498 



BOTANY 



form of delicate filaments, or being provided with appendages which 

 serve as floats and make them extremely buoyant (Fig. 465). Others, 

 like the Peridineae, are ciliated and thus capable of independent 

 locomotion. 



B 



Land Plants 



When the ancestors of the terrestrial plants left the water, the con- 

 ditions encountered were very different from those which they had 

 left behind. The comparatively stable conditions of life in water 

 were exchanged for far more variable ones for which the plant had 

 to provide, and the struggle for existence was far keener, and soon 

 resulted in a much greater diversity of types than was possible in 

 the more uniform aquatic environment. 



Being no longer surrounded by a dense medium, the plant must 

 either lie prostrate on the earth, or must develop mechanical tissues 

 which enable it to maintain itself erect in the rarer medium in 

 which it is growing. Moreover, protection has to be provided 

 against undue loss of water from the cells. We find, therefore, that 

 all land plants have their exposed surfaces provided with a more or 

 less perfectly impervious cuticle. 



The transition from the aquatic to the terrestrial condition was not, 

 probably, a sudden one, but took place gradually, much as it still 



takes place in some aquatic or amphi- 

 bious forms. There are a good many 

 fresh-water Algae which are able to live 

 on moist earth quite as well as in the 

 water. Thus species of Vaucheria, one 

 of the fresh-water Siphonese, often form 

 dense felts on moist earth, and another 

 form, Botrydium, regularly grows upon 

 moist clay soil. These forms, however, 

 require an abundant supply of mois- 

 ture, and are quickly killed if they are 

 allowed to dry up even for a short 

 time. 



Still more suggestive is the case of 

 a few Liverworts. Ricciocarpus natans 

 (Fig. 466) is a simple Liverwort which 

 ordinarily grows as a floating aquatic, but if the water subsides so as 

 to leave it stranded on the mud, the plant will root itself in the mud 

 and begin a more vigorous growth than when floating free in the 

 water, and very often it does this preliminary to forming its repro- 

 ductive organs. It is very probable that in some such way as this 

 the first genuine terrestrial plants had their origin. 



FIG. 4<!6. Ricciocarptis natans. 

 A, floating form. B, terrestrial 

 form. 



