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,’ 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.® 
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. 
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