LEAVES 



551 



milfoils, and many water plants, are peculiarly favorable for light reception, 

 because the sifting of the light between the leaf divisions enables it to impinge 

 upon the leaves beneath. The aggregate surface exposed in a day is much greater 

 than in a colony of plants with large simple leaves, because of the lighting at 

 different levels, due to the shifting of the sun, and to the pliancy of the leaves in 

 wind or water currents. Probably the amount of leaf surface lighted at any given 

 moment is also greater than in the case of simple leaves, because of reflection from 

 one leaf surface to another. Plants with numerous small leaves, such as pines and 

 spruces, have the same general effect as do plants with divided leaves, and they 

 exhibit the same sort of light-sifting. Probably the lower leaves of plants with 

 divided or small leaves are less likely to suffer injury from shade than are similar 

 leaves in sunflowers and in other plants with large entire leaves. In some plants 

 (as Monstera and various oaks) the upper leaves are more divided than those 

 beneath, thus facilitating light penetration; in quite as many plants, however, the 

 lower leaves are more divided than the upper. There is no evidence in either case 

 that light has any causal relation to leaf division ; any advantage that may come in 

 the way of light reception is to be regarded as purely incidental. 



4. AIR CHAMBERS AND STOMATA 



Gaseous exchanges in plants. The chief gas movements in plants 

 are associated with respiration, carbohydrate synthesis, and transpira- 

 tion. Respiration, involving the 

 absorption of oxygen and the 

 emission of carbon dioxid, takes 

 place in nearly all plants at all 

 times, though it is slight, or even 

 wanting, in " resting " organs, 

 such as seeds. Carbohydrate 



FIG. 790. A tangential longitudinal 

 section near the upper surface of a leaf 

 'of the century plant (Agave americana), 

 showing palisade cells in cross section; 

 note the small intercellular air spaces; 

 highly magnified. 



FIG. 791. A cross section through the 

 stem of a water milfoil (Myriophyllum), 

 showing large and symmetrically arranged air 

 chambers ; note also the centrally placed con- 

 ductive region (i>), whose cells are relatively 

 undifferentiated ; considerably magnified. 



synthesis, involving the absorption of carbon dioxid and the emission 

 of oxygen, is confined essentially to chlorophyll-bearing organs in 



