CHAPTER XXII 



THE ANGIOSPERMAE : LEAVES 



When we turn to the consideration of the angiospermic leaf we are confronted 

 by the same difficulty of definition that we found in the case of the stem but 

 in an accentuated degree, inasmuch as the range of morphological variation 

 is much greater among leaves than among stems. Whatever formal 

 definition of a leaf we attempt to frame, it will be possible to find some cases 

 which appear to contradict it. 



The science of comparative morphology, which arose from Goethe's 

 famous " Essay on Metamorphosis," was based upon the concept of organ 

 categories, to which we referred in the last chapter ; that is to say, on the 

 principle that the plant body is built up of a limited number of kinds or 

 categories of organ, primarily distinct from each other, but related, in the 

 architecture of the plant, in a fundamentally uniform way, no matter how 

 variable their external appearance may be. According to the principle of 

 metamorphosis which Goethe formulated, any one kind of organ may vary, 

 according to its position and function, so far from the normal that it may 

 become difficult to recognise. Nevertheless it should not, so the argument 

 runs, take over wholly the character of an organ of a different category, and 

 it was considered to be the business of the morphologist to trace out and 

 determine its true nature. So rooted did this idealistic theory become that 

 controversies were carried on, sometimes without any acceptable conclusion, 

 regarding the placing of such structures as the ovule in their proper category 

 of leaf or axis. The most familiar type of leaf, namely the green foliage leaf, 

 was tacitly assumed to be the ideal form, and all other " leaf " organs were 

 supposedly metamorphosed foliage leaves. 



With the growth of our knowledge of plant evolution such formal ideas of 

 morphology have become obsolete. Goebel, in his great " Organography of 

 Plants," was the first to break away from them by directing attention first 

 and foremost to the functional aspect of organs and their relationships to the 

 plant's conditions of life. He was careful, however, to avoid the naive 

 adaptationism which confuses post hoc with propter hoc and seeks to relate 

 every observed structure to some supposed function in relation to the 

 environment. The complexity of plant structure is not necessarily a direct 

 reflection of the complexity of the environment. Even with our limited 

 knowledge of the life conditions of plants it is quite evident that similar 

 requirements have been met by plants in an infinity of different ways, which 

 are not physiologically predetermined. We must combine both the 

 morphological and the physiological viewpoints if we are to attain a natural 

 understanding of plant structures. 



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