Angiosperms
Phylum Magnoliophyta; flowering & fruiting plants
The angiosperms are the most successful and dominant plants on the Earth with over 250,000 species. Angiosperms evolved duringthe Early Cretaceous (130 million years ago), and survived the extinction event that killed the non-avian dinosaurs. As their name indicates, they are plants that produce showy reproductive structures called flowers, which attract pollinators seeking a food source such as nectar or pollen. Animal-pollination is not unique to angiosperms, whereas wind-pollination is more common in gymnosperms. The pollinated flower (pollen is transferred to the stigma) leads to fertilization of the ovules inside the pistil. After fertilization, the ovules develop into seeds, and the pistil matures into a ripe fruit. Angiosperms have a wide range of fruit/seed dispersal mechanisms allowing them to populate the Earth.
Ecology and Form
Vascular plants
Possess xylem with vessel elements (except Amborella)
Possess phloem with sieve elements.
Seed-bearing plants (spermatophytes), but unlike other seed plants, angiosperms have a double integument around ovule
Flowering plants
Determinate stem with 4 whorls of modified leaves (i.e. sepals, petals, stamens, pistils)
Fruiting plants: ovary becomes fruit
Sporophyte is dominant, photosynthetic phase
Gametophyte is highly-reduced but still multicellular
Female ♀ gametophyte is 7 cells, located in ovule
Male ♂ gametophyte is located in pollen; germinate into the pollen tube and 2 sperm nuclei
Life Cycle
Flowers, Fruits, and Inflorescences: Review Botany 1: Reproduction
Ovule (♀) development
Like all plants, meiosis produces 4 spores from a spore mother cell
In seed plants, the "female spores" are large and called megaspores
During development, only 1 megaspore remains functional, and the others abort
The nucleus of this megaspore goes through mitosis three times to produce 8 nuclei (2 x 2 x 2).
The result is the following nuclei:
1 egg (after fertilization will become the embryo)
2 synergids (next to the egg on the micropylar end)
3 antipodals (at end opposite of egg; on the chazal end)
2 polar nuclei (in the central cell)
Each nucleus is separated into its own cell, so that 1 cell = 1 nucleus, except...
...the 2 polar nuclei that are partitioned into one large cell called the central cell
This central cell is considered dikaryotic
During fertilization, the central cell will become the endosperm (see below)
These seven cells (egg, synergids, antipodals, central cell) are the female gametophyte (megagametophyte)
Evolution has greatly reduced the female gametophyte
Pollen (♂) Development
Each anther is composed of several microsporangia (usu. 4), which produce pollen
Each pollen grain contains the male gametophyte, which is just a tube-cell nucleus, and a generative cell.
The tube-cell nucleus will go through many mitotic divisions to create the pollen tube
The generative cell will go through mitosis, once, to create two (2) sperm nuclei. These sperm cells lack flagella.
Double Fertilization
This is a process by which there are two fertilization events inside the ovule; it is unique to angiosperms and the gnetophytes
Like most plants, there is one fertilization event that involves a sperm cell fusing with an egg cell to create an embryo (which is diploid)
The other fertilization event involves another sperm cell that fuses with the central cell to create the endosperm, which is triploid in nature
The triploid endosperm serves as high-energy tissue to nourish the embryo
N.B. The nutritive tissue in gymnosperms is the haploid female gametophyte
Gymnosperms vs. Angiosperms
In the fossil record and extant plants, evolution has produced flower-like and fruit-like structures in gymnosperms groups
What is defining feature that separates gymnosperms from true angiosperms?
Since angiosperms are defined by an enclosed carpal (fruit), this means that pollination occurs differently in both groups
Where does pollination occur?
Gymnosperms
Pollen lands directly on ovule, even if ovule is in a cone
Angiosperms
The ovules are enclosed in a carpal (a modified protective leaf known as the megasporophyll)
Pollen lands on the carpal or pistil; usually on the stigma.
Pollination does not occur on the ovules; pollen must grow through a pollen tube toward the ovules.
Monocot evolution
¼ of all flowering plants are monocots (e.g. orchids, grasses, sedges, palms, aroids, etc.)
