34 



LEAF ADJUSTMENT 



20). Possibly these blades are driven into this position because 

 they are more sensitive and the direct sunlight upon the broad 

 surface of the blade would be injurious. This certainly appears 

 to be the case in young leaves when emerging from the bud. 

 Note the character of the foldings and the erect positions of such 

 leaves (Fig. 21). The leaves of the horse chestnut assume at 



Fig. 20. 



Fig. 21. 



Fig. 20. Shoot of wild lettuce, Lactuca, with leaves turned edgewise to 

 the light. 



Fig. 21. Position of young leaves of hickory: A, scales of bud curving 

 back showing the tightly folded leaves. B, later stage, leaves unfolding but 

 still erect. 



least three different positions during their growth. The folding 

 of such leaves and their erect positions expose less surface. They 

 consequently lose less heat and moisture, and the delicate grow- 

 ing cells are better protected against the intense sunlight. You 

 must notice as the cells mature and become better protectet that 

 their irritability changes and they respond in a different way 

 to light and gravity, etc. , They begin to unfold and turn away 

 from their erect positions and at maturity assume a fixed posi- 

 tion to light, etc., that is quite different from the original posi- 



