^6 THE LEAF 



plete turns round the stem which the spiral line traces. For 

 example, the angular divergence of the leaves on a pear shoot 

 is I : selecting any one leaf as a starting point, the spiral line 

 passes twice round the stem by the time that it reaches the 

 next leaf on the same row, and in doing so touches the bases 

 of iive leaves. To determine the leaf-arrangement upon any 

 particular shoot, it is necessary to observe the bases of the 

 leaves and not the blades, as the position of the latter is affected 

 by external conditions, especially by light and the force of 

 gravitation. Occasionally the stems become twisted during 

 growth, and the leaves are consequently displaced from their 

 normal position. 



The orderly arrangement of the leaves upon stems is de- 

 pendent on the internal forces of the living plant. By growing in 

 this manner all the leaves become equally exposed to light and 

 air, and interfere very much less with each others requirements 

 in this respect, than would be the case if the leaves were disposed 

 irregularly. 



Ex. 41. — Examine and describe the leaf-arrangement upon the shoots of all 

 common farm plants, trees, and weeds. 



5. Bud-arrangement. — As buds arise normally in the axils of 

 leaves, it follows that the arrangement of buds upon trees in 

 winter will be similar to that of the leaves during the previous 

 summer. A careful recognition of the position and arrangement 

 of buds upon the shoots of plants is of some importance in the 

 practice of pruning, where buds are required to produce branches 

 growing in some particular direction. 



For the arrangement of the buds upon the chief shrubs and 

 trees, see pp. 61-66. 



6. Leaf-fall : ' Evergreens.'— In most of the broad-leaved trees 

 and shrubs of temperate regions the leaves produced from buds 

 in spring usually last only one growing-season, and then all fall 

 off before the plants enter a state of rest in the following winter. 



