By the end of this module, students should be able to
For more than 3 billion years, the earth's terrestrial surface was lifeless. Geological evidence indicates that an algal scum of cyanbacteria-like organisms first formed thin coatings on land 1.2 billion years ago. In the Ordovician period, around 450 million years ago, the first land plants appeared. These began to diversify in the late Silurian Period, around 420 million years ago; the results of their diversification are displayed in remarkable detail in an early Devonian fossil assemblage from the Rhynie chert (which is in Scotland). This chert preserved early plants petrified in volcanic springs in cellular detail. By the middle of the Devonian Period, most of the features recognized in plants today were present, including roots, leaves, and secondary wood; by late Devonian times, seeds had evolved.
Late Devonian plants (385 million years ago) had thus reached a degree of sophistication that allowed them to form forests of tall trees. Evolutionary innovation continued after the Devonian period. Most plant groups were relatively unscathed by the Permo-Triassic extinction event, although the structures of communities changed. This may have set the scene for the evolution of flowering plants in the Triassic (200 million years ago), which exploded in the Cretaceous and Tertiary periods. The last major group of plants to evolve were the grasses, which became important in the mid-Tertiary Period, from around 40 million years ago. The grasses, as well as many other groups, evolved new mechanisms of metabolism to survive the low CO2 and warm, dry conditions of the tropics over the last 10 million years.
Photosynthesis, conducted by land plants and algae, is the ultimate source of energy and organic material in nearly all ecosystems. Photosynthesis radically changed the composition of the early earth's atmosphere, which as a result is now 21% oxygen. Animals and most other organisms are aerobic, relying on oxygen; those that are not are confined to relatively rare anaerobic environments. Plants are the primary producers in most terrestrial ecosystems and form the basis of the food web in those ecosystems. Many animals rely on plants for shelter, as well as oxygen and food.
The plants that are most familiar to us are the multicellular land plants, called embryophytes. These include the vascular plants: plants with full systems of leaves, stems, and roots. They also include a few of their close relatives, often called "bryophytes," of which mosses and liverworts are the most common.
Land plants, or embryophytes, share many characteristics with photosynthetic protists. Green algae are the closest relatives of land plants. With a few exceptions among the green algae, all such forms have cell walls containing cellulose, have chloroplasts containing chlorophylls a and b, store food in the form of starch, undergo closed mitosis without centrioles, and typically have mitochondria with flat cristae. There are two recognized classifications of green algae: Chlorophyta and Charophyta. Land plants share four additional characteristics only with Charophyta, suggesting that these two groups are most closely related:
It is widely accepted that embryophytes evolved from Charophycean Algae or a common ancestor of these algae. In other words, the tremendous diversity we see in land plants today, from mosses to redwoods and orchids, all descended from a single common ancestor that colonized land 430-470 million years ago. Uncertainty remains concerning the precise relationships between embryophytes and their algal relatives, but the comparison of similarities between land plants and charophytes may provide clues to the specific genes that allowed a lineage of photosynthetic organisms to colonize land so successfully and give rise to land plants.
Biologists think that the first land plants on earth evolved from shallow freshwater algae, much like Chara some 400 million years ago. The charophyte green algae include the most complex forms of photosynthetic cells other than those of land plants. The point where these non-algal plants begin and algae stop is usually taken to be the presence of reproductive organs with protective cell layers, a characteristic not found in the other alga groups.
Terrestrial habitats have advantages and disadvantages. On land, sunlight energy and CO2 are more available than in water. Mineral nutrients are abundantly available in soils for those organisms that have adaptations to absorb those nutrients. When plants first colonized land, herbivores and pathogens were essentially non-existent there.
Changing environmental factors such as heat, cold, drought, or floods presented great challenges to colonizing species. Plants did not develop the ability to move and escape unfavorable environmental situations as animals did, but plants became phenotypically plastic, meaning they could display a number of observable characteristics, and have an impressive array of genes that aid in adapting to changing conditions. It is hypothesized that this large number of genes can be partly explained by plant species' need to adapt to a wider range of conditions. In adapting to terrestrial life, plants had to find ways to obtain nutrients and release waste products without dehydrating. In addition, plants needed to develop structural support to counteract gravity, as well as protection against environmental factors like over-hydration, freezing, and strong winds.
There are many different kinds of plants. However, all plants share some features in common. Plants are all eukaryotic, multicellular autotrophs. Photosynthesis is their autotrophic mechanism and their chloroplasts contain chlorophyll a and b. Their cells have cell walls made out of the polysaccharide, cellulose. For the most part, plants are adapted for life on land. Finally, plants employ a life cycle called "alternation of generations," in which plants alternate between a haploid and a diploid generation.
Date last modified: August 30, 2016.
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