88 



PHYLLOTAXIS OR LEAF- ARRANGEMENT. 



are produced (figs. 1C8, 197), they are verticillate (verto, I turn), and 

 the circle of leaves is then called a verticil or whorl. When leaves are 



196 



opposite, the pairs which are next each other, but separated by an in- 

 ternode, often cross at right angles (fig. 196 a J), or decussate (decusso, I 

 cut crosswise), following thus a law of alter- 

 nation. The same occurs in verticils, the 

 leaves of each whorl being alternate with 

 those of the whorl next to it; or, in other 

 words, each leaf in a whorl occupying the 

 space between two leaves of the whorl next 

 to it. There are considerable irregularities, 

 however, in this respect, and the number 

 of leaves in different whorls is not always 

 uniform, as may be seen in Lysimachia 

 vulgaris. 



170. When a single leaf is produced at a 

 node, and the nodes are separated so that 

 each leaf occurs at a different height on the 

 stem, the leaves are alternate (fig. 198). 

 The relative position of alternate leaves 

 varies in different plants, although it is 

 tolerably uniform in each species. In fig. 195, leaf 1 arises from a 



Fig. 195. Portion of a branch of a Lime tree, with four leaves arranged in a distichous manner, 



the spiral and two leaves. 



Fig. 196. Opposite, decussate leaves of Pimelea decussata. a, A pair of opposite leaves, b, 

 Another pair placed at right angles. 



Fig. 197. Leaves of Lysimachia vulgaris, in verticils or whorls of three. The leaves of each 

 verticil alternate with those of the verticils next it. In this plant the number of the leaves in a 

 verticil often varies. 



