546 cook: morphology and evolution of leaves 



Many leaves have a basal pulvinus, with a layer of absciss 

 tissue to form an articulation with the internode, when the leaf 

 separates at maturity. In other leaves the joint that provides 

 for the detachment of the leaf is between the foot and the petiole, 

 as in the case of the peaches, plums, cherries, and related plants. 

 In Magnolia virginiana there is a pronounced development of the 

 entire upper surface of the foot as a pulvinus, which has a special 

 function in lifting the winter bud scales in the spring. 



The organ that is usually described as a true petiole may 

 prove to be a foot in cases where there are pulvini at both ends, 

 as in cacao. In the patashte tree, a relative of cacao, the petioles 

 of the leaves of the fruiting branches have the structure of pul- 

 vini for their whole length. 8 In the cotton plant also there are 

 pulvini at both ends of the petiole. Other reasons for consider- 

 ing the cotton petiole as a foot may be found in the fact that 

 while the margins of the leaf blade never show any tendency to 

 become decurrent, the petiole is often united with the margins 

 of enlarged stipules like those that form the involucral bracts. 



The suppression or extreme reduction of the blade in the for- 

 mation of the involucral bracts of the cotton plant might also 

 be considered as an indication that the blade, in spite of its 

 much greater size, still behaves in some respects as an appendage 

 of the sheath. That the general course of evolution has been 

 in the direction of enlarging the blade and reducing the sheath 

 element may be considered as at least a partial explanation of 

 the fact that enlargement of the stipules is almost invariably 

 accompanied by reduction of the blade of the leaf. If only one 

 stipule is enlarged, a lobe is likely to be wanting on the same 

 side of the blade. 9 



SUMMARY 



The leaves of angiosperms show a primary division into two 

 morphological elements, (1) a basal sheath supporting (2) an 

 expanded blade, as represented in the leaves of palms, grasses, 

 and many other plants. The organs that are usually described 



8 Cook, O. F. Branching and flowering habits of cacao and patashte. Contr. 

 U. S. Nat, Herb., 17: 609-625, pis. 44-54. 1916. 



9 Cook, O. F. Brachysm, a hereditary deformity of cotton and other plants. 

 Journ. Agric. Research, 3: 387-400, pis. 53-62. 1915. 



