OF THE GENUS ULEX. 31 



in the axil of one of which a short spine was produced. The 

 uppermost spine on the branch was 12 mm. long, being more 

 exposed than the lower one, and had four foliage leaves, in the 

 axils of three of which secondary spines were found and in the 

 fourth a flower. The arrangement of the leaves, both on the 

 spines and on the accessory branches, was very similar to that 

 which has been described as occurring in U. Etiropceus, and 

 exhibited the same kind of variation. 



This variation in the arrangement of the leaves is difficult to 

 explain, being probably due to many causes, some of which we 

 can perhaps indicate, and some of which we are ignorant. It is 

 determined, first of all, by their development at the apex of the 

 stem, the new leaf appearing just where there is the greatest 

 amount of space between those previously formed. This explains, 

 of course, the formation, at the base of a branch, of leaves in 

 pairs, the two first leaves being developed opposite each other and 

 the succeeding ones exactly midway between them. It depends, 

 secondly, upon the size of the leaves, variation being introduced 

 when the newly developed leaves vary in size from those preceding 

 them. And variation is also probably caused when flowers are 

 produced in the axils of the leaves instead of spines. 



The size of the leaf mainly depends, of course, upon the con- 

 ditions which have caused their reduction to spines, but it also 

 depends to a slight extent upon their situation on the plant, the 

 spiny character not being so well developed in leaves formed in 

 the shade as in those exposed directly to light. We shall, however, 

 consider this question again when dealing with the development 

 of the seedling. 



The anatomical structure of the stem is correlated with the 

 reduction of the leaves to spines. As the assimilative function of 

 the leaves is reduced, that of the stem becomes more perfect. In 

 order to eff*ect this, both the normal stems and the spiny branches 

 are longitudinally ridged, so that the amount of stem surface 

 which becomes thus exposed is in extent considerably more than 

 doubled. If we examine a transverse section of 'a primary spine 

 under the microscope, we shall notice that the ridges are very pro- 

 minent, with the hollows between them very pronounced (see 

 Fig. 12). The ridges are supported by bands of strengthening 



