TYPES OF DIMORPHISM OF LKAVES TN COTTON. 



21 



shaped (li{?. lo), wliilo on tlic fertilo l)ian('hcs of tlio same ])lant one 

 stijuile may be much l)roa(UM- than tlie other (fig. 14). Broadening of 

 one of the stipules is a usual and apj)arently quite normal charac- 

 teristic of tlie Eo^'ptian cotton. (PI. III.) It also appears in a 

 related African type from the Niam Niam, in the upper valley of the 

 Wliite Nile. On tlie other hand, the enlargement of the stipules and 

 the correspon(hng reduction of the petiole and blade of the leaf some- 

 times represents a distinctly abnormal tendency, accompanied by fie- 

 quent abortion of tlie flower buds. In such cases the leaves of the 

 fruiting branches become rechiced and more or less intermediate in 

 form between the normal 

 leaves of the fertile 

 branches and the invo- 

 lucral bracts that inclose 

 the flower buds. (PI. IV. ) 



These abnormal inter- 

 mediate forms of leaves 

 illustrate the nature of tlie 

 transformation that ha-; 

 taken place in the speciali- 

 zation of the involucre of 

 the cotton ])lant. Each 

 of the three bracts that' 

 compose the external in- 

 volucre represents a leaf 

 with the blade much re- 

 duced, the petiole entirely 

 suppressed, and the 

 stipules greatly enlarged 

 and united with the blade. 

 In the abnormal inter- 

 mediate forms of leaves a 

 reduction of the petiole 

 and blade is usually ac- 

 companied by a corresponding increase of the stipules, though one 

 is generally much larger than the other. (Fig. 15.) 



In contrast with the other leaves of the plant, the bracts might be 

 considered as an extreme case of dimorphism, since the differences of 

 form are much greater than those of the difl'erent types of foliage 

 leaves. The occurrence of the intermediate forms between bracts and 

 foliage leaves is also quite rare. Under some conditions of growth 

 such intermediate forms seldom or never occur, but under other 

 conditions, or at the end of the season, the normal specializations of 



221 



Fig. 10.— Leaf of "Katteree's Favorite," an "okra" variety of 

 American Upland Cotton. (Reduced.) 



