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- 

 tophyte, 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 the Palmella-form of 

 one of the lower Algce, there may exist a clothing of jelly; 

 or, as in Diatom, the walls may take the form of silicious 

 valves variously sculptured. The contained matter may be 

 partly or wholly here green, there red, and in other cases 

 brown. 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 contact with the 

 environment. 



When we remember that this trait is one which these 

 simple living bodies have in common with bodies that are 

 not living when we remember that each inorganic mass 

 eventually 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 conclusion that, in the one case as in the 

 other, the contrast is due to the unlike actions to which the 

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