THE LEAVES OF VASCULAR PLANTS. 279 



spires ; such are the floral leaves, and consequently the 

 flowers of some Aloes. I have counted thirteen parallel 

 spires in the flowers of the male catkins of the Cedar of 

 Lebanon. 



In all these cases of multiple spires, they follow their 

 course around the stem, parallel to one another. Their 

 direction is sometimes from right to left, sometimes the 

 contrary way in different species, and sometimes it pre- 

 sents variations in the same species. Thus Bonnet has 

 counted seventy-five examples of Chicory ( Chicorium. 

 Intyhus) where the spire went in the first direction, 

 forty-eight where it was in the second, and one in which 

 Loth were united. 



Plants which have the leaves disposed in many spires, 

 are almost always species with long narrow leaves, such 

 as Pinus, Euphorhium, &c. But the other dispositions 

 have not, in general, any connexion either with the size 

 or the form of the leaves. We can only say, that when 

 they are large they are generally more distant, and the 

 pairs, verticils, or spires, are nearer one another when 

 the leaves are small. 



All this distribution of the leaves is connected with 

 the functions of these organs. They are destined to 

 decompose carbonic acid gas, and to evaporate the super- 

 abundant water ; and physiology informs us that these 

 two functions are caused almost exclusively by the 

 action of solar Kght. In order that this action may 

 be properly performed, it is necessary^ either that the 

 leaves be far apart from each other, or that with a given 

 distance they cover one another as little as possible. In 

 fact, we have seen that in all the different systems of the 

 position of leaves, it results that those which arise im- 

 mediately above the others never exactly cover them. 

 In the least favourable cases, the third covers the first, 

 and the fourth the second ; in several others, it is the 



