30 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 

 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 protected 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- 

 tion. The leaves of many plants do not have a fixed position 

 but show more or less motility during their entire life. Inter- 

 esting examples are seen in the sensitive plants and in members 

 of the bean and oxalis family where marked changes result in 

 the position of the leaves from alterations in the intensity of 

 light, temperature, or moisture. Many of these plants fold their 

 leaves when the temperature falls at sundown. Many flowers 

 close in the same way. These changes are popularly termed sleep 

 movements. They are caused not only by the changes in the 

 intensity of the light but also by changes in temperature and 

 moisture. In the case of these leaves the cells in the swollen 

 organ (the pulvinus) at the base of each leaf are kept full of 

 water so long as the temperature and light exert a suitable stimu- 

 lus upon the plant, but when the conditions are unfavorable 

 certain of the cells on one side of the pulvinus lose water and 

 contract while the cells on the opposite side of the pulvinus re- 

 main rigid; consequently, the leaves droop and fold in various 



