102 



PHYLLOTAXIS OK LEAF-AERANGEMENT. 



as it were one, and then several leaves may be produced at the same 

 height on the stem. When two leaves are thus produced, one on 



Fig. 211. Fig. 212. 



each side of the stem or axis, and at the same level, they are called 

 opposite (fig. 212) ; when more than two 

 are produced (figs. 183, 213), they are 

 verticillate (verto, I turn), and the circle 

 of leaves is then called a verticil or whorl. 

 When leaves are opposite, the pairs which 

 are next each other, but separated by an 

 intemode, often cross at right angles (fig. 

 212 ab), or decussate (decusso, I cut cross- 

 wise), following thus a law of alternation. 

 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 ia Lysimachia vulgaris (fig. 213). 



Fig. 211. Portion of a branch of a Lime tree, with four leaves arranged in a distichous man- 

 ner, or in two rows, a, The branch with the leaves niunbered in their order, n being the 

 node, and m the Intemode or merithal. 6 Is a magnified representation of the branch, 

 showing the cicatrices of the leaves and their spiral arrangement, which is expressed by i, or 

 one turn of the spiral and two leaves. Fig. 212. Opposite, decussate leaves of Pimelea 

 decussata. a, A pair of opposite leaves, b, Another pair placed at right angles. Fig. 

 213. Leaves of Lysimachia vulgaris, in verticils or whorls of three. The leaves of each ver- 

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

 a verticil often varies. 



