PHYLLOTAXIS OE LEAF-AEEANGEMENT. 



103 



When a single leaf is produced at a node, and the nodes are sepa- 

 rated so that each leaf occurs at a different height on the stem, the 

 leaves are alternate (fig. 214). The relative position of alternate 

 leaves varies in different plants, although it is tolerably uniform in 

 each species. In fig. 211, leaf 1 arises from a node, n; leaf 2 is 

 separated by an internode, m, and is placed to the right or left ; while 

 leaf 3 is situated directly above leaf 1. The arrangement in this case 

 is distichous (&h, twice, and erl^og, order), or the leaves are arranged 

 in two rows. In fig. 215, on the other hand, the fourth leaf is 

 directly above the first, and the arrangement is tristiehous (rgs/j, three, 

 and eri^og, order). The same arrangement contiaues throughout the 

 stems, so that in fig. 215 the 7th 

 leaf is above the 4th, the 10th 

 above the 7th ; also the 5 th above 

 the 2d, the 6th above the 3d, and 

 so on. There is thus throughout 

 a tendency to a spiral arrangement, 

 the number of leaves in the spire 

 or spiral cycle, and the number of 

 turns, varying in different plants. 

 In plants whose leaves are close to 

 each other, the spiral tendency is 

 easily seen. In the Screw pine 

 (Pandanus odoratissimus), in the 

 Pine-apple family, and in some 

 Palms, as Copernicia cerifera, the 

 screw-like arrangement of the 

 leaves is obvious. This mode of 

 development prevails in all parts 

 of plants, and may be considered 

 as depending on their manner of 

 growth in an upward and at the same time in a lateral direction. 

 Alternation is looked upon as the normal arrangement of all parts of 

 plants. This arrangement is liable to be interrupted by many causes, 

 so that its distinct existence cannot be always detected. 



In a regularly-formed straight branch covered with leaves, if a 

 thread is passed from one to the other, turning always in the same 

 direction, a spiral is described, and a certain number of leaves and 

 of complete turns occur before reaching the leaf directly above that 

 froin which the enumeration commenced. This arrangement has been 

 expressed by a fraction, the numerator of which indicates the number 



Fig. 214. 



Fig. 214. Part of a branch of a Cherry with six leaves, the 6th being placed vertically 

 over the first, after two turns of the spiral. This is expressed by ^ or the quincunx, a 

 The branch, with the leaves numbered in order, h, A magnified representation of the brai^ch, 

 showing the cicatrices of the leaves or their points of insertion, and their spiral arrangement. 



