388 Journal of Agricultural Research vonii, no. 5 



The cotton plant has a definite dimorphism of branches, no flowers or 

 bolls being produced on the main stalk or the vegetative branches.* The 

 main stalk and the vegetative branches of cluster cottons are not short- 

 ened, but are often longer in brachytic varieties than in those that have 

 normal fruiting branches. The leaves of the main stalk and vegetative 

 branches of cluster cottons are of normal form, but they are larger, of 

 thicker texture, and have longer petioles than those of normal long- 

 jointed varieties. The axillary buds of noncluster varieties usually 

 remain dormant or produce long vegetative branches, but in cluster 

 cottons they commonly develop into short branches and produce one or 

 two bolls. These differences may be considered as direct results of the 

 failure of the fruiting branches to make normal growth, for similar changes 

 occur in noncluster varieties, in plants that have been severely pruned, 

 and also in seedlings that lose their terminal buds through insect injuries 

 or by abortion.- 



The abnormalities that accompany the shortening of the fruiting 

 brandies in cotton are shown in the fonns of the leaves and the involucral 

 bracts. The leaves of short-jointed fruiting branches often become 

 smaller and more bractlike, while the bracts are often enlarged and leaf- 

 like, and show definite indications of the stipulac and foliar elements 

 that are completely fused in normal bracts. 



Each of the involucral bracts of a cotton plant represents a modified 

 leaf. The specialized form of the bract results from having the stipules 

 of the leaf greatly enlarged, the petiole entirely suppressed, and the 

 blade reduced and united with the enlarged stipules. The abnormal 

 leaflike bracts and bractlike leaves of cluster cottons show all the stages 

 between normal leaves and normal bracts. In such abnormalities there 

 is usually an obvious relation between the reduction of the blade or the 

 suppression of the lobes of the blade and the enlargement of the stipules 

 of the same leaf, and also between abnormalities of the leaf and of the 

 involucral bracts of the same intemode. This can be understood by 

 comparing Plate LIII, which shows an abnormal reduced leaf and an 

 abnormal enlarged bract, with Plate LIV, which shows a leaf and a bract 

 of normal size and proportions. The converse relation is that when the 

 bracts take a more leaf-like fonn, with an enlargement of the middle or 

 blade element of the bract, it is almost always accompanied by a 

 reduction of the stipular elements, as shown in the leaf-like bract in 

 Plate LIII. When one side of a leaf is reduced or has the lobe sup- 



• The dimorphic specializations of the branches and leaves of Uie cottott plant have already been de- 

 scribed. (Cook, O. V. Dimorphic branches in tropical crop plants ... U. S. Dept. Asr. Bur. Plant 

 Indus. Bui. 198, 64 p . t) tit;.. 7 pi. 1911. Cook, O. F. Dimorphic leaves of cotton and allied plants in 

 relation to heredity. U. S. Dcpl. Aer. Bnr. Plant Indus. Bui. in, 59 p., 18 fig., s pi. 1911.) 



' Abortion of terminal buds is a frequent result of a peculiar disorder of cotton seedlings previously 

 described. (Cook. O. F. Leaf-cut, or tomosis, a disorder of cotton seedlines. In U. S. Dept. Aer. Bur 

 Plant Indus. Circ. 1^0. p. 29-34. r fie- 191.1-) 



