260 UNIVERSITY OF COLORADO STUDIES 



ment. Here, as in most cotyledons, the spongy parenchyma consists 

 of rather regular cells not bulged out variously as in the leaf. A few 

 multicellular trichomes are present toward the base of the leaf lamina 

 and are continued in greater number along the petiole. 



Cotyledon stalk and lea} petiole. — As the cotyledons are practically 

 sessile perhaps no useful comparison can be made with the leaf petiole. 

 However, sections of the extreme narrowed basal part of the cotyledon 

 were made. These show the vascular bundles entering the cotyledon 

 in the form of a flat interrupted arc. In the leaf petiole a similar arc 

 of bundles is found but in addition there are also other bundles above. 

 These, in the first few leaves of the seedling, are not numerous but in 

 leaves taken from old trees they form two or more arcs parallel to the 



main one. 



Ipomoea coccinea Linn. 



General. — The cotyledons of this species are broadly trapezoidal 

 with the base rounded and slightly heart-shaped while the apex has a 

 broad, shallow sinus. The first leaf is somewhat cordate with sagittate 

 base. Later leaves are narrower and more markedly sagittate. 



Epidermis as seen in surface view. — Both surfaces of the leaves and 

 cotyledons have stomata and the cells are wavy in outline. As usual, 

 stomata are very abundant on the lower surface of the leaf, less so on 

 the upper surface and on the cotyledon. The guard cells are flanked 

 by narrow cells which usually differ considerably in size. While the 

 the cotyledons are glabrous the leaves have scattered pubescence. 



Internal structure. — A comewhat irregular three-layered palisade is 

 found in the cotyledon, which is thicker than the leaf and evidently 

 modified as a storage organ. There is only one layer of palisade cells 

 in the leaf and these cells are quite long. No special differences are to 

 be noted on the spongy parenchyma of the two structures. 



Cotyledon stalk and lea} petiole. — Both are channeled above and 

 rounded below. Neither one is greatly flattened. In the cotyledon 

 stalk there are two principal vascular bundles placed close together 

 and in some specimens almost fused. A well-defined endodermis with 

 starch grains lies below these bundles. In addition to these principal 

 bundles there are two very small ones, one on each side, running close 



