Reproduction On Land

A primitive land plant requires several adaptations in order to reproduce on land. One of these is a method to move gametes from one gametophyte to another in order to effect fertilization, and the other is some method of spore dispersal in which the dispersal units are protected from the desiccating environment. In addition, it is important to move the sporo-phytic reproductive units up off the substrate, both to prevent infection by microbes and presumably to disperse spores further, in order to colonize new substrates. Ancestral algae produce motile gametes and spores in an aqueous environment. In the most primitive land plants, gamete transfer is still dependent on water, as it was in the algae, but spores of even the earliest land plants are already protected by a wall of sporopollenin, which persists through all the subsequent land plant lineages. Thus, land plant sporophytes produce spores or seeds that resist desiccation and can be transported great distances by abiotic or biotic vectors. The simplest method to raise dispersal units off the substrate is a sporangium positioned terminally on upright axes, and numerous examples of the earliest bona fide land plants show terminal sporangia on tiny unbranched or branched naked axes (Edwards and Wellman, 2001). As will be seen in later chapters, the vascular plant sporophyte in higher plants has become so dominant that the gametophyte phase is completely dependent on the sporophyte for its survival. In the more specialized vascular plants, that is the seed plants, water as a medium for fertilization is no longer necessary. Whether we are speaking of a cycad, club moss, lily, or beech tree, each has basically the same life cycle pattern: a dominant, diploid sporophyte capable of producing spores as a result of meiosis, with each spore germinating to produce a haploid gametophyte (or gameto-phytes) that produce two types of gametes—egg and sperm. The life cycle is complete when the two gametes fuse to initiate the diploid phase of the new sporophyte (Graham, 1985).

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