doors called stomata. Stomata can be on both sides of the leaf, but are usually concentrated on the bottom of the leaf to limit water loss due to evaporation.

Leaves on the same plant can have different shapes. This occurs if some of the leaves were produced by the juvenile plant and later leaves were produced when the plant matures. Leaf thickness and size also varies within a plant, depending on whether the leaf is in the sun most of the day (a "sun leaf") or in the shade most of the day (a "shade leaf"). Sun leaves are smaller and thicker because they pack a bunch of chloroplasts into a small space. Since shade leaves don’t get as lighter, they tend to be bigger but thinner, with a wider surface area to capture what light they do receive.

The pattern of veins on a leaf is usually determined by whether the plant is a monocot or eudicot. Monocots have a few parallel veins running down their leaf, which is called parallel venation. Eudicots typically have networks of veins, which is called net venation.

The way leaves are arranged on a plant isn’t something you probably think about every day, or even at all. However, just as pastry chefs pay attention to where every frosted detail goes when the Food Network has a cake challenge, plants care about where their leaves go. Sometimes leaves are very neatly arranged in an alternating pattern. Other plants try to optimize the light their leaves receive by spreading their leaves out around the stem, in an effort to limit the amount of shade they receive from their own leaves. The arrangement of a plant’s leaves on the stem is called phyllotaxy.

Though most leaves perform photosynthesis, some plants have modified their leaves to do other things. Cacti are a great example of this: those prickly spines are actually leaves. The green part of a cactus is actually a photosynthetic stem. By producing leaves that are thin and pointy, cacti reduce water loss, which could be quite substantial in a desert, and discourage animals from eating their precious leaves or stems. Onions are also modified leaves, but they store food for the plant. Other plants, including many desert succulents (e.g. Aloe vera) use their leaves as storage organs, keeping water in them. Pea plants have tendrils, which are vine-like extensions of leaves. Tendrils can cling to other plants or structures to support the pea plant. Other plants, such as poinsettas, have colored leaves to help attract pollinators.

Many people are familiar with the vibrant autumn leaf displays of the northern United States. Red, orange and yellow leaves fill up the woods and later, people’s lawns. Why do leaves get all dressed up in pretty colors just before they fall from the tree?

The autumn leaves you see are senescing—getting old. Most angiosperms in cold climates can’t keep their leaves through the winter; there just isn’t enough sunshine or water to keep up their metabolism in the winter, so they salvage what nutrients they can before the leaves drop off the tree. The bright colors you see are the pigments that are left after the tree has reabsorbed what it can from its leaves.

Colored compounds called anthocyanins make reds, and other pigments called carotenoids appear yellow or orange. These pigments have been there all along, but are only visible once the plant stops photosynthesizing and stops making chlorophyll. In other climates where leaves don’t change color all at once, they still senescence and drop off the tree. Often old leaves turn yellow, but an individual plant may only have a few yellow leaves at any given time.

Just like curly-haired people sometimes straighten their hair and straight-haired people sometimes curl their hair, plants like to change their appearances too. Some plants can change the way they hold their leaves throughout the day to maximize the sunlight they receive. Lupines always have their leaves fanned out to receive direct sunlight. Other plants, particularly those that live near the equator and get more intense sunlight, change the orientation of their leaves to minimize the solar radiation they get. These plants line their leaves up so that the sun hits the leaf edge rather than the top part of the blade.

Brain Snack

Stone plants are plants whose leaves are mostly underground. Only the tops of the leaves are visible, and the leaves look like rocks:

Plant Reproduction

Bryophyte Reproduction

Bryophytes are primitive plants that don’t have seeds or vascular systems. Because they lack a mechanism for moving water through their bodies, bryophytes are pretty small. When was the last time you saw a tall moss? Exactly. Mosses, liverworts and other bryophytes are so tiny that it is easy to ignore them most of the time, even if they are under your feet. Mosses actually are underfoot a lot—check out the cracks in the sidewalk and you’ll probably find moss there. If you’re lucky, you might get to see a gametophyte wearing a sporophyte like a hat. Now that would be an interesting fashion statement.

Gametophytes don’t "wear" sporophytes, exactly; they don’t put them on in the morning and change out of them at night. What really happens is that the sporophyte grows out of the top of the gametophyte, so it looks like a headdress of some kind.

