282 RELATION OF PHYLLOTAXIS TO MECHANICAL LAWS. 



phyllotaxis is very rare (c/. Abies sp.) ; and the fact that such a 

 degree of " dorsiventrality " may be legitimately regarded as a 

 secondary specialisation is suggested by the curious fact that in 

 the great majority of such phyllotaxis systems the construction is 

 symmetrical of the distichous (1 + 1) type, the decussate (2 + 2), and 

 very rarely (3 + 3) (cf. Salvinia, Catalpa syringaefolia) ; * thus 

 giving, in the first case, two rows of leaves on the upper side of 

 the shoot, and in the second case four lateral rows of the Selaginella 

 type. But on comparing the orientation of such lateral shoots 

 with the normal examples of distichous and decussate orientation, 

 it will be noted that while in the former case the leaves may lie 

 either in the transverse or median planes, and in the latter are 

 cruciately orientated in these same planes, the assumption of 

 eccentric development is accompanied by a displacement of the 

 growth-centre in such a manner that the leaf-members are always 

 left on the flanks of the now bilateral construction, and the growth- 

 centre is displaced towards the upper side in such a way that 

 distichy is replaced by two rows of leaves now apparently on the 

 upper surface, and decussation by an accurate displacement at 45°, 

 giving a diagonal orientation; while the (3 + 3) system displaces 

 at 30°, with the result that three anisophyllous rows are left on 

 either flank. In fact, as opposed to the use of the term '' dorsi- 

 ventral," the more immediate purpose of the operation would 

 really appear to be the creation of a right and left side, so that one 

 would be as fully justified in calling a " dorsiventral " shoot 

 zygomorphic, as a " zygomorphic " flower dorsiventral (cf. Goebel). 



Such displacements have been variously described in terms of 

 angular changes and "displacements," according as the general 

 effect is judged by the eye ; f but there can be little doubt that 

 the same general construction principles continue to obtain, and 

 that the scheme of fig. 97 conveys a very good summary of the 

 facts of the case, the orthogonal intersection lines of the respective 

 systems remaining unaffected. For example, it is clear that the 

 greater the eccentric specialisation of a distichous shoot, the more 

 nearly would the two rows of leaves appear as a single median line 



* Goebel, Organography, p. 108. 

 t Cf. Ibid., p. 112. 



