610 - Heredity and Evolution 



Fig. 31-17. Corn, infected by corn smut (Ustilago 

 zeae), another parasitic basidiomycete. (Photo by 

 Benjamin Koehler; from The Plant World.) 



groups, especially the gymnosperms and 

 angiosperms, finally did evolve a fertiliza- 

 tion mechanism in which the sperm do not 

 need water while they are being transmitted 

 to the eggs (Chap. 12). 



Plant Embryos and Their Protection. The sec- 

 ond problem — nurturing the zygote during 

 the early vulnerable stages of development — 

 began to reach a solution very early. All Em- 

 bryophyta (see below) are characterized by 

 the possession of multicellular reproductive 

 organs. Especially important are the arche- 

 gonia (p. 205). In the archegonium, the 

 zygote and the developing embryo are pro- 

 tected and nurtured until the young plant 

 can shift for itself. This development is the 

 primary basis for grouping all terrestrial 

 species into one large subkingdom — the Sub- 

 kingdom Embryophyta. All members of this 

 large group possess multicellular archegonia, 

 in which the embryos develop. Also all Em- 

 bryophyta possess multicellular sperm-form- 

 ing organs (antheridia) and multicellular 

 spore-forming organs (sporangia). And lastly, 

 the Embryophyta all display a regular alter- 

 nation between the diploid asexual sporo- 

 phyte generation and the haploid sexual 

 gametophyte generation (Chap. 12). 



The Bryophyta (Literally, Moss Plants). 

 This phylum (about 23,000 species) of rela- 

 tively small primitive land plants includes 

 two classes: (1) the Hepaticae, or liverworts 

 (Fig. 13-4), and (2) the Musci, or true mosses 

 Fig. 12-11). 



Fig. 31-18. Reindeer moss (Cladonia 

 rangiferina). These and other lichens can 

 grow on very poor soil and in very cold 

 regions. (From The Plant World.) 



