CHAPTER II. 



THE MORPHOLOGICAL COMPOSITION" OF PLANTS. 



§ 180. Evolution implies insensible modifications and 

 gradual transitions, which render definition difficult — which 

 make it impossible to separate absolutely the phases of 

 organization from one another. And this indefiniteness of 

 distinction, to be expected a priori, we are compelled to 

 recognize a posteriori, the moment we begin to group morpho- 

 logical phenomena into general propositions. Thus, on 

 inquiring 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 primitive kind are formed, not of bricks, but out of 

 unmoulded clay; and second, that though other houses con- 

 sist mainly of bricks, yet their chimney-pots, drain-pipes, and 

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