16 CIRCULAR NO. 109, BUREAU OF PLANT INDUSTRY. 



young leaf and the fruiting branch of each internode have a period 

 of very rapid growth when they take nearly upright positions and 

 throw the delicate terminal shoot out of the perpendicular. The 

 petiole of a young leaf often becomes thicker, for a time, than the 

 next joint of the stalk. Subsequent thickening of the joints pushes 

 the leaves and branches over to the side, so that the zigzag form of 

 the young terminal shoots is gradually lost and the mature stalk 

 becomes straight and upright. In cluster varieties the mature 

 stalks may still show the zigzag form, which is doubtless connected 

 with the fact that such varieties have an accelerated development of • 

 short, thick fruiting branches. Thus, if the zigzag form were to be 

 accepted as a sufficient indication, the main stalk might be consid- 

 ered sympodial as well as the fruiting branches, but there is no occa- 

 sion for such a theory, in view of the fact that the mam stalk has no 

 floral buds or branches opposed to the leaves. 



CONCLUSIONS. 



There is no apparent necessity to consider the normal development 

 of the cotton plant as resulting in any truly sympodial structure. 

 The pseudosympodial form of the fruiting branches arises from the 

 fact that a flower, instead of a branch, is developed from the extra- 

 axillary bud, and from the further peculiarity that the fruiting 

 branches have a stronger tendency to develop one joint at a time. 

 But notwithstanding these differences the two kinds of branches 

 may be considered as homologous in other respects. The floral buds 

 have an extra-axillary position on the internodes of the fruiting 

 branches, corresponding to the extra- axillary position of the buds 

 that produce the fruiting branches on the mam stalk or the vegeta- 

 tive limbs. Transformations of branches from the fruiting to the 

 vegetative form afford additional evidence that the two types of 

 branches are not so fundamentally different as the sympodial theory 

 Would imply. Changes from one form of branches to the other are 

 readily induced by differences of external conditions. 



[Cir. 109] 



