The Profound Effect on the Structure of Plants 73 



in the discovery, identification, collection and arrangement of the 

 various special structures of plants, which can then be preserved 

 in some such manner as our picture illustrates (figure 24). 



Thus it is evident that, on the one hand, the three primary 

 plant parts, leaf, stem and root, though developed with a 

 structure adaptive to the very particular function of photo- 

 synthesis or food-making, have in many cases become trans- 

 formed into other parts of very different ecological significance 

 and structure; while, on the other hand, and correlatively, all 

 of the great number of highly specialized parts performing other 

 functions can be traced back to an origin morphologically in the 

 three primary plant parts. This interlocking relationship of 

 morphological origin with ecological meaning, of morphology 

 with ecology, can perhaps be made clearer by use of a diagram 

 such as is given herewith (figure 25). 



Although I ought now to end this long chapter, I will continue 

 far enough to answer two questions which I am sure have arisen 

 in the mind of the reader. Thus, he will surely be wondering 

 why it is that some plants make their tendrils, for instance, from 

 leaf-blades, others from petioles, others from stipules, others from 

 stems, and others even from roots. The most reasonable answer 

 appears to be this, that when a plant, owing to a change of habit 

 forced on it by a change of environment, develops a need for a new 

 organ, that organ is made by a transformation of the part which 

 happens to be most available for the purpose, often some part 

 which the change of habit has happened to set free from its 

 former use; and sometimes that most available part will be one 

 thing and sometimes another. In the second place the reader 

 will wonder why some plants should abandon their leaf-blades 

 as foliage, and then proceed to replace them by petioles, stipules, 

 stems, or even roots, which are for the purpose converted physi- 

 ologically and structurally into leaves. In answer it may be said 

 that the abandonment of the leaf-blade, as will be shown in the 

 chapter on Protection, usually accompanies exposure to very dry 



