Leaves in Relation to Light 71 



light of reduced intensity. They receive an amount of light 

 comparable to that received by the shade plants found in 



Fig. 41. Various positions taken by leaflets of lima bean: A, position in 

 intense light ; B, position in diffuse light ; C, position in darkness. 



forested ravines. Submerged leaves, too, are of very soft tex- 

 ture, and are quite without mechanical tissue in the veins, so 

 that they are unable to support themselves when hfted from 

 the water. They are kept upright in the water by their buoyancy. 

 Summary. Light has marked effects upon the positions, the 

 color, and the structure of leaves. Leaves tend to be placed 

 directly outward from the nodes to which they are attached, but 

 light affects them during their development, and most leaves 

 come to occupy positions that have more relation to the light 

 than to the stem which bears them. The position of leaves 

 and the movements of leaves are determined by differences in 

 water content and in the rate of growth on opposite sides of the 

 stems and petioles that support them. 



PROBLEMS 



1. Why do house plants flourish best at south windows in the winter time? 



2. What part of full sunlight is received by a plant that stands near a window? 



3. Why do gardeners shade lettuce plants in midsummer? 



4. What other condition, besides light intensity, is affected by shading? 



5. Why is tobacco that is intended to be used for cigar wrappers usually grown 



under canvas or beneath lattice frames? 



