cook: moephology and evolution of leaves 539 



the course of specialization in leaf-forms 



Other specialized forms of leaves, such as sepals, bracts, and 

 scales of subterranean rootstocks, may also be considered as 

 representing primitive sheaths or bud scales, rather than as reduc- 

 tions from the fully developed type of foliage leaves. From this 

 point of view the foliage leaves appear to be the most specialized. 

 Yet in developing the foliage leaves plants have not lost the 

 ability to produce the simpler organs — sheaths, bud scales, or 

 bracts. The plant body is a succession of different kinds of 

 internodes, or metamers, bearing different kinds of leaves. At 

 one end of the series are the cotyledons or seed-leaves, at the 

 other the carpels or fruit-leaves, with many intermediate stages 

 between the different kinds of foliar and floral organs. 



As stamens are often transformed into petals, so we may think 

 of cotyledons and foliage leaves as sterile carpels performing 

 vegetative functions. Some of the species of Sterculia have broad 

 leaflike carpels that persist and remain green long after the seeds 

 have been shed. That the stamens and carpels of the different 

 families of plants are generally more alike than the leaves or other 

 parts of the plant body is more easily understood when we con- 

 sider the evolution of plants as a process of intercalation of more 

 numerous and more specialized forms of metamers. Plants like 

 the junipers, pines, and eucalypts have two distinct kinds of 

 foliage leaves, showing clearly that a double evolution of these 

 organs has taken place. 



The succession of different kinds of leaves, the classical exam- 

 ple used by Goethe in presenting the idea of evolutionary changes 

 in the forms of the same organ, may not be less significant for 

 the strictly morphological purpose of understanding the origin 

 and homologies of the structural elements of the leaves. The 

 changes that take place in passing through the succession of leaf- 

 forms, during the development of the plant, are also of interest 

 for the study of heredity. 1 



1 Cook. O. F. Dimorphic leaves of cotton and allied plants in relation to heredity. 

 U. S. Dept, Agric, Bur. PI. Ind., Bull. 221. 1911. Heredity and cotton breeding. 

 U. S. Dept, Agric, Bur. PL Ind., Bull. 256. 1912. 



