PHYLLOTAXY, OR LEAF-ARRANGEMENT. 119 



CHAPTER IV. 



PHYLLOTAXY, OR LEAF-ARRANGEMENT. 



SECTION I. THE DISTRIBUTION OF LEAVES ON THE STEM. 



231. PHYLLOTAXY (or PhyUotaxis) is the study of the distri- 

 bution of leaves upon the stem and of the laws which govern it. 

 The general conclusion reached is, that leaves 



are distributed in a manner to economize space 

 and have a good exposure to light, &c., and 

 that this economy on the whole results from 

 the formation of leaves in the bud over the 

 widest intervals between the 

 leaves next below. 1 Leaves 

 are arranged in a consider- 

 able variet} 7 of ways, which 

 all fall under two modes, the 

 Verticillate and the Alter- 

 nate (13), but which may 

 also be termed the Cyclical ( 

 and the Spiral. 



232. Alternate leaves are 

 those which stand singly, 

 one after another ; that is, 

 with one leaf to each node 



or borne 



on one height of stem. Verticillate leaves are 

 those with two or more at the same height of/ 

 stem, circularly encompassing it, i. e. forming! 

 a Verticil or Whorl. Verticillate and whorled 

 are synonymous terms to denote this arrange- 

 ment. These two kinds of leaf-arrangement 

 are commonly ranked as three, viz. alternate, 

 opposite, and whorled. But the opposite is 

 only the simplest case of the whorled, being 



1 For the most comprehensive discussion of phyllotaxy in connection with 

 development, and in view of these relations, see Hofmeister, Allgemeine 

 Morphologie, 11, and Chauncey Wright, Mem. Amer. Academy, ix. 389. 

 FIG. 236, Alternate, 237, Opposite, 238, Verticillate or whorled leaves. 



