22 DIMORPHIC BRANCHES IN TROPICAL CROP PLANTS. 



Each of the three bracts that inclose the bud of the cotton plant 

 represents a specialized leaf formed by enlarged stipules united with 

 a greatly reduced blade. In Egyptian cotton it often happens that 

 the leaf subtending a flower bud does not retain its normal size and 

 shape, but becomes more or less intermediate between a leaf and a 

 bract. One or both of the stipules may be enlarged and united with 

 the blade, or the blade may remain separate, wiih the stalk more or 

 less shortened. 



The formation of these abnormal organs shows, as in the case of 

 the branches, a failure to maintain the normal specialization of the 

 parts. The processes of growth that should take place only in the 

 bracts are partly anticipated in the formation of the leaf, the result 

 being an intermediate expression of the leaf and bract characters. 

 Plants that have the bractlike leaves are also likely to have leaf- 

 like bracts, more deeply divided at the apex than the normal bracts, 

 and often deeply lobed or cleft nearly to the base. 



The liability of the normal specializations to break down may be 

 connected in a general way with the fact of dimorphism of the 

 branches. The fertile branches can be looked upon as inflorescences 

 that have approached the vegetative form and tend to revert to more 

 determinate conditions. The dimorphism of the branches, in such 

 plants as cotton and coffee, means that there are two kinds of vege- 

 tative internodes, one forming branches devoted to purely vegetative 

 purposes, the other somewhat infermediate between vegetative and 

 reproductive internodes. Individual internodes which are accessory 

 to the reproductive internodes occur in many plants, just below the 

 flowers. The fruiting forms of specialized branches are made up of 

 such intermediate or slightly specialized internodes. 



The practical siginficance of the abnormalities of the involucre is 

 the same as in the case of the branches. The disturbance of the 

 normal processes of growth are shown to have affected more than 

 the mere external form of the plants. The flower buds that follow 

 the abnormal bractlike leaves are almost invariably aborted, and 

 if the number of such abnormalities is large the plant becomes un- 

 productive or even completely sterile. Such abnormalities have been 

 particularly abundant in the Dale variety of Egyptian cotton, both in 

 1008 and 1909, but the 1909 planting from seed raised in Arizona in 

 1908 shows a much larger proportion of normal individuals than 

 among the plants grown from imported seed. Some of the plants of 

 the Dale cotton have the strict upright form of the so-called limb- 

 less' varieties of Upland cotton, and some produce no flower buds in 

 the normal place on fruiting branches, but only from buds of short 

 axillary branches that appear to represent transformed leaf buds, 

 all other buds being completely aborted. Sometimes all of the buds 

 abort and the whole plant remains completely sterile. 



198 



