ARRANGEMENT OF LATERAL MEMBERS. 



15 



If it is required to determine the arrangement of the leaves 

 (phyllotaxis) on a stem, it is necessary to find the leaf which is 

 exactly above the one, numbered 0, selected as a starting-point, and 

 then to count the number of leaves which are met with in following 

 the shorter spiral round the stem between thesfc two leaves. The 

 number of the leaf which lies in the same orthostichy is the 

 denominator of the fraction of divergence, and the numerator is the 

 number of turns made by the spiral between 

 the two leaves. 



When the number of orthostichies is 

 greater than 8, it becomes very difficult to 

 detect them, particularly when the leaves are 

 closely arranged as in the rosette of the 

 House-Leek, the florets in the capitulum of 

 the Sunflower, or as the scales in a Mr-cone. 

 Another set of lines lying obliquely then strike 

 the eye, called parastichies, which also run 

 round the stem in a spiral, but touch only some 

 of the leaves ; for instance, in Fig. 8, the line 

 which connects the leaves 3, 6, 9, and 12. It 

 is evident that the number of parallel para- 

 stichies must be as great as the difference 

 between the numbers of the leaves in any 

 one such line. Thus in Tig. 8, again, another 

 parastichy connects the leaves 2, 5, 8, 11, 

 and so on ; and a third, the leases 1, 4, 7, 

 10, etc. From this it is possible to deduce 

 a simple method for ascertaining the phyl- 

 lotaxis in complicated cases ; the parastichies 

 which run parallel in one direction are 

 counted, and the leaves in one of them are 

 numbered according to the above-mentioned 

 rule ; by repeating the process in another 

 system of parastichies which intersects the 

 first, the number of each leaf will be found. 



The commonest divergences are the follow- 

 ing : 



i, i, *, I, TV, A, M- 



This series is easy to remember, for the numerator of each 

 fraction is the sum of those of the two preceding, and it is the 

 same with the denominator. There are, however, divergences 



FIG. 8. Diagram of a 

 stem the leaves of which 

 have the constant diver- 

 gence of f, the leaves of 

 the anterior surface are 

 indicated by their inser- 

 tions, those of the pos- 

 terior by circles ; they 

 are connected by eight 

 orthostichies, I, I ,11, IT, 

 etc. 



