GOSSYPIUM 433 



and phellogens are produced which form a protective layer of 

 phellem. The first phellogens are of cortical origin, while later 

 ones are differentiated in the pericyclic parenchyma. The outer 

 layers of the axis may become quite corky, accompanied by the 

 development of lenticels. As secondary thickening proceeds, a 

 large number of thick-walled fibers are produced in the secondary 

 phloem. The secondary xylem is not heavily lignified, and the 

 soft woody tissue disintegrates rapidly. 



Ontogeny of the Leaf. — The early development of the leaf 

 and the stipules has been reported by Gore (i8) as follows: 



"The leaf primordium and stipule primordia arise practically simul- 

 taneously from the apical meristem of the primary axis. (Fig. 1x3, 

 A, B.) The leaf primordium is rounded and collar-like at first, later 

 tapering somewhat up to the rounded tip. As the embryonic leaf 

 grows upward it becomes bluntly pointed. . . . There is little evidence 

 of differentiation into petiole and blade at this stage, except the slight 

 beginning of the plicate folding characteristic of the lamina of a young 

 leaf. No epidermal hairs are present at this stage, although they 

 begin their development early. The third young leaf from the tip 

 is completely covered with them. 



"The tip of this first protuberance becomes the median lobe of the 

 palmate leaf; its base develops into the petiolar region. There soon 

 develops on the slightly incurved margins of this primordium two 

 pointed protuberances located about halfway between the tip and 

 its base. These are the primordia of the two lateral lobes of the leaf. 

 The young leaf, consisting of three lobes, now proceeds to develop 

 in length; continued inward plicate folding, as the lobes increase 

 laterally in size, forms a cuplike portion. (Fig. 113, B, C.) If addi- 

 tional lobes occur they arise in the same way and are similar in all 

 respects. (Fig. xxt,, D, £.) Seven lobes is the usual number for 

 leaves on the main axis, while on the fruiting branches of Pima and 

 Sea Island varieties the leaves usually have three. In Mebane five- 

 lobed leaves are commonly found on the fruiting branches. Soon 

 the tips of the individual lobes meet and the lamina develops rapidly. 

 The main veins appear as definite ridges on the abaxial surface of the 

 leaf and are very thick in proportion to the rest of the leaf. The 

 venation is palmate. 



"Simultaneously with or shortly after the initial leaf primordium 

 becomes distinct, two stipular flanking protuberances arise. (Fig. 

 2-Z3, B.) These are pointed and concave at first, but as they grow 

 they widen out from the base somewhat, later become toothed, fold 

 inward very slightly, and keep pace with the growth of the leaf. 

 They are not folded or plicate as is the young leaf but remain more or 

 less flattened, their interlocking hairs holding them tightly appressed 

 to the leaves." 