In this image of stagshorn moss, the green leafy part is the gametophyte and the tall spindle on top is the sporophyte.

The first thing bryophytes need to reproduce is water. Since they usually live in places that are moist at least some of the time, this isn’t really a problem for bryophytes. However, they still wait until a rainy period to reproduce, because they need water to carry sperm to the eggs.

Bryophyte reproduction happens in two ways, like with other plants. Remember all that alternation of generations stuff? Asexual reproduction occurs when a sporophyte releases spores, and sexual reproduction happens when gametes fuse and form a zygote.

When a bryophyte spore settles somewhere, it grows into a gametophyte. Gametophytes are green and leafy, but small. A gametophyte’s reproductive organs are called antheridia (male) and archegonia (female). The antheridia and archegonia look like little umbrellas sticking up from the plant. Antheridia make sperm and archegonia make eggs. As Rihanna says, when it’s raining more than ever, the eggs stay under the umbrella; meanwhile the sperm take a free ride on the rainwater and seek out eggs.

In the liverwort Marchantiapolymorpha, the antheridia look like umbrellas:

After sperm and egg join in fertilization, the zygote grows into a sporophyte. This is still happening on the gametophyte, which makes the sporophyte generation completely dependent on the gametophyte generation.

Sporophytes make spores in the plant’s spore factory, called a sporangium. The spores are then released from a capsule on top of the sporophyte.

Fern Reproduction

Ferns are seedless vascular plants. Instead of seeds, they grow from spores. Spores grow into gametophytes, which in ferns are very tiny and short-lived. The gametophytes release sperm to fertilize eggs, and fertilization happens right on top of the gametophyte. The sporophyte grows into a new fern plant, and produces spores to complete the life cycle.

A few things make this simple story a little more complicated:

Spores are produced in a sporangium (plural = sporangia). This is the spore factory, and that word will come back to haunt you. Sporophytes are diploid (2N), but since they produce spores through meiosis, spores are haploid (N). Sporangia occur on the underside of fern fronds in little clusters called sori (singular = sorus). If you lift up a fern frond, you will often see little dots in neat lines. These are sori, which will release spores when the time is right. Spores are carried by the wind (dispersed) to new locations.

The densely packed brown dots are sori, clusters of spores underneath fern leaves.

The fern’s sperm are produced in the sperm factory, or antheridium. Meanwhile, the female gametes, eggs, are housed in an archegonium. Gametophytes and gametes are haploid (N).

Sperm have flagella, which are whip-like tails that allow them to swim. In order to swim, they need water. This means ferns can only successfully reproduce in wet places, or after a rain. After fertilization, the sporophyte grows up on top of the gametophyte. A mature sporophyte makes spores, and the life cycle starts over again.

Gymnosperm Reproduction

Gymnosperm means "naked seed," a name that reflects the fact that gymnosperms have no fruits to protect their seeds. However, the presence of seeds isn’t the only thing that separates gymnosperms from ferns; a lot of changes happened along the way in the evolution of seed plants. These changes include:

• the seed

• reduced gametophytes

• heterospory

• ovules

• pollen grains

First, the seed. What is a seed? A seed consists of an embryo and its food supply. A seed has the potential to grow into a new plant, but isn’t considered a separate plant until it disperses and germinates. Because a seed has a protective coating in the form of a seed coat, it can lie dormant for months or years until conditions are right for it to germinate. Spores don’t enjoy the same relaxed schedule as seeds because they are usually made of only one cell, with no protective coating.

An example of a gymnosperm seed is the pinyon pine. Those round things in the middle are the seeds, with the remnants of a cone surrounding them.

(Public domain image from National Park Service, retrieved from)

Reduced gametophytes: We know gametophytes are essential for plant life cycles. In ferns, the gametophytes lived on their own, were photosynthetic, and were small compared to the sporophyte. In gymnosperms, the gametophytes are even smaller, only visible through a microscope. They aren’t photosynthetic and don’t live on their own: instead, gametophytes live on the sporophyte. There are also two kinds of gametophytes, bringing us to the next subject…

Heterospory: In seed plants, there is no one-size-fits-all spore. Instead, there are megaspores, which grow into female gametophytes, and microspores, which grow into male gametophytes. Each sporangium makes only one kind of spore, so the sporangia are either megasporangia or microsporangia. Where do we find these sporangia? On pinecones! And because not every gymnosperm is a pine, we also find sporangia on cones of other species, such as firs, redwoods, and spruces.

