CHAPTER XIV 



PHYSIOLOGY (Continued) 



Relation to Environment 



THERE is every reason to suppose that the most primitive plants 

 were unicellular forms which were aquatic in their habit. Some of 

 the simpler existing fresh-water Algae probably are very much like 

 these ancestors of the higher plants. Somewhat higher are the 

 related filamentous or thallose forms, represented by some of the 

 Conf ervoideae ; and related to them, but differentiated in another 

 direction, are the numerous marine Algae. 



Owing to the dense medium in which they grow, all submersed 

 aquatic plants are unprovided with the mechanical tissues which 

 give firmness to plants growing in the air, and consequently these 

 Algae, when removed from the water, collapse completely. Being 

 exposed on all sides, too, to the nutrient medium, the water with its' 

 dissolved food elements may be absorbed at any point of the plant's 

 surface, and roots, when present, serve merely for attachment. The 

 surrounding water also serves as a vehicle for the transport of the 

 reproductive bodies, both the non-sexual spores and the sexual 

 cells, or gametes. Both may be provided with cilia, which may be 

 present also in the vegetative cells of some of the most primitive 

 forms like the Peridineae and Volvocaceae, which retain through life 

 their animal character of active locomotion. This feature, as has 

 been shown, persists in the male reproductive cells of all but the 

 highest plants. 



The formation of temporary, freely locomotive stages, becoming 

 later stationary, is shared with plants by many low animals, such as 

 the Corals, which, like the Algae, live in a medium that is abundantly 

 supplied with their food elements. 



The conditions in fresh water are much the same everywhere, and 

 we find the lower types of plants growing in fresh water to be much 

 alike in all parts of the world. As the conditions have probably 

 changed but little from very remote times, it is reasonable to suppose 

 that most of these simple fresh-water organisms are of very ancient 

 origin. Comparing the fresh-water Algae with other plants, we may 

 recognize two principal categories of the latter marine plants and 

 land plants, the former being principally Algae, more or less remotely 

 related to the fresh-water types. 



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