THE OUTER TISSUES OF PLANTS. 235 



growth of a body which may be regarded as an individual 

 cell/' supplies a still-better example. Among 



aggregates of the second order the like connexion is 

 displayed in more various modes but with equal con- 

 sistency. As, before, the Puff-ball served to exemplify the 

 primary physiological differentiation of outer parts from 

 inner parts; so, here, it supplies a simple illustration of 

 the way in which the differentiated outer part is re-dif- 

 ferentiated, in correspondence with the chief contrast in its 

 relations to the environment. The only marked unlikeness 

 which the cortical layer of the Puff-ball presents, is that 

 between the portion next the ground and the opposite portion. 

 The better- developed Fungi exhibit a more decided hetero- 

 geneity of parallel kind. Such incrusting Algce as Ralfsia 

 cleusta furnish a kindred contrast ; and in the higher Algce 

 it is uniformly repeated. Phsenogams display 



this physiological differentiation very conspicuously. That 

 earth and air are unlike portions of the environment, sub- 

 jecting roots and leaves to unlike physical forces, which entail 

 on them unlike reactions ; and that the unlike functions and 

 structures of their respective surfaces are fitted to these 

 unlike physical forces ; are familiar facts which it would be 

 needless here to name, were it not that they must be counted 

 as coming within a wider group of facts. 



Is this unlikeness between the outer tissues of the attached 

 ends and those of the free ends in plants, determined by 

 their converse with the unlike parts of the environment ? 

 That they result from an equilibration partly arising in 

 the individual and partly arising by the survival of indivi- 

 duals in which it has been carried furthest, is inferable 

 a priori ; and this d priori argument may be adequately 

 enforced by arguments of the inductive order. A few 

 typical ones must here suffice. The gemmulea 



of the MarcJiantia are little disc-shaped masses of cells 

 composed of two or more la} r ers. Their sides being alike, 

 there is nothing to determine which side falls loweraiost 



