CHAPTER XI 

 DEFINITIONS OF THE ORGANS 



Not uncommonly an organ is defined as the tool of a function. 

 Of course from the evolutionary point of view this conception cannot 

 hold, because the same organ in a plant often at different times 

 subserves very diverse functions. For example, the stem may 

 function as a leaf and the leaf as a root. From the standpoint of 

 the doctrine of descent the value of a particular organ in the course 

 of evolution is assigned, not so much on the basis of what it does, 

 as on that of what it is. In other words, an organ is known mainly 

 by its organization. 



In the case of the vascular plants there are a number of distinct 

 structural units to which the name of organ is applied. It is not 

 by any means clear that the different organs of plants were always 

 as distinct from one another as they are at the present time. For 

 example, there is some reason to believe that the root may manifest 

 the primitive structural features of vascular plants, and in the 

 course of time the stem may have become differentiated from the 

 root, arriving finally at that degree of distinctness which at present 

 definitely separates it from the root. It has, moreover, often been 

 suggested in recent years that the leaf of the higher vascular plants 

 was originally of the nature of a branch and that its distinguishing 

 features are the product of later evolution. It would be going 

 beyond the range of an elementary work like the present one to 

 discuss the question of the origin of organs, particularly as the data 

 are extremely meager and not always easy to interpret. Disre- 

 garding, consequently, any speculations as to the appearance of the 

 organs of the higher or vascular plants, we may proceed at once 

 to the enumeration of those which are generally accepted by 

 anatomists. 



The parts or organs of the higher plants are usually distinguished 

 as three namely, root, stem, and leaf. To these may be added a 

 fourth, the sporangium or spore sac. Each of the organs named 



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