30 LEAF ADJUSTMENT 



reacts t" these stimuli that its blade is exposed at right angles 

 to the rays <>f light, but in the iris, many grasses and rushes 



the bhuks an.- nearly erect. Quite a large number of plants bear 

 their leaves edgewise. The giant trees of Australia i 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 I 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 wax- 

 to light and gravity, etc. They begin to unfold and turn awa\ 

 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. Main 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 abo by changes in temperature and 

 moisture. In the case of these leaves the cells j M the swollen 

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

 water so lon-- as the temperatun 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 OH the opposite side of the pulviiu: 



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



