Terrestrial habitats presented very different environments, and consequently selective pressures, to colonizing plants. As we examine plants in the next several labs, you will discover that many aspects of their biology have been modified through time. Such modifications have accounted for a variety of adaptations to land. Plant reproductive systems reflect changes resulting from the loss of the ancestral aquatic habitat. In water the sperm of one individual could swim to other individuals. Water in a terrestrial environment, however, varies from place to place and time to time. Reproduction depending upon the release of sperm into water became modified enough that gametes could be exchanged on land. With each new group of plants that evolved, the difficulty of exchanging gametes was solved in many different and perhaps increasingly more efficient ways. Similar trends in the structure and elaboration of the primary organs of plants and their overall life form can also be observed. Different groups of plants evolved which utilized a variety of solutions and combinations of solutions and were able to survive on land.

(A) The influences of Sexual Reproduction on Genetic Variation

How did this variety come about over so short a period of time? This asks the evolutionary question of how plants maintained so much genetic variation within populations that they were selected for the variety of ways of coping on land. One of the most effective ways of maintaining and providing genetic variation is through sexual reproduction: the fusion of two nuclei and the meiotic division that follows. This increase in genetic variation is due largely to the increase in the number of meiotic events in the life cycle. The more meiotic events (that is, number of cells dividing by meiosis), the more the chromosomes and genes are shuffled into new gene combinations. New gene combinations in individuals lead to genetic variation in a population. Thus, very different organisms with different genetic makeup have come to occupy and survive in the varied land habitats.

Compare the generalized life cycles. In land plants, for each zygote that is formed there can be many meiotic events generated. Compare this to the one meiotic event generated for each zygote formed in the life cycle of algae or fungi. A 1n-dominated life cycle with no 2n-development past a zygote must produce populations orders of magnitude larger in size to produce as much variation.

(B) Gamete/spore dispersal in a dry environment

The major problem to overcome was the loss of an aquatic matrix as a means for the exchange of gametes. Gametes can be particularly sensitive to their environment and in most cases even moderate drying can inhibit reproductive success. The same problem arises in considering the dispersal of spores. In the algae, the spores produced by the diploid zygote (or sporophyte in some cases) is flagellated and simply swims to a suitable location to develop. Spores must also be modified for successful dispersal and development on land.

(C) Ideas to keep in mind

When examining the various plant groups, keep the following ideas in mind:

1) Plants exhibit alternation of generations, that is, independent haploid (gamete-producing gametophyte) and diploid (spore-producing sporophyte) organisms. Consider if one generation or the other is more sensitive to the physical environment or to fluctuations in habitat moisture. Compare this generation between groups and consider its relative exposure to the environment.

2) Consider how gametes are moved from one individual to the other. Compare between different plant groups the sensitivity and relative exposure to the environment involved in transporting sperm to the egg or in dispersing spores.

3) Once the gametes fuse and an embryo is formed, consider its location and exposure to the environment. Compare between groups.