32 



LEAF ADJUSTMENT 



reacts to these stimuli that its blade is exposed at right angles 

 to the rays of light, but in the iris, many grasses and rushes 

 the blades are nearly erect. Quite a large number of plants bear 

 their leaves edgewise. The giant trees of Australia {Eucalyptus) 

 and the so-called compass plants are familiar examples (Fig. 

 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 



/ II 





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 



