HIGHER LAND PLANTS 



167 



independent in lower taxa but a sporophyte parasite 

 in the higher taxa; gametophyte organization varies 

 from a cylindrical body with some tissue organiza- 

 tion, to a membranous structure reminiscent of cer- 

 tain liverworts and horned liverworts, to a single cell; 

 sporophyte functionally independent at maturity, 

 containing vascular tissues (central cylinder, veins, or 

 stele) and many other tissues grouped into organs; 

 organs especially in the form of leaves, stems, and 

 roots (roots absent in Psilopsida); over-all structure 

 associated with whether plants produce a single kind 

 of spore or two kinds of spores (apparently the ances- 

 tral condition); two-spore species produce male 

 spores (smaller) which form a strictly male gameto- 

 phyte and female spores (larger) which form a strictly 

 female gametophyte; living "primitive" species gener- 

 ally with one spore (includes Psilopsida; Lycopsida, 

 except for certain fossil orders, the Isoetales and the 

 Selaginellales; Sphenopsida, except for a few fossil 

 forms; Filicineae, except for certain fossils and the 

 Marsileales and, probably, the Salviniales). 



The life cycle of Tracheophyta involves a haploid 

 spore (one set of chromosomes) which develops into a 

 haploid gametophyte. The gametophyte may or may 

 not constitute a distinct plant that is independent of 

 the sporophyte. Gametophytes produce sperm and 

 eggs, and a sperm fertilizes an egg to form a diploid 

 zygote (two sets of chromosomes). The zygote divides 

 mitotically, producing a diploid embryo sporophyte 

 which eventually develops into the mature sporo- 

 phyte. The mature sporophyte completes the life 

 cycle when it produces haploid spores. Therefore, the 

 life cycle is diplobiontic with two adult generations, 

 gametophyte and sporophyte (Figure 11.1). 



G.iiMETOPHVTE 



In all subphyla except the three species constituting 

 the living psilopsids, there is a tendency toward re- 

 duction in size, complexity, and duration of the game- 

 tophyte generation (see the many life cycle diagrams 

 in the remainder of this chapter). Therefore, the 

 known gametophyte reductions in lycopsids, sphe- 

 nopsids, and pteropsids are a case of distinct parallel 

 evolution because each group has evolved in its own 

 way to a simplified gametophyte. This parallel and 

 apparently directional evolution (orthogenesis) seems 

 to be explained by a past need for all land plants that 

 survived to assume a more terrestrial existence. This 

 probably is the case because gametophytes are rela- 

 tively more fragile and susceptible to environmental 



-ADULT SPOROPHYTE (2n) 



\ 



SPORANGIUM 

 SPORE-FORMING 

 CELLS (2n) 



\ 



OSIS 



\ 



SPORE (n) 



I 



mitosis 



/ 



EGG SPERM 



GAMETOPHYTE (n) 



Figure 11.1 Lite cycle of vascular plants, simplified to the point of 

 being suitable for bryophytes also. 



extremes than are sporophytes. The gametophyte is 

 less able to do without water than is the sporophyte. 

 Moreover, this "gametophyte problem" apparently 

 was solved in the same general way. In each sub- 

 phylum, reduced size and complexity enabled a much 

 shorter gametophyte generation and, therefore, less 

 dependence upon the dangers of varying climatic 

 and/or environmental conditions. The culmination 

 of this line of development is found in the angio- 

 sperms where the male gametophyte is reduced to a 

 single cell within a pollen grain and the female game- 

 tophyte is reduced to an embryo sac within the ovule 

 of a flower structure, the pistil. However, in both 

 cases the gametophytes still are associated with modi- 

 fied spores. 



More specifically, the non-seed-producing plants 

 (pteridophytes) that form only one kind of spore 

 regularly develop part to all of their gametophytes 

 outside the spore, but seed plants (spermatophytes) 

 and pteridophytes forming two types of spores not 

 only tend to produce distinct male and female game- 

 tophytes, but the gametophytes when mature are 

 largely to completely within the spores. In this latter 

 process, any male spore develops into the male game- 



