CONSTANT PHYLLOTAXIS. 



107 



reversal of parastichies may be observed (c/. Saxifraga umbrosa, 

 Gyperus) ; but such phenomena as the transitory variation of an 



Fig. 43. —Sedum elegans, Lej. Section of shoot-apex (6 + 10). 



(8 + 13) Pine-cone to (8 + 12), implying the formation of four genetic- 

 spirals, or the bijugate variation (6-1-10) of a (5 + 8) system 



