34 6 BOTANY OF THE LIVING PLANT 



way, the method is not generally applicable, and the analysis itself 

 bears evidence of its artificiality. It should be realised that the 

 evolution of the Higher Animals and of the Higher Plants has pursued 

 distinct lines. The two Kingdoms cannot be assumed to have 

 adopted like methods of advance. 



In Plants lower in the scale the categories of parts are less clearly 

 defined than in the Higher Flowering Plants, and in the simplest 

 types the several parts cannot be distinguished at all. This suggests 

 that such categories of parts do not represent any essential plan of 

 construction applicable to plants generally, but that they are con- 

 sequences of progressive evolution in organisms that meet in a similar 

 way the requirements of a life common to them all. It raises also 

 the question whether on the one hand all the parts classed together, 

 such as leaves, had really a common origin by Descent. There is 

 reason to think that leaf-like bodies have originated separately in 

 numerous distinct phyletic lines : for instance, among the Higher 

 Algae, in the Bryophytes, and in Vascular Plants. In fact, while 

 we group those parts that are held as homologous into certain morpho- 

 logical categories, such as leaf or axis, these are not to be held as 

 definite or of universal application. They are rather to be regarded 

 as being based upon such uniformity of result as has been achieved 

 in Descent by various types of organisms. This view gives a true 

 or evolutionary basis to the distinction between homology and 

 analogy. Those parts are held to be " homologous " which had 

 fallen relatively early in Descent into such relationships as char- 

 acterise that category of parts to which they are referred : those 

 are held to be only " analogous " which have undergone their specific 

 modification relatively late in Descent, after the characters defining 

 their morphological category had already been acquired. 



Morphological Categories of Parts. 



Classifying the parts of the Higher Plants by the relations which 

 they bear to the whole plant-body, and not merely according to 

 function, they are found to fall into certain categories, and the parts 

 so grouped together are held to be " homologous " one with another, 

 whether they be borne on the same or on different individuals. 

 The seedling plant consists of Root and Shoot (Figs. 2, 4). The 

 primary shoot is the whole product of the plumule on germination 

 and the primary root is the product of the radicle of the seedling. 

 Each of these parts is distinct from the other in origin and character, 



