180 GENERAL BOTANY 



being a leaf. If it is a leaf, then the leaf -like organs 

 which arise on it are pinnae and not leaves, and this 

 is confirmed by the fact that there are no buds in their 

 axils, as there would be if the thorn were a branch, 

 and they were really leaves. Compare PITHECO- 

 LOBIUM DULCE (p. 174). 



If we examine this plant when new growth is taking 

 place at the beginning of the hot weather, we shall 

 find the spines, and the stipules also, soft and green. 

 Sometimes the stipules do not fall off, but harden and 

 become thorns like the main rachis. The very young 

 pinnae curve upwards at first with the minute leaflets 

 folded flat along the upper side of the rachis. 



The conversion of the main rachis of the leaf into' 

 a thorn, and the reduction in the size of the leaflets, 

 is an adaptation to zerophytic, or dry land and air, 

 conditions. Corresponding with the reduction in the 

 size of the leaflets the rachises of the pinnae have 

 become broad, and being much harder and tougher 

 than the leaflets, are not so liable to be dried up 

 and withered by a hot wind. Notice too, that when 

 a branch has been picked for some time, or while 

 it is on the plant if the day be hot, the leaflets 

 fold flat along the upper surface, their upper sides 

 downwards. 



A similar folding of the leaflets occurs in a num- 

 ber of plants with compound leaves, and further 

 reference to it was made in chapter xiii. We cannot 

 doubt that it is of a protective nature, serving to 

 prevent the leaflets being scorched by the hot mid- 

 day sun when the air is dry, or suffering when for 

 any reason, the supply of water is diminished. A 



