THE PLANT KINGDOM 



245 



1. The Multicellular Plant. — The first great forward step 

 which, like all first steps, was probably a long time in being 

 accomplished, consisted of the union of these simple cells into 

 colonies (Fig. 135). The two daughter-cells formed at a division 

 remained attached to one another 

 instead of separating, and thus 

 arose small cell-groups or aggrega- 

 tions such as we still may see 

 among the lowest algae. The 

 individual cells forming these 

 groups might cohere variously — in 

 spherical masses, in threads, or in 

 sheets. Through a still more in- f 

 timate union between their members 

 these cell colonies gradually de- 

 veloped into definite multicellular 

 plants, various in size and shape 

 and probably much like some of the 

 simpler seaweeds of today. The 



A C 



Fig. 135. Fig. 136. 



Fig. 135. — The beginnings of a multicellular plant. A simple alga, Pleuro- 

 coccus, in which the plant body is commonly a single cell, but in which the 

 daughter cells following cell di\dsion may remain united in very simple colonies. 



Fig. 136. — The beginnings of differentiation. A thread-like or filamentous 

 alga, Oedogonium, in which the cells are no longer all alike but have begun to be 

 differentiated. One is modified as a holdfast (C), others as male sexual organs 

 (a) and others as female sexual organs (o). 



way was thus opened for the production of those very large and 

 complex plant bodies with which we are most familiar. 



2. Differentiation. — The evolution of the many-celled plant 

 was soon followed l>y another and equally important advance, the 

 beginning of differentiation (Fig. 136). The primitive single cell 

 performed all the functions which we now associate with the 

 entire plant, such as absorption, photosynthesis, and reproduc- 

 tion. Soon after the multicellular individual had arisen, how- 



