CHAPTER NINE 



LEAVES IN RELATION TO LIGHT 



Fig. 29. Vertical branch of 

 magnolia. Note the alternate 

 arrangement of the leaves. 



Leaves grow from points variously arranged on stems that 

 have all sorts of positions. If these leaves grew out in random 



directions, many of them would re- 

 ceive httle light. But an examination 

 of a plant shows its leaves arranged 

 in positions which display them ad- 

 vantageously to the hght. The raised 

 leaves of the pumpkin, the mosaics 

 formed on the sides of buildings by the 

 leaves of vines, and the successive tiers 

 of leaves on a beech, maple, or dog- 

 wood illustrate diilerent arrangements 

 by which large numbers of leaves are 

 efficiently displayed to light. Evidently 

 something controls the positions of leaves on a plant. 



Growth influenced by light. Light affects growth in all organs 

 of the plant, including the leaf. The amount of light received 

 by a leaf blade not only affects the growth of the blade, but also 

 the petiole, and in some plants the adjoining stem. The influence 

 of the hght, during the growth of leaves, is such that when they 

 are mature most leaves are favorably placed with respect to hght. 



The arrangement 

 of leaves on stems. 

 Leaves develop from 

 somewhat thickened 

 places on the stems, 

 called the nodes. 

 Each node may bear 

 one, two, or several 



leaves. AcCOrdmg ^^^ ^^^ Horizontal branch of magnolia. Compare leaf 



to the number of positions with those of Figure 29. 



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