You probably have spent much more of your life thinking about ice cream cones and sno-cones than plant cones. Pinecones aren’t as tasty as frozen sweets, but have very important jobs to do, so we can’t blame them too much. The technical name for plant cones is strobili (sounds like a type of pasta, doesn’t it?), and they are the site of all the sporangia. Male cones (microstrobili) hold microsporangia, and female cones hold megasporangia. Male cones are temporary structures that exist only long enough to make and release pollen, but female cones can grow for years while the seeds they hold develop.

Here is a picture of male pine cones. See how small they are?

Ovules: As we already established, a megasporangium produces a megaspore, and they are stuck on a spore-bearing leaf (a sporophyll) on the sporophyte. A layer of sporophyte tissue surrounds and protects the megasporangium and megaspore. This layer of tissue is called an integument. The integument, the megasporangium, and the megaspore together make up the ovule. An ovule is where a female gametophyte develops and produces eggs. The integument will later give rise to the seed coat that protects the seed.

Pollen grains: Once the microsporangium produces a microspore, that microspore develops into a pollen grain that consists of a male gametophyte and a tough pollen wall that protects the gametophyte while the pollen grain disperses. When a pollen grain lands in an appropriate place (i.e. an ovule of the same species), the pollen grain germinates and a pollen tube grows. Sperm travel down this pollen tube into the ovule.

If you get seasonal allergies, you can probably blame your runny nose and itchy eyes on the wind-pollinated plants that release their pollen into the air and straight into your nasal passages. Just tell those plants that they would be better off finding animal pollinators and to leave you alone! Gymnosperms are wind pollinated, and some angiosperms, such as grasses, have evolved wind pollination even though their angiosperm ancestor was not wind-pollinated.

Why on earth would they do this? Wouldn’t it be better to have a direct pollen transfer from a specific animal rather than making lots of extra pollen and hoping it lands on another plant of the same species? It turns out pollen is cheap to make, energetically speaking, especially compared to sugary nectar and showy flowers. So some plants have become successful by letting the wind do the work. And if you think about where grasses live, in fields or open prairies, it seems likely that the pollen will land on the right spot.

OK so where is all this pollen going? And what exactly is pollen, anyway? Remembering the Alternation of Generations section, we learned that there are two parts of a plant’s life cycle, the gametophyte generation and the sporophyte generation. These in turn, bear gametes and spores. Pollen contains sperm, which is a gamete, and the sperm will fertilize an ovule, which is the plant’s egg. Put the two gametes together and…voila! We get a zygote that develops into a sporophyte. All the angiosperms and gymnosperms are sporophytes when visible to the naked eye. We’ve accounted for the gametes and the sporophytes, but where are the spores and the gametophytes?

Gametophyte spores come in two types: microspores and megaspores.

Microspores are the male spores, which develop into microgametophytes. In both gymnosperms and angiosperms, the male gametophyte is contained in the pollen grain. So pollen is really just an immature male gametophyte, which will mature when the pollen lands on a stigma and starts growing a pollen tube.

Megaspores are female spores. A megaspore divides and grows into a megagameophyte, also known as the female gametophyte. This happens on the sporophyll, which in a pine tree is just one scale of the pine cone. The gametophytes are tiny. The female gametophyte produces eggs.

Angiosperm Reproduction

Flowering plants, or angiosperms, are the dominant plants on Earth today. They evolved 220 million years after the first seed plants appeared, but quickly dominated plant life because of some major improvements:

• Flowers, which attract pollinators and improve the plant’s mating success

• Fruits, which protect the seed and help with seed dispersal

Flowers

Flowering plants are masters of deception. You wouldn’t know it, with their perky stalks and brightly colored flowers or their tasty fruits, but there is a lot going on behind their innocent smiles—plants are manipulating us. We aren’t the only ones they dupe, either. Lots of animals are falling for their little tricks. Believe it or not, plants are tricking animals into helping them mate and carrying their offspring around for them. And you thought a rose is a rose is a rose.

