DOR SI1/ENTRAL SHOOTS. AN 1 SOPH YLL Y IN SPERM A PH YTA 1 1 1 



which plagiotropous shoots arise with leaves which are equal in size so 

 long as they are in the soil. There are therefore differences like those 

 we find in Selaginella. In Elatostemma sessile, etiolated shoots also 

 show anisophylly. It is quite clear that in all these cases the anisophylly 

 cannot be directly caused by 

 external factors. Vegetative 

 shoots arise only in the axils 

 of the larger leaves, and as 

 these leaves are arranged in 

 two apparent rows on the 

 mature shoots, a flat dorsi- 

 ventral branching system is 

 developed like that of most 

 species of Selaginella, with 

 which plants Elatostemma has 

 also in common the asymmetry 

 of the leaves. It is easy to 

 find amongst the Urticaceae 

 examples of less marked 

 anisophylly which of course 

 indicate a less degree of bio- 

 logical importance, but I prefer 

 to cite some more examples 

 from other cycles of affinity. 

 2. Melastomaceae. The 

 genus Centradenia includes 

 species possessing varying 

 degrees of anisophylly. The 

 differences in the leaves are 

 extremely well marked in 

 Centradenia inaequilateralis 

 whose sickle-like leaves are 

 conspicuous on the plagio- 

 tropousshoot-system (Fig. 65). 

 The construction of the leaf- 

 shoot evidently points to a 



FlG. 65. Centradenia inaequalateralis. The leaves stand 

 in cross-pairs and are very asymmetric. In each leaf-pair one 

 leaf is always much larger than the other, and the larger leaf 

 only has an axillary shoot. 



shaded habitat for the plant 



in which it receives the rays 



of light at a right angle. 



Centradenia floribunda shows the anisophylly to a less extent, especially on 



the orthotropous shoots ; the larger leaf here has a length of 6 cm. and 



a breadth of 1-4 cm., whilst the smaller has a length of 5 cm. and a 



breadth of i-i cm. The sickle-like curvature of the leaf is only slightly 



