46 Science of Plant Life 



eyes under water knows that it is dark at a comparatively 

 slight depth. Hence submerged plants always grow in light 

 of reduced intensity. They receive an amount of light com- 

 parable to that received by the shade plants found in forest 

 ravines. Submerged leaves, too, are of very soft texture, 

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

 they are unable to support themselves when lifted from the 

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

 which is due to the greatly enlarged air spaces among the 

 mesophyll cells. 



Summary. Light has a marked effect upon the positions, 

 the color, and the structures 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. Some 

 leaves vary their positions constantly as the light, changes. 



Leaves of plants grown in weak light, including those of 

 submerged plants, usually have the mesophyll made up wholly 

 or largely of spongy cells. Leaves of plants grown in intense 

 light usually have the mesophyll made up wholly or largely 

 of palisade cells. 



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 ? 