Even though angiosperms would have us believe that flowers exist for the sole purpose of sending them to loved ones on birthdays and Mother’s Day, we know that plants have another motive: reproduction.

Just like animals and the other plants in this section, angiosperms reproduce with a sperm and an egg. But unfortunately for angiosperms, they don’t get to enjoy romantic walks on the beach and candlelit dinners before the sperm and egg meet. Most can’t just release their precious pollen to the wind and hope it lands on another plant of the same species. Angiosperms are stuck relying on a middleman for their reproductive success: a pollinator.

The pollen carries the genetic material that gets transferred from one plant to another, but pollen doesn’t have legs so it can’t walk around looking for mates. Instead, the pollen is picked up, usually by a flying animal such as a bee, bird or bat (the pollinator), and gets transferred from plant to plant as the pollinator looks for nectar, which is a sugary liquid flowers secrete. In the process of drinking delicious sweet nectar from flowers, the pollinator inadvertently carries pollen from one plant to another. Once the pollen arrives it fertilizes the plant’s ovary and a seed begins to develop.

Source

Plants that rely on animal pollinators are more successful than plants that use wind or water to transfer genetic material: this is why so many angiosperms have evolved and prospered. Even though angiosperms have fancy fruits and flowers, they still have gametophytes, gametes, spores and sporophytes. The angiosperm life cycle looks like this:

If you can ignore the flower for the moment, you’ll notice that on the sides of the diagram, a spore is just developing into a gametophyte, which makes gametes. This is similar to what we’ve seen before. On the right side is the microspore, which is the male side of things, and on the left side is the megaspore, holding down the female camp. When egg and sperm meet at the bottom, the ovule is fertilized and a seed and fruit form.

Now for the flower. Let’s look at the structure of a flower to figure out where the pollen starts and where it ends up. Don’t be scared of all the terms! They can’t hurt you!

The four main parts of a flower are the:

• sepals

• petals

• carpels

• stamens

Sepals

Sepals help protect the flower bud before it opens, since they are often thicker than petals and can enclose the bud completely. Sepals are often, but not always, green. In orchids, the sepals look just like the petals and can have complicated multi-color patterns.

Petals

Petals are the eye-catching part of the flower, and come in every color of the rainbow. Petals attract insects and other pollinators, and sometimes have hidden messages on them. Seriously. When viewed under ultraviolet light (which birds and bees can see usually, but humans can’t), some petals show nectar guides that point pollinators to the nectar.

Sometimes they look like landing strips for bees, and they often change color after the flower has been pollinated. This way, bees don’t waste their time looking for nectar in places where it has already been taken, and the flowers that haven’t been pollinated yet have a better chance of flagging down their pollinator.

Carpels

The carpels, which are collectively called the pistil, are the female reproductive parts. Carpels are made up of ovaries, which contain reproductive cells called ovules. The ovules will develop into seeds after fertilization. On top of the ovary is a stem-like thing called the style, and the stigma, which is sticky and receives the pollen. One way to remember where the pollen lands are that the stigma is sticky.

In this picture of, you can clearly see the taproot:

Once a seedling gets its roots going and grows some new leaves, it is pretty much established. However, it’s not ready to start a family yet; plants have juvenile and adult stages just like humans, though plants get to skip those awkward middle school years. Plant development is dictated by the all-important shoot apical meristem.

At some point determined by environmental conditions and hormones internal to the plant, the juvenile plant will become an adult. When the plant passes from its juvenile phase to its adult phase, the shoot apical meristem starts producing adult leaves that do boring adult things like balance their checkbooks and listen to talk radio. The shift to adulthood only affects new growth on the plant—leaves that already exist and were made during the juvenile phase retain their youthful look.

Two important factors for flowering, photoperiod and hormones, are discussed in their own sections. However, there are also genetic controls on flowers. When the plant becomes an adult, the apical meristem converts into a floral meristem. Somehow the apical meristem needs to develop all the flower parts. Good thing there’s an instruction booklet.

The instruction booklet for floral development is DNA, which also gives directions for how to make everything else in the plant. First, a stimulus called florigen moves up the vascular tissue to the shoot apical meristem. This causes certain genes to be expressed.

