2io Plant Life and Evolution 



Excessive Light. The effect of too strong light 

 is injurious and may result in the destruction of the 

 delicate tissues exposed to it. Hence arises the ne- 

 cessity in many plants for protection against exces- 

 sive illumination, and as this is usually associated 

 with high temperature, and consequently rapid 

 transpiration, it is not always easy to determine how 

 far certain structures are connected with excessive 

 light, and how far with protection against heat and 

 loss of water. Thus the thickened cuticle of the 

 leaves, and the dense covering of hair often found 

 in many plants exposed to hot dry air, are probably 

 protective against both light and heat rays, and the 

 same is true of the vertically placed leaves of Eu- 

 calyptus and Manzanita. This also holds good for 

 the great reduction of surface seen in many plants 

 of arid regions. In some species of broom and 

 asparagus, and the cacti, the leaves are nearly or 

 quite absent, and the small twigs develop chlorophyll 

 in their outer tissues and replace the leaves. The 

 amount of green tissue is thus greatly reduced, but 

 the activity of these cells is much greater owing 

 to the more intense light, and at the same time the 

 transpiration surface is correspondingly reduced. 



Certain effects apparently due directly to other 

 factors may be found ultimately to be the result of 

 light. Thus it has been found that plants having 

 bright-colored flowers, when forced into bloom at 

 high temperatures in a greenhouse, will develop pale- 

 colored or even white flowers. Klebs, and others 



