cook: morphology and evolution of leaves 545 



the blade, as the element that is between the petiole and the 

 internode, or as the element to which the stipules are attached. 

 Thus in the peaches, almonds, plums, and other members of the 

 family Amygdalaceae, the petiole is articulated at its base to 

 the foot, to which the stipules are attached. The petiole is 

 deciduous with the blade, but the foot persists for another season 

 and functions as a bud scale. The foot is present also in apples, 

 pears, and roses, but falls with the leaf, there being no joint at 

 the base of the petiole. 7 The joint is present in Oxalis and in 

 many leguminous plants. 



Objection might be taken on etymological grounds to using 

 the word foot for an element that in many plants is smaller than 

 the petiole, which term means a small foot. It does not seem, 

 however, that this is likely to cause confusion, since the obvious 

 signification of foot is in relation to the lowest, most truly basal 

 portion of the leaf. Absence of the foot is to be considered as a 

 specialization of leaf structure, and it will be interesting to deter- 

 mine the status of the organ in the different families. Even 

 though not present in the foliage leaves, the foot may still be 

 represented in the bud scales, bracts, or other organs which from 

 our present point of view appear to be more primitive and less 

 specialized than the foliage leaves. That the cotyledon is some- 

 times called the nursing foot, or simply the foot, hardly con- 

 stitutes an objection to the use of this word, with other leaves, 

 for the element that corresponds to the cotyledons and primitive 

 bladeless sheaths of seedlings. 



pulvini and articulations 



Pulvini and articulations represent special forms of tissue con- 

 nected with the ligule or base of the blade in grasses and palms, 

 and found in corresponding positions in other families of plants. 

 The chief function of pulvini is to control the position of the 

 leaf blade, which is accomplished by varying the turgidity or 

 water pressure in the rather loose cells of which the pulvini are 

 composed. A flexible pulvinus is in the nature of a joint. 



7 Cook, O. F. Jointed leaves of Amygdalaceae. Journ. Wash. Acad. Sci.,2: 

 218-220. 1912. 