The general idea for how genes control flowers is called the ABC model. In the ABC model, there are three switches that can be turned on in a cell to dictate what floral part develops there. The switches are really groups of genes, and gene expression is equivalent to flipping the switch on.

The A group by itself causes sepals to develop. The C group by itself causes carpels to form. When A and B are both turned on, petals form, and when B and C work together, stamens develop. Scientists have figured out what these genes do by studying mutant plants that are can’t express one of the genes. Unfortunately, the mutant plants don’t have superpowers and won’t be joining the X-Men anytime soon, but they have at least helped us figure out how flowers form.

Photoperiodism

Contrary to popular belief, photoperiodism is not a modern art movement; it refers to the day length plants need to trigger flowering. Why is this important? As you already know, many plants require pollinators to transfer their pollen.

Pollinators such as birds and bees are more active in summer than winter, so plants that rely on certain birds and bees are better off if they wait until the long days of summer to open up their flowers. Thephotoperiod is the number of hours of daylight in a 24-hour period. In the northern hemisphere, winter days have less sunlight than summer days, so they have a shorter photoperiod. The leaf is the site of light perception, and only a small amount of leaf surface needs to receive light to maintain the plant’s internal clock.

The idea that day length could influence flowering arose in the 1920s, when researchers in Maryland were trying to breed large-leaved tobacco plants of the variety Maryland Mammoth. Though the researchers manipulated temperature, moisture, and soil nutrients, they couldn’t get the plants to flower. In the winter the researchers moved the plants into a greenhouse where they wouldn’t freeze and at last, they flowered.

The researchers learned by manipulating light that the tobacco plants would flower only when there were less than 14 hours of daylight. This earned Maryland Mammoth the label, "short-day plant," because it needed a day shorter than some critical length to flower. Other short-day plants include chrysanthemums, poinsettias, Easter lilies, and some soybeans.

The opposite of a short-day plant is a long-day plant. In order to flower, these plants need light periods that are longer than a certain time. Examples of long-day plants include spinach, radishes, lettuce, and irises.

Some plants aren’t picky about their photoperiod and will flower no matter how long they get sunlight—these are called day-neutral plants, and include tomatoes, rice and dandelions. Other plants are extremely picky and require certain conditions in addition to photoperiod. Some plants that grow in northern climates need a specific photoperiod and prolonged cold temperatures to stimulate flowering. This process, called vernalization, is required of cabbage, celery, magnolias and beets.

After a few more decades of manipulating plant flowering periods, researchers realized that flowering responds to the length of the dark period (night), not the length of light. For example, one short-day plant they used was cocklebur (Xanthium strumarium), which required days that were shorter than 16 hours to flower. If researchers interrupted the light period by putting the plant in total darkness for a brief time, the plant still flowered. However, if they interrupted the dark period by briefly turning on the lights, the plant didn’t flower. The researchers concluded that the short-day plants are actually long-night plants because they need a period of uninterrupted darkness that is longer than a certain length.

The length of the night necessary for the plant to flower is called the critical period. For the Maryland Mammoth, the critical night period is ten hours because it needed a daylength shorter than 14 hours and there are 24 hours in a day.

Plant stem

Stem showing internode and nodes plus leaf petioles

A stem is one of two main structural axes of a vascular plant, the other being the root. The stem is normally divided into nodes and internodes. The nodes hold buds which grow into one or more leaves, conifer cones, roots, other stems, or flowers (inflorescences); the internodes distance one node from another. The term "shoots" is often confused with "stems"; "shoots" generally refers to new fresh plant growth including both stems and other structures like leaves or flowers. In most plants stems are located above the soil surface but some plants have underground stems. A stem develops buds and shoots and usually grows above the ground. Inside the stem, materials move up and down the tissues of the transport system.Stems are often green.Stems have thin pipes to carry water around the plant.Stem consists of xylem and phloem tissue.

Stems have four main functions which are:^[1]

• Support for and the elevation of leaves, flowers and fruits. The stems keep the leaves in the light and provide a place for the plant to keep its flowers and fruits.

• Transport of fluids between the roots and the shoots in the xylem and phloem.

• Storage of nutrients.

