GLASGOW SOCIETY OF FIELD NATUEALISTS. 203 



Passages might be quoted from the works of Livingstone, Wallace, 

 and Darwin, to prove that thorns are characteristic of dry, stony 

 soils, whilst well watered districts are remarkable for the almost 

 total absence of such defensive weapons. The common pear tree, 

 if allowed to grow wild, exhibits many branches which ordinarily 

 bear leaves aborting into spines. The little Scotch rose (Rosa 

 spinosissimaj, which grows in dry sandy places, becomes much less 

 prickly if grown in moist garden soil. The common bramble, 

 when grown on stony waste places, possesses prickles both on its 

 stem and leaves, but when growing high up on moist mountain 

 sides is entirely devoid of them. The inability of the wild pear 

 tree to produce leafy branches is to be explained by the difficulties 

 the roots experience in obtaining a sufficient supply of mineral 

 matter from the stony soil ; for, since a much larger amount of 

 inorganic matter is required for leaf-formation than for the pro- 

 duction of wood, we should expect that poor soil would have the 

 effect of diminishing the number of leaves. Possibly this may 

 also be the reason for leaves splitting up in dry soil, 



IV. In his address to the Biological section of the British 

 Association in 1876, Mr Wallace, when discussing the distribution 

 of plants and animals in oceanic islands, was led to speculate on the 

 object attained by the presence of aromatic leaf glands in plants, 

 in connection with the observed absence of conspicuously coloured 

 and scented flowers from insular floras, as determined by the 

 paucity of insect life. He cited Mr. Darwin's opinion, that leaf 

 glands bearing essential oils are a protection against the attacks of 

 insects where these abound, and would thus not be required in 

 countries where insects were scarce, but took exception to this 

 view on the ground that highly aromatic plants are characteristic 

 of deserts all over the world. He also gave numerous examples 

 and quoted many authorities proving aromatic plants to enjoy no 

 immunity from insect attacks ; neither did Mr. Wallace consider 

 that scented leaves could be much in the way of attraction, 

 supplementary to that of a scented corolla, so he leaves us in 

 doubt as to the original office of these aromatic leaf glands. 



It is to the physicist rather than the biologist we must turn 

 for a solution of this difficulty. Professor Tyndall has made an 

 important series of experiments on the absorptive power of 

 different gases for radiant heat, from which it appears that under 



