CHAPTER XI 



LEAF ARRANGEMENT FOR EXPOSURE TO SUN AND AIR; 

 HELIOTROPIC MOVEMENTS OF LEAVES AND SHOOTS 



111. Leaf arrangement. 1 Leaves are quite generally arranged 

 so as to secure the best possible exposure to the sun and air. 

 This, in the vertical shoots of the elm, the oak (Fig. 100), the 

 apple, beech, and other alternate-leaved trees, is quite consistent 

 with their spiral arrangement. In horizontal twigs and branches 



MA 



FIG. 100. Leaf arrangement 

 of the oak 



FIG. 101. Leaf arrangement 

 of European beech 



of the elm, the beech (Fig. 101), the chestnut, the linden, and 

 many other trees and shrubs, the desired effect is secured by the 

 arrangement of all the leaves in two flat rows, one on each side 

 of the twig. The rows are produced, as is easily seen on exam- 

 ining such a leafy twig, by a twisting about of the leafstalks. 

 The adjustment in many opposite-leaved trees and shrubs con- 

 sists in having each pair of leaves cover the spaces between 

 the pair below it, and sometimes in the lengthening of the lower 



i See Kerner and Oliver, Natural History of Plants, Vol. I, pp. 390-424. 



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