CHAPTER II. 



DIFFERENTIATIONS BETWEEN THE OUTER AND INNER 

 TISSUES OF PLANTS. 



268. The simplest plant presents a contrast between its 

 peripheral substance and its central substance. In each pro- 

 topbyte, be it a spherical cell or a branched tube, or such 

 a more-specialized form as a Desmid, a marked unlikeness 

 exists between the limiting layer and that which it limits. 

 These vegetal aggregates of the first order may differ widely 

 from one another in the natures of their outer coats and in 

 the natures of their contents. As in a Palmella, there may 

 exist a clothing of jelly ; or, as in Diatom, the walls ma^y 

 take the form of silicious valves variously sculptured. The 

 contained matter may be here green, there red, and in other 

 cases brown or black. But amid all these diversities there is 

 this one uniformity a strong distinction between the parts 

 in contact with the environment and the parts not in con 

 tact with the environment. 



When we remember that this trait is one which these 

 simple living bodies have in common with bodies th-it are not. 

 living when we remember that each inorganic mass even 

 tually has its outer part more or less differentiated from 

 its inner part, here by oxidation, there by drying, and else 

 where by the actions of light, of moisture, of frost ; we can 

 scarcely resist the ccnclus .on that, in the one case as in 



