8 ARRAXGEMENT OF PARTS IN THE COTTON PLANT. 



ARRANGEMENT OF THE LEAVES ON THE MAIN STALK OF THE 



COTTON PLANT. 



The leaves of the cotton plant are not set at random along the stalks 

 and branches, but are regularly arranged in ranks and spirals. In 

 plants of normal growth it is usually easy to see that each leaf is 

 directly above or below some other leaf and that there are three, five, 

 or eight of these vertical ranks of widely separated leaves. 



The regular spiral arrangement of the leaves is found on the main 

 stalk and the vegetative branches. It is not apparent on the fruiting 

 branches, for these have the joints twisted so that the leaves appear 

 to stand in two -rows. 



The spiral arrangement of the leaves around a main stalk or a vege- 

 tative branch can be understood hy considering the relation of any 

 given leaf to the one next higher on the stalk. An imaginary line that 

 would connect the insertions- of the leaves would form a regular spiral, 

 since it continues around the stalk in the same direction, to the right 

 on some plants and to the left on others. On plants with three-ranked 

 leaves the spiral makes one turn around the stalk in going from any 

 given leaf to the next leaf that is directly above it. If the leaves are 

 in five ranks the spiral makes two turns in going up to the next leaf on 

 the same rank, while with eight-ranked leaves three turns are made. 



Many individual plants will be found with their stalks so bent or 

 twisted that no regular leaf arrangement is apparent, but in the ma- 

 jority of cases it is easy to ascertain which of the systems is followed. 

 The regularit}^ of the spiral is also destroyed if the growth of the stalk 

 has been interrupted by dry weather or other injuries that cause the 

 formation of very short joints. In such cases the direction of the 

 spiral may even appear to be reversed. 



Instances of such irregularities have been brought to our attention 

 in cotton raised at Palestine, Tex., in the season of 1909, by Dr. D. N. 

 Shoemaker, of the Bureau of Plant Industry. Some of the stalks had 

 two or three sections of shortened joints and apparent reversals of the 

 spirals. A possible explanation may be found in the fact that boll 

 weevils, which were unusually abundant in the early part of the sea- 

 son, often eat out the terminal buds of the young plants. If the 

 growth of the stalk were continued by an axillary bud it might be 

 expected that the direction of the spiral would often be reversed, for 

 the vegetative branches often differ from the main stalk in the direc- 

 tion of the spiral. 



Botanists who have made special studies of the arrangement of 

 leaves have found it convenient to describe the different systems by 

 fractional numbers. The numerator of the fi'action shows the num- 

 ber of turns that the spiral makes in passing from any given leaf to the 

 next member of the same rank, while the denominator indicates the 



