56 



PRACTICAL BOTANY 



oaks, corn, and most kinds of hard-wood trees and of herba- 

 ceous seed plants. There are many varieties of spiral arrange- 

 ment, the simplest 

 being that of corn 

 and other grasses, 

 in which the sec- 

 ond leaf is on the 

 opposite side of 

 the stem from the 

 first, and the third 

 is directly over the 

 first. More com- 

 plicated is the spi- 

 ral of our common 

 fruit trees (Fig. 

 41) ; and yet more 

 complicated those 

 of pines (Chapter 

 XVIII), of house- 

 leeks, and of many 

 other plants. 



53. Arrangement on vertical stems in relation to overshadow- 

 ing. On a vertical stem usually no leaf directly overlies the one 

 next beneath it. In the opposite arrangement overshadowing 

 is partially prevented by having the leaves of each pair overlie 

 the spaces between the two leaves directly below them (Fig. 

 39). In the various spiral arrangements the third, the sixth, 

 or the ninth leaf, and so on, comes directly over the first. 



Since every foliage leaf usually bears in its axil a leaf bud 

 which may develop into a branch, it follows that branch arrange- 

 ment must depend upon and resemble leaf arrangement. 1 It is 

 clear, therefore, that the branches cannot usually directly over- 

 lie each other. This is plainest in the case of opposite leaves 

 and branches, but it is fairly evident in many other cases. 



1 Branch arrangement is often obscured by the dying of most of the 

 branches (Sect. 60). 



FIG. 41. Alternate arrangement of leaves 

 An apple twig in the autumn 