• Production of new living tissue. The normal life span of plant cells is one to three years. Stems have cells called meristems that annually generate new living tissue.

Stem structure

Flax stem cross-section, showing locations of underlying tissues. Ep = epidermis; C = cortex; BF = bastfibres; P = phloem; X = xylem; Pi =pith

Stem usually consist of three tissues, dermal tissue, ground tissue and vascular tissue. The dermal tissue covers the outer surface of the stem and usually functions to waterproof, protect and control gas exchange. The ground tissue usually consists mainly of parenchyma cells and fills in around the vascular tissue. It sometimes functions in photosynthesis. Vascular tissue provides long distance transport and structural support. Most or all ground tissue may be lost in woody stems. The dermal tissue of aquatic plants stems may lack the waterproofing found in aerial stems. The arrangement of the vascular tissues varies widely among plant species.

Dicot stems

Dicot stem with primary growth have pith in the center, with vascular bundles forming a distinct ring visible when the stem is viewed in cross section. The outside of the stem is covered with an epidermis, which is covered by a waterproof cuticle. The epidermis also may contain stomata for gas exchange and multicellular stem hairs called trichomes. A cortex consisting of Hypodermis (collenchyma cells) and Endodermis (starch containing cells)is present above the pericycle and vascular bundles.

Woody dicots and many nonwoody dicots have secondary growth originating from their lateral or secondary meristems: the vascular cambium and the cork cambium or phellogen. The vascular cambium forms between the xylem and phloem in the vascular bundles and connects to form a continuous cylinder. The vascular cambium cells divide to produce secondary xylem to the inside and secondary phloem to the outside. As the stem increases in diameter due to production of secondary xylem and secondary phloem, the cortex and epidermis are eventually destroyed. Before the cortex is destroyed, a cork cambium develops there. The cork cambium divides to produce waterproof cork cells externally and sometimes phelloderm cells internally. Those three tissues form the periderm, which replaces the epidermis in function. Areas of loosely packed cells in the periderm that function in gas exchange are called lenticels.

Secondary xylem is commercially important as wood. The seasonal variation in growth from the vascular cambium is what creates yearly tree rings in temperate climates. Tree rings are the basis of dendrochronology, which dates wooden objects and associated artifacts. Dendroclimatology is the use of tree rings as a record of past climates. The aerial stem of an adult tree is called a trunk. The dead, usually darker inner wood of a large diameter trunk is termed the heartwood and is the result of tylosis. The outer, living wood is termed the sapwood.

Monocot stems

Stems of two Roystonearegia palms showing characteristic bulge, leaf scars and fibrous roots, Kolkata, India

Vascular bundles are present throughout the monocot stem, although concentrated towards the outside. This differs from the dicot stem that has a ring of vascular bundles and often none in the center. The shoot apex in monocot stems is more elongated. Leaf sheathes grow up around it, protecting it. This is true to some extent of almost all monocots. Monocots rarely produce secondary growth and are therefore seldom woody, with Palmsand Bamboo being notable exceptions. However, many monocot stems increase in diameter via anomalous secondary growth.

Gymnosperm stems

All gymnosperms are woody plants. Their stems are similar in structure to woody dicots except that most gymnosperms produce only tracheids in their xylem, not the vessels found in dicots. Gymnosperm wood also often contains resin ducts. Woody dicots are called hardwoods, e.g. oak, maple and walnut. In contrast, softwoods are gymnosperms, such as pine, spruce and fir.

Fern stems

Most ferns have rhizomes with no vertical stem. The exception is tree ferns, with vertical stems up to about 20 metres. The stem anatomy of ferns is more complicated than that of dicots because fern stems often have one or more leaf gaps in cross section. A leaf gap is where the vascular tissue branches off to a frond. In cross section, the vascular tissue does not form a complete cylinder where a leaf gap occurs. Fern stems may havesolenosteles or dictyosteles or variations of them. Many fern stems have phloem tissue on both sides of the xylem in cross-section.

Relation to xenobiotics

Foreign chemicals such as air pollutants,^[3] herbicides and pesticides can damage stem structures. In the case of herbicides many chemicals act by surficial effects, and other agents cause damage through uptake of the chemical herbicide, which is typically a complex organic chemical.

Economic importance