148 



THE VEGETATIVE ORGANS 



leaves. The leaves are often reduced to stiff, radiating 

 spines {l.s) forming a gauze-like covering to the surface, 

 which acts as a most effective light-screen and protects 

 the chlorophyll of the stem from the too powerful rays of 

 the sun. Thus, sunlight is an important factor in deter- 

 mining the position of stems and leaves, and there is a 



Fig. 96. Stems which perform the Functions of Leaves. 

 1, Acacia ; 2, Smilax ; 3, Cactus ; l.s, leaf-spines ; r, leaf-like 

 petioles (phyllodes) ; Pc, leaf-like stem (phylloclade) ; s.l, scale- 

 leaf. 



tendency for plants growing in dry and very sunny places 

 to take on strange shapes and exhibit curious devices which 

 help the plant to survive under trying conditions. 



Protective movements against radiation and transpiration. 

 Movements which have for their object reduced radiation 

 and transpiration are also very common. Note how young 

 leaves emerge from the buds in spring in, e.g., the Elm 

 (see Fig. 198, 1), Beech (see Figs. 75 and 76), Lime, and 

 Horse-Chestnut (see Fig. 68). At first they are erect, then 

 as they unfold and grow they bend over and hang down- 

 wards with their under surfaces applied one to another so 



