cook: morphology and evolution of leaves 541 



a line of separation of the two portions of the primitive leaf 

 that became specialized. Even this indication of definite spe- 

 cialization of parts is lacking in many plants, as among the 

 lilies, orchids, and plantains, where the leaves still appear as 

 simple sheaths with a gradually broadened distal portion to serve 

 as a blade, but no distinct separation of special parts. The idea 

 of leaf blades arising as outgrowths from leaf sheaths appears 

 more probable, or at least less fantastic, when we consider such 

 a case as the leaf of Smilax, where the end of the sheathing base 

 produces a pair of slender tendrils several inches in length, in 

 addition to the blade of the leaf. It may not be without sig- 

 nificance that rudimentary blades of abnormal or reduced leaves 

 often assume very slender, tendril-like forms. 



THE EVOLUTION OF STIPULES 



It has been supposed that stipules were developed from basal 

 lobes of leaf blades. This interpretation was suggested by Ward 

 and later adopted by Berry on the basis of evidence drawn from 

 fossils and abnormal leaf-forms of Liriodendron, but these can 

 be understood as intermediate stages between normal leaf-forms 

 and bud scales. It seems unnecessary to suppose that stipules 

 began with Liriodendron, or that the stipules of Liriodendron are 

 not homologous with those of other genera and families. 3 



The very general occurrence of stipules or stipular structures 

 is in itself a fact that must make it difficult to credit the idea of 

 recent development or independent derivation of such organs, 

 either in the family Magnoliaceae or in the many and widely 

 different families of plants that have stipules. The stipules of 

 Liriodendron may be more primitive than those of other Magno- 

 liaceae in retaining more of the vegetative functions of the primi- 

 tive sheath, but their nearly complete separation from the petiole 

 and from each other may be considered as a specialization, since 

 partially united stipules occur. Union between the stipules and 

 the petiole is shown in many of the reduced leaves or the large 

 floral bud scales that have small petioles and blades. Union 



3 Berry, E. W. The origin of stipules in Liriodendron. Bull. Torrey Bot. 

 Club, 28: 493. 1901. 



