The Development of Land Flora

Today’s most primitive land plants, the liverworts and hornworts, have patterns in their DNA that indicate a connection to a group of complex green algae that are found only in freshwater, which suggests that land plants arose in freshwater lakes and streams or from river deltas-not from the ocean shore. But what might have caused a plant to adapt to the harshness of a land environment? The most reasonable scenario suggests that early land plants lived along streams or in ponds that dried up during part of the year. Their first adaptation was the spore, a reproductive cell that could survive this dry period. Carried by the wind, the microscopic spore could be scattered across wide areas, germinating (starting to grow)—if lucky -in a distant pool of quiet water or moist mud. Such spores are the first evidence of plants living on land.

Earlier, blue-green bacteria probably lived on damp soil and in shallow pools over hundreds of millions of years. They were probably joined by green algae and fungi to form puddles of green glop in moist depressions and along the edges of ponds and streams. These lineages (lines of descent) may have populated land surfaces for many millions of years before more complex land plants arose. And they, too, had to develop sporelike stages to survive drying out and achieve dispersal. Unfortunately, because they did not possess resistant tissues, these early forms of land flora left no trace in the fossil record. The first “true” land plants were probably flat liverwort- like plants with little leaves only one cell layer thick. Again, these first land plants are remembered in the rocks only by their spores. Being able to withstand the physical stresses of drying and rehydration, the tough spore wall is also resistant to decomposition. Thus, spores became an important element in the fossil record. Algal and fungal spores are quite different from these. These early spores are first recorded in the rocks dated at around 470 million years ago, and they are usually found as groups of four (called tetrads). By about 430 million years ago, spore tetrads declined in abundance and separate individual spores with a Y-shaped (trilete) configuration on one side became common.

About 410 million years ago, with plant spores diversifying, the rocks preserved the first fragmentary evidence of larger plants. It was not until more complex land plants had evolved desiccation-resistant surfaces and a strong erect structure that a really three-dimensional terrestrial vegetation came into being. Waxy surfaces were a major innovation in the advent of larger terrestrial plants. In addition to reducing water loss through outer surfaces, a waxy layer protected against ultraviolet radiation, microbial attack, and corrosive chemicals. Unfortunately, such protection also presents a problem: how can a plant absorb carbon dioxide from the air if it’s covered with wax? Moist interior cell surfaces must be exposed to air in order to absorb carbon dioxide for photosynthesis (the process through which plants obtain energy by using sunlight, carbon dioxide, and water). Inevitably, exposing moist cells to air will also result in water loss by evaporation from within the plant. And because carbon dioxide concentrations are low in air, a great deal of water will be lost in the process of absorbing carbon dioxide. (This is why plants require a lot more water to survive than do animals with a similar volume of active tissue.) The solution to this plant problem came in the form of special little pores called stomates on the surfaces of the plant: pores that could be opened for gas exchange, and closed when conditions became too dry. The other major adaptation for life on land was a ‘plumbing system” to conduct water from the roots to the parts of the plant active in photosynthesis. The vascular/plumbing system includes thick-walled cells that quickly become dead, empty, and open at the ends. Laid out end-to-end these empty cells resemble a system of pipes, facilitating the flow of water through the plant. These same dead thick-walled cells would also serve as architectural support. With a vascular system and stomates in place, the earliest erect plants were able to draw water up to the aerial parts of the plant and provide strength in the face of wind and weather. 

完整版题目和答案请付费后查阅：