Monocots appear relatively early in the evolution of angiosperms
Apomorphies include a single cotyledon in the seed, and an scattered appearance of vascular bundles in stem cross-sections (atactostele)
Monocot growth forms are wide-ranging; mostly herbaceous; none have true wood
Tree-like forms have evolved at least 4x in evolution
Central trunk with branches (e.g. Yucca, Pandanus)
Central trunk with leaves attached (e.g. palms)
Monocot with woody, secondary growth (i.e. Dracaena draco), which is another independent origin of wood
Short trunk with rosette of massive leaves (e.g. Agave, Musa, Ravenala)
Succulent, leafy rosettes (e.g. Agave, Aloe, bromeliads)
Bulbs with modified storage leaves (e.g. Allium, Lilium, etc.)
Vines and vine-like herbs (e.g. Dioscorea, Smilax, Tillandsia, Philodendron)
“Grassy forms” found in the Poales (e.g. Grasses, Sedges, Rushes)
Basal meristem that produces new growth at ground level
Many have efficient wind pollination and dispersal
C4 carbon fixation pathway in many grasses and sedges
Orchids (Orchidaceae): Largest of all plant families with 25–30K species
Greater variety of pollinators than any other family
Special petal, labellum, is governed by different genes than other petals, which may allow for greater floral diversity than other angiosperms
Diversity
A.N.A. Grade
Amborellaceae (e.g. Amborella)
Nymphaeales (e.g. Nymphaea, Cambomba, Trithuria)
Austrobaileyales (e.g. Austrobaileya, Illicium, Trimenia)
Magnoliids
Canellales (e.g. Canella, Drimys)
Laurales (e.g. Sassafras, Camphora, Persea, Cinnamomum, Idiospermum, Calycanthus)
Magnoliales (e.g. Magnolia, Liriodendron, Asimina, Myristica)
Piperales (e.g. Piper, Peperomia, Saururus, Asarum, Aristolochia)
Chloranthales (e.g. Chloranthus, Sarcandra)
Monocots (e.g. Zea, Bambusa, Zingiber, Musa, Areca, Arisaema, Acorus, Pandanus, Lilium, Dioscorea, Aspargus)
Ceratophyllales (e.g. Ceratophyllum)
Eudicots (based upon APG IV)
Clade Core eudicots
Clade Superrosids
Clade Rosids
Clade Fabids
Clade Malvids
Clade Superasterids
Clade Asterids
Clade Campanulids
Clade Lamiids
The form, color, and scent of flowers correlated to pollinator preference
Ethnobotany
Agriculture and artificial selection have greatly shaped angiosperm evolution: selecting better-tasting, less toxic plants
Only about 300 species have been domesticated to any degree
Only a little over 100 species account for 95% of the world human food budget in terms of total calories consumed
About 20 are of major importance
Grasses: wheat, rice, maize, sugar cane
Legumes: common bean, soybean, lentil, peanut, pea
Starchy staples: white potato, sweet potato, yam, cassava, banana
Oil seeds, fruits, & vegetables: coconut, sunflower, tomato, orange, apple, cabbage/mustards, onions
Geologic Age
Early Cretaceous - present
Additional Resources
Study shows orchid family emerged in northern hemisphere and thrived alongside dinosaurs for 20 million years (Phys.org 22Feb2024)
└ Pérez-Escobar et al. (2024) The origin and speciation of orchids
Grasses take evolutionary shortcut by borrowing genes from their neighbors (Phys.org 5Oct2023)
└ Raimondeau et al. (2023) Lateral gene transfer generates accessory genes that accumulate at different rates within a grass lineage
Nature's great survivors: Flowering plants survived the mass extinction that killed the dinosaurs (Phys.org 12Sep2023)
Evolutionary origin of the Asteraceae capitulum: Insights from Calyceraceae (Pozner et al. 2012, AJB)
Bennetittales (extinct gymnosperm with flower-like cones)
Hydatellaceae: The Other Basal Angiosperms (In Defense of Plants, Feb 2018)
Pollen competition (In Defense of Plants, May 2019)
Flowers are changing color in response to climate change (Science Sep 28, 2020)