THE LEAVES OF VASCULAR PLANTS. 293 



been presented a curious example of inequality in 

 Ruellia anisophylla : — one of two opposite leaves is very 

 small and narrow, and, as it were, abortive in com- 

 parison with the other ; but symmetry is also met with in 

 this irregularity, for on comparing the successive pairs, 

 the small leaf is found alternately on both sides. 



Stipules present analogous phenomena ; thus it hap- 

 pens, although rarely, that those of the two sides of a 

 leaf are unequal in form and size. This is very remark- 

 able in Ervum Ervilia, in which one of the stipules of 

 each leaf is small, entire, scarcely apparent, whilst the 

 other is large and cut. There are some plants where 

 they do not seem to exist on one side: such are the 

 stipulary tendrils of several Cucurbitaceas, supposing 

 that these are true stipules; such are especially the 

 stipulary spines of some Capparideae, such as Capparis 

 heteracantha, See. 



Section XI. 



Of the History of Leaves at different Periods of their 



Existence. 



The leaves arise upon the young shoots, and all of 

 them are already existing there, and more or less de- 

 veloped, at the moment when this young shoot begins 

 to appear ; necessarily they are very close to each other, 

 very small, and reduced, thus to speak, to their fibrous 

 skeleton. At this period sometimes the outer leaves, 

 altered in their development by the action of the air, 

 take the appearance of scales, and serve as coverings to 

 the inner leaves, and to the young shoot itself; some- 

 times, on the contrary, they are developed as the inner 



