DIFFERENTIATKiN OF TISSUES 



281 



is in the sporophyte of the Pteridophytes and Spermato- 

 phytes that this differentia tion of tissues becomes extreme, 

 and tissues are organized which differ decidedly from 

 parenchyma. This differentiation means division of labor, 

 and the more highly organized the body the more tissues 

 there are. 



All the other tissues are derived from parenchyma, and 

 as the work of nutrition and of reproduction is always 

 retained by the parenchyma cells, the derived tissues are 

 for mechanical rather 

 than for vital purposes. 

 There is a long list of 

 these derived and me- 

 chanical tissues, some of 

 them being of general' 

 occurrence, and others 

 more restricted, and 

 there is every gradation 

 between them and the 

 parenchyma from which 

 they have come. We 

 shall note only a few which are distinctly differentiated 

 and which are common to all vascular plants. 



149. Paxencliyina. — The parenchyma of the vascular plants 

 is typically made up of cells which have thin walls and whose 

 three dimensions are approximately equal (Figs. 264, 265), 

 though sometimes they are elongated. Until abandoned, 

 such cells contain very active ^jrotoplasm, and it is in them 

 that nutritive work and cell division are carried on. So 

 long as these cells retain the power of cell division the 

 tissue is called merisfem, or it is said to be merisfemalic, 

 from a Greek word meaning ''to divide."' When the cells 

 stop dividing, the tissue is said to be permanent. The 

 growing points of organs, as stems, roots, and leaves, are 

 composed of parenchyma which is meristematic (Figs. 266, 

 274), and meristem occurs wherever growth is going on. 



Fig. 264. Parenchyma and sclerenchyma from 

 the stem of Pteris, in cross-.sc'Ction. — Cham- 

 berlain'. 



