CHAPTEE 11. 



TEE MORPHOLOGICAL COMPOSITION OF PLANTS. 



§ 180. Evolution implies insensible modifications and 

 gradual transitions, wMcli render definition difficult — wliicb 

 make it impossible to separate absolutely the pbases of or- 

 ganization from one another. And this indefiniteness of 

 distinction, to be expected a j^riori, we are compelled to re- 

 cognize a postejiori, the moment we begin to group morpho- 

 logical phenomena into general propositions. Thus, on in- 

 quiring what is the morphological unit, whether of plants or 

 of animals, we find that the facts refuse to be included in any 

 rigid formula. The doctrine that all organisms are built up 

 of cells, or that cells are the elements out of which every 

 tissue is developed, is but approximately true. There are 

 living forms of which cellular structure cannot be asserted ; 

 and in living forms that are for the most part cellular, there 

 are nevertheless certain portions which are not produced by 

 the metamorphosis of cells. Supposing that clay were the only 

 material available for building, the proposition that all houses 

 are built of bricks, would bear about the same relation to the 

 truth, as does the proposition that all organisms are composed 

 of cells. This generalization respecting houses would be 

 open to two criticisms : first, that certain houses of a primi- 

 tive kind are formed, not out of bricks, but out of unmoulded 

 clay ; and second, that though other houses consist mainly of 

 bricks, yet their chimney-pots, drain-pipes, and ridge-tiles 



