CHAPTEE XV 



DIFFERENTIATION OF TISSUES 



148. Introductory. Among the simplest Thallophytes 

 the cells forming the body are practically all alike, both as 

 to form and work. What one cell does all do, and there 

 is very little dependence of cells upon one another. As 

 plant bodies become larger this condition of things can not 

 continue, as all of the cells can not be put into the same 

 relations. In such a body certain cells can be related to 

 the external food supply only through other cells, and the 

 body becomes differentiated. In fact, the relating of cells 

 to one another and to the external food-supply makes large 

 bodies possible. 



The first differentiation of the plant body is that which 

 separates nutritive cells from reproductive cells, and this is 

 accomplished quite completely among the Thallophytes. 

 The differentiation of the tissues of the nutritive body, 

 however, is that which specially concerns us in this chapter. 



A tissue is an aggregation of similar cells doing similar 

 work. Among the Thallophytes the nutritive body is prac- 

 tically one tissue, although in some of the larger Thallo- 

 phytes the outer and the inner cells differ somewhat. This 

 primitive tissue is composed of cells with thin walls and 

 active protoplasm, and is called parenchyma, meaning 

 "parent tissue." 



Among the Bryophytes, in the leafy gametophore and 



in the sporogonium, there is often developed considerable 



dissimilarity among the cells forming the nutritive body, 



but the cells may all still be regarded as parenchyma. It 



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