Deserts 



In other respects they are also very interesting. 

 Such a solid pillar as the Cereus or giant cactus of 

 America has to withstand considerable wind pressure. 

 It has an interesting lattice-like arrangement of woody 

 fibres which is ingeniously adapted to give support 

 against the wind, and yet to allow of the contraction 

 and expansion which we have already mentioned. 



The use of these spines or thorns is obvious enough. 

 Antelopes or guanacoes, or even rats and mice, would 

 surely break into them when perishing of thirst. Mr. 

 Darbishire, 5 however, considers that the glancing thorns 

 are intended to reflect off the sunshine, and that this 

 is also the explanation for the hairs of many Mesem- 

 bryanths. 



The Euphorbias of the Canary Islands and many 

 desert fringes are quite efficiently protected by their 

 acrid and poisonous milky juice. A drop on the skin 

 may produce a blister, or even total blindness if it 

 reaches the eyes. 



The grey hairs or cotton-wool which shrouds and 

 swathes many desert plants is another interesting char- 

 acter ; it is also found in, for instance, the common cud- 

 weeds (Gnaphalium and Filago). Such hairs will greatly 

 hinder the loss of water by transpiration ; they also 

 diminish by about one-half the amount of light that falls 

 upon the leaf-surface. 6 



The flannel-like or dense tangled mass of hairs which 

 coats the Edelweiss, South African Helichrysums, and 

 the Zillas of the Egyptian desert are therefore distinctly 

 serviceable. 



Very young leaves belonging to all sorts of plants 

 are often covered with hairs whilst still enclosed in the 

 bud, but when they unfold and expand they often lose 

 all trace of them. In these desert plants the primitive 

 hairs seem to have remained and indeed increased. 



*74 



