CHAPTER NINE 



The Main Lines of 

 Evolution among Land Plants 



It is almost a point of definition for the algae that they are aquatic 

 plants, although some of them have invaded moist habitats on land. The 

 useful fossil record begins with the Cambrian period of the Paleozoic 

 Era, over 500,000,000 years ago, and the Cambrian record, so far as plants 

 are concerned, consists entirely of a wide variety of algae and bacteria. 

 In fact, the early Paleozoic is often referred to as the Age of Algae and 

 Invertebrates. But fossils from the Silurian period, beginning about 360,- 

 000,000 years ago, include primitive land plants, and it is probable that 

 their first appearance was in the preceding period, the Ordovician. When 

 these first colonists left the waters to invade the more difficult but more 

 varied habitats on land, the algae remained the dominant members of the 

 earth's flora. But soon ( geologically speaking ) the land dwellers surpassed 

 their aquatic progenitors. One of the crucial problems which had to be 

 solved before plants could invade the land was the protection of the 

 zygote against drying. In all land plants, this is accomplished, with im- 

 portant differences in the details, by the retention of the zygote and the 

 developing embryo within the sex organs of the maternal plant. For this 

 reason, the land plants are known collectively as the subkingdom Em- 

 bryophyta. This subkingdom includes only two phyla, the Bryophyta and 

 the Tracheophyta, but the latter is greatly varied. 



THE BRYOPHYTES 



The phylum Bryophyta is a relatively small group comprising the mosses, 

 the liverworts, and the horn worts. These are the amphibians of the plant 

 world, for they have met only minimum re(|uirements of adaptation 

 to the terrestrial environment. Thev are restricted to wet habitats, and all 

 of them require water for reproduction, at least as a film over the surface 

 of the plant in which sperm can swim. The bryophytes share with other 

 land plants certain adaptations which permit them to utilize terrestrial 

 habitats. As already mentioned, the embryos, which are always multi- 

 cellular, are retained within the female sex organs, thus protecting them 



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