Meade: Positions and Movements of Cotton Leaves 445 



guished, like the callus, by a reddish 

 color. The callus and thickened bases 

 of the veins have a function in shifting 

 the individual lobes, and may act inde- 

 pendently to a certain extent, although 

 generally functioning together so as to 

 cause the blade to face the direct 

 light. 



INDEPENDENT AND SIMULTANEOUS 



ACTION OF THE TWO PULVINI 



WHICH ADJUST THE LEAF 



BLADE IN CHANGES IN 



DIRECT ILLUMINATION 



The leaves of the cotton plant are 

 heliotropic, that is, the blades move 

 in such a way as to keep the upper sur- 

 face perpendicular to the direct rays 

 of light. The leaves gradually shift 

 their positions as the direction of the 

 light changes, leaves that are differ- 

 ently situated on the plant moving in 

 different directions. The changes in 

 position are governed by the pulvini, as 

 already noted. 



Both governing organs may func- 

 tion at the same time or each may act 

 independently. The distal pulvinus 

 may raise or lower individual lobes of 

 the blade, or shift the whole blade 

 while the basal pulvinus alters the 

 position of the whole leaf. Under 

 direct light the leaves are in constant 

 but gradual motion. Thus the leaves 

 are in a different position in the after- 

 noon than in the morning, the blades 

 facing east in the forenoon and west 

 in the afternoon. 



Movements are most varied on the 

 side of the plants away from the direct 

 light, where many of the leaves are in 

 the shade. Here a single leaf usually 

 requires the aid of both the distal and 

 basal pulvini to adjust the blade and 

 petiole. Some turn to the right and 

 some to the left while others either rise 

 or fall, according to the exposure to 

 the strongest light. The blade or the 

 petiole may take almost any position, 

 the blades always moving to face the 

 direct light. Often the blade extends 

 straight out from the petiole, or is 

 forced against the petiole on the under 

 side. The petiole may take either an 

 acute or an obtuse angle to the per- 

 pendicular. 



POSITION OF 

 LEAVES UN- 

 DER DIFFUSED 

 LIGHT 



In the dif- 

 fused light of 

 cloudy wea- 

 t h e r the 

 leaves show 

 practically no 

 movement 

 or differences 

 in position 

 and for this 

 reason the 

 posi t i on of 

 the leaves 

 common un- 

 der diffused 

 light is con- 

 sidered the 

 typical day 

 position. Un- 

 der such con- 

 ditions, re- 

 gardless of 

 the angle of 

 the stems on 

 which they 

 are borne, the 

 petioles stand 

 at an angle of 

 about 45 de- 

 grees from 

 the vertical 

 and the leaf- 

 blade is usu- 

 ally flat, but 

 on plants 

 with large 

 leaves they 

 are somewhat 

 folded at the 

 sinuses. 



NIGHT POSI- 

 T I O N OF 

 LEAVES DIS- 

 TINCT FROM 

 THAT ASSUMED 

 DURING DAY 



The leaves 

 take a posi- 

 tion at night 



Petiole of a cotton leaf to indicate location of 

 distal and basal pulvini. (Fig. 7.) 



