DEFINITIONS OF THE ORGANS 141 



existence or various needs on the part of the plant. In general, 

 however, the possession of nodes and internodes serves as a suffi- 

 cient diagnostic character, since this feature is rarely obliterated 

 even by the most extreme conditions to which the organ is exposed. 



THE LEAF 



This organ has a double importance, because it not only sub- 

 serves the vegetative functions present in the case of root and stem, 

 but is also charged with the extremely important office of repro- 

 duction. The leaf from the purely vegetative aspect is of con- 

 siderable evolutionary significance, since, although much less 

 conservative than the root, it is much more retentive of ancestral 

 characteristics than is the stem. Its chief evolutionary value, as 

 will appear in subsequent chapters, is in connection with the 

 unraveling of the relationships of the older and lower gymno- 

 sperms. In more modern types the anatomy of the root 

 generally throws more light on the relationships than that 

 furnished by the leaf. Reproductively the foliar organs are of 

 great importance in connection with taxonomic arrangement. In 

 fact, at the present time the angiosperms are almost universally 

 classified on the features presented by their reproductive leaves, 

 and in the case of the higher gymnosperms anatomical features of 

 the vegetative structures are only beginning to receive adequate 

 consideration in connection with investigations as to evolutionary 

 sequence. 



As a reproductive structure the leaf is best considered under the 

 particular groups to be discussed in subsequent chapters, as no 

 general statements can be advantageously made at the present 

 stage. As a purely vegetative organ the leaf is readily distinguished 

 from the stem, of which it forms an appendage, by the absence of 

 nodes and internodes. It is not easy to summarize the anatomical 

 structure of the leaf, since it is to a large degree variable in passing 

 from lower to higher groups. A useful criterion is its dorsiventral 

 organization, which is usually present, at least in the region of the 

 expanded part of the leaf or blade. Another outstanding feature of 

 the leaf is the fact that it is an axillating organ, and as a conse- 

 quence other appendages are commonly axillary to it. The 



