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figure 6.36 Frederick Orpen Bower. (Courtesy A. C. Scott.)

pteridologist F. O. Bower (1908) (FIG. 6.36), views the sporophyte as gradually evolving away from a parasitic dependence on the gametophyte to become a physiologically independent, photosynthesizing organism.

The fossil record provides no direct evidence as to which of these ideas is correct as it is not possible to know the sequence of events that took place after the spores of early land plants germinated. Both ideas do, however, allow for subsequent patterns of plant evolution regardless of which theory may be correct. Those supporting the antithetic theory suggest that the diploid phase was initially better adapted to life on land than the haploid phase (Keddy, 1981). As a result the diploid phase continued to evolve adaptations that contributed to increasing organ differentiation, whereas the haploid phase remained small. Proponents of the homologous theory argue that, although both phases were initially identical, environmental influences provided selective pressures, so that each phase continued to evolve different levels of specialization and reproductive strategy.

Why is the diploid phase better adapted to the terrestrial environment? One answer is that water is necessary for fertilization in the most primitive land plants. Sperm cells of primitive extant plants are motile (perhaps a retention of an algal characteristic) and require the presence of water to reach the egg cell. Water may be in the form of a film of dew, a swampy environment, or splashing raindrops. Whatever the source, water is essential to fertilization. Once the new sporophyte (diploid plant) has developed, spores are produced, released, and carried away by air currents. Even though only a small proportion of the diploid plant body is used for spore production, the absolute number of spores produced can be quite large. Spores may be distributed randomly over wide areas, with each capable of producing a haploid gametophyte. In general, the larger the number of spores produced, the greater the probability that some will settle in places suitable for germination and the eventual production of gametes. It should be pointed out, however, that in some extant plants not all spores produced are viable, and this no doubt happened in the fossil record as well.

Our understanding of early land plants is based primarily on features of the sporophyte and dispersed spores. The discovery of gametophytes from the Lower Devonian Rhynie chert has provided important details about the gametophyte phase of some early land plants and these will be discussed in Chapter 8 (Remy, 1982). In these plants the gametophytes are all free-living, autotrophic organisms that differ morphologically from the sporophyte. It is not known, for example, whether any of the early land plants possessed a life history in which the gametophyte phase existed in an aquatic environment, whereas the sporophyte occupied a terrestrial habitat. Perhaps a separation of the two life history phases in the early evolution of land plants is one reason why the sporo-phyte has become the dominant phase in vascular plants today (but see Bennici, 2005).

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