III. KINGDOM PLANTAE
D. Gymnosperm and Angiosperm Seed Plants
Modern seed plants are divided into 2 groups: gymnosperms and angiosperms. Angiosperms are the most successful land plants. They comprise 235,000 species and dominate large areas of land, including hardwood forests and grasslands. Human agriculture is almost entirely devoted to the production of angiosperm seed plants and angiosperm seeds provide the vast majority of human food, directly or indirectly. Accordingly, the first half of LIFE 2023 was devoted to the structure and function of angiosperm plants.
Gymnosperm plants have existed on the land for well over 300 million years. In the present, conifers are the most successful group of gymnosperms and can form large forests in temperate and boreal regions of Earth. Other gymnosperm groups are not very successful in the present but provide fascinating evidence of earlier seed plant strategies for life on land. Extant gymnosperm seed plants include the following groups.
The cycads resemble ferns because of their leaf structure and their whorled leaf arrangement. To the touch, however, they are very stiff and tough and their large cones make clear that they are gymnosperms. In the Jurassic period, cycads were dominant members of the land flora and likely provided much of the food for herbivorous dinosaurs. In the present, there are less than 300 species of extant cycads, many of them endangered species. A common cycad is Cycas revoluta, the "sago palm.
The ginkgos consist of a single species, Ginkgo biloba. Ginkgos were known from fossils for many years and thought to be extinct until living specimens were discovered under cultivation in China. Ginkgos are now widely distributed as ornamental plants and a source of herbal medicines. Ginkgos have distinctive leaves that are deciduous like those of angiosperm trees. Ginkgos also have seeds with a fleshy, fruit-like layer known for its evil smell. Ginkgos are either "male", with cones that produce pollen, or "female", with ovules. Most of the ornamental ginkgos planted around the world are cone-bearing to avoid the production of piles of smelly ginkgo fruits in the urban settings where they are often planted.
Gnetophytes
The gnetophytes include genus Ephedra, also known as Mormon Tea. The herbal stimulant "ephedrine" is extracted from Ephedra. It is an unusually strong herbal medication and now a controlled substance in the US to reduce the risks of misuse. Gnetophytes now consist of 3 genera and less than 70 species.
Conifers are the most successful group of gymnosperm plants in the present day. There are only about 720 species of conifers but they dominate vast tracts of land in the northern boreal forest of Earth.
1. Characters shared between gymnosperm and angiosperm seed plants
Gymnosperm seed plants share several characters with angiosperm seed plants that are not shared with seedless plants or algae. These include a haploid gametophyte enclosed and protected by the diploid sporophyte, lignified xylem that exhibits secondary growth to increase girth, well-developed roots, complex leaves, pollen, and seeds.
2. Distinguishing characters
Gymnosperm seed plants have several characters not shared with angiosperm seed plants. These include a larger female gametophyte consisting of 2,000 cells with archegonia, cones with exposed seeds on the cone scales ("gymnosperm" means naked seed"), seed reserves that consist of the haploid female gametophyte and the many diploid sporophyte siblings generated by polyembryony, and xylem that includes only tracheids for movement of water.
In contrast, angiosperms have female gametophytes composed of only 7 cells (8 nuclei) with no archegonia, a seed reserve consisting of the polyploid endosperm resulting from double fertilization, large-diameter vessels in their xylem rather than just tracheids, flowers that promote efficient pollination by insects in many cases, and fruits that promote distribution of seeds. These features of angiosperm plants are believed to confer several advantages for survival in the harsh terrestrial environment.
Polyploid endosperm of angiosperm seeds may be a better source of stored food than the haploid gametophyte and diploid sporophyte siblings used by gymnosperms. Higher ploidy levels of the endosperm cells may allow greater synthesis of carbohydrates, lipids, and proteins needed by the young sporophyte as it germinates.
Vessels are larger in diameter than tracheids and can rapidly supply growing leaves with water from the soil. As a rule, angiosperms grow faster than gymnosperms, in part because of their vessels.
