PHYIXOTAXIS OR LEAF-ARRANGEMENT. 



89 



node, n; leaf 2 is separated by an internode or merithal, 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 (S<j, twice, and 

 ffT/%o?, order), or the leaves are arranged in two rows. In fig. 199, on 

 the other hand, the fourth leaf is that directly above the first, and the 

 arrangement is tristichous (f^e^ three, and (tripos, order). The same 

 arrangement continues throughout the stems, so that in fig. 199 the 

 7th leaf is above the 4th, the 10th 

 above the 7th ; also the 5th above 

 the 2nd, the 6th above the 3rd, 

 and so on. There is thus through- 

 out a tendency to a spiral arrange- 

 ment, the number of leaves in the 

 spire or spiral cycle, and the num- 

 ber of turns varying in different 

 plants. In plants whose leaves 

 are close to each other, the spiral 

 tendency is easily seen. In the 

 ScreAv pine (Pandanus odoratis- 

 simus), in the Pine-apple family, 

 and in some Palms, as Corypha 

 cerifera, the screw-like arrange- 

 ment of the leaves is obvious. This 

 mode of development prevails in all 

 parts of plants, and may be con- 

 sidered as depending on then- man- 

 ner of growth in an upward and at 

 the same time in a lateral direction, 

 the normal arrangement of all parts of plants. 



171. 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 from 

 which the enumeration commenced. This arrangement has been 

 reduced to mathematical precision,* and Braun has expressed it 

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

 turns, and the denominator the number of leaves in the spiral cycle. 

 Thus, in fig. 198 a J, the cycle consists of five leaves, the Gth leaf 

 being placed vertically over the first, the 7th over the 2nd, and so 

 on ; while the number of turns between the 1st and Gth leaf is two : 



Fig. 198. Part of a branch of a Cherry with six leaves, the sixth being placed vertically over 

 the first, after two turns of the spiral. This is expressed by -J or the quincimx. a, The branch, 

 with the leaves numbered in order. 6, A magnified representation of the branch, showing the 

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



* For a full account of .Phyllotaxis, see Bravais Mem. snr la Disposition Ge"ometrique di-s 

 Feuilles. Annales des Sciences Naturelles. Jan. and Feb. 1837. 



198 



Alternation is looked upon as 



