CHAP, i.] PLANT ARCHITECTURE. 15 



their full development, a condition of things impossible 

 so long as the act of reproduction necessitated the loss 

 of the parent form as in the Pleurococcus type. 



Closely following the division of multicellular plants 

 into vegetative and reproductive portions, we meet with a 

 corresponding differentiation in each part ; for example, 

 the vegetative portion of many seaweeds consists of parts 

 resembling in appearance the root, stem, and leaves of 

 the higher flowering plants, and although the specializa- 

 tion of these parts is nob so complete in the seaweed as 

 in the flowering plant, yet we have indicated in the lower 

 forms those types of structure that by degrees become the 

 most prominent characteristics of plant life. 



In seaweeds the root-like portion, which is frequently 

 a discoid body, only performs the mechanical function of 

 fixing the plant to one particular spot, and is in no way 

 concerned with the absorption of food as in the higher 

 plants; the flattened out portion of the seaweed again, 

 although not the exact equivalent of the leaf of a flower- 

 ing plant, is nevertheless expanded into a thin sheet for 

 the same purpose, that of exposing a large area in con- 

 nection with the assimilation of food ; thus we have 

 shadowed in amongst the simplest of plants all the most 

 important structural features of the Vegetable Kingdom. 



There is no evidence of any preconceived scheme in 

 connection with the gradual extension or evolution of the 

 plant world from primitive types. Keeping in view the 

 fact that protoplasm is the only vital portion of a plant, 

 it follows that the possibility of a given plant to live 

 under varying conditions of environment depends on the 

 limits under which its protoplasm can perform the various 

 functions collectively constituting life. As illustrating 