Flowers are an extremely important feature. They induce insects and other animals to assist with pollination between plants that are far apart, which decreases the chance of inbreeding and increases genetic diversity in the population. Flowers also increase the efficiency of pollination, since animals often prefer specific flowers and thus exchange pollen between plants of the same species. The combination of flowers for pollination and seeds that can survive years of unfavorable conditions allow angiosperms to be seasonal annuals in regions with harsh winters that would otherwise kill them. Populations of seasonal annuals die out entirely every year but survive as a species because flowering assures successful pollination in a single growing season and because seeds can survive in the soil until conditions are suitable for germination. Flowers are proposed as one of the primary reasons that angiosperms have come to dominate the harsh terrestrial environment. Interestingly, one group of angiosperms, the grasses, have reverted to wind pollination in many cases.
Fruits promote distribution of seeds by animals who eat the fruits and distribute undigested seeds in their dung as they move about.
G. Great Moments in Plant History
To conclude our discussion of plant biology and how it came to be what it is now, let's review what is known about the history of plant life on Earth. It is one of the great stories of the universe and must certainly contain insights that humans have yet to fully understand or put into practice.
1. 2.5 billion years ago
The beginning of plant evolution occurred in this pre-Cambrian time period with the acquisition of plastids by endosymbiosis. The plastid endosymbiont is believed to have been an early cyanobacterium. When it acquired plastids, the plant line diverged from animals forever. 2.5 billion years of separate evolution accounts for how plants differ from animals in so many fundamental aspects of their biology.
2. 410 million years ago
Fossils of the earliest land plants have been dated to 410 million years ago. Multicellular life in the seas was already diverse by this time but the land remained un-colonized after 3 billion years. Plants were the first organisms to invade this new habitat, likely accompanied by fungi and followed shortly after by insects. The earliest plants were little more than stems with sporangia, for example Cooksonia.
3. 375 million years ago, The Coal Age
In less than 50 million years, plants were well established in low-lying swamps near coastlines. Many of the species present were seedless tracheophytes comparable to those in the present but some much larger. In the warm, moist conditions believed to prevail in the ancient "coal swamps", swimming sperm and lack of seeds were unlikely to be a liability to survival. The earliest gymnosperms were also present in the coal swamps
4. 248 million years ago, the Great Permian Extinction
The greatest mass extinction in Earth's history occurred at this time and the coal swamps disappeared, along with many species of seedless trachophytes. There is much debate about what caused the Great Permian Extinction. One element may have been have been the formation of Pangea, a supercontinent composed of many modern continets pushed together. The coalesence of continents could have reduced coastline habitat and reduced low-lying swamp area by uplift. A cooler, drier climate may also have contributed to extinction of many seedless tracheophytes.
5. 180 million years ago, the Age of Cycads
After the Coal Age, gymnosperms prospered and became major components of the land flora. Cycads flourished at this time and may have provided food for the largest herbivorous dinosaurs.
6. 120 million years ago
Fossils of the earliest flowering plants resemble modern day magnolias. They date to the Cretaceous period and diversified rapidly. Flowering plants tend to grow faster than gymnosperms and flowering shrubs may have fed herds of ceratopsian dinosaurs. The had beak-like mouths that they may have used to crop the angiosperm shrubs of the Cretaceous.
7. 65 million years ago, the K-T extinction
A mass extinction of dinosaurs and other organisms marks the end of the Cretaceous period. Many plants survived this extinction event but angiosperms began to dominate the land flora in many parts of the world.
8. 20 million years ago
During this cooler, drier period of Earth's history, the first grasses appear. Grasslands and grazing animals are now important elements of life on Earth. C4 photosynthesis also appeared as a strategy for coping with low CO2 levels of this time.
9. The present
Recent history has been a dynamic period in the history of plant life. The dominance of humans has precipitated another mass extinction resulting from loss and degradation habitat but some species of plants have thrived by domestication. Selective breeding of these has produced great phenotypic diversity and the equivalent of new species. From this point forward, the history of plant life will be determined by human choices.