PLANT PHYSIOLOGY 155 
adopted different devices to prevent excessive transpira- 
tion. Most of: them, eg. Pinus, Eucalyptus, Aloe, 
have their stomata sunk in pits, instead of being on the 
surface; their leaves have also a very thick cuticle. 
Some, like the Silver Tree, have a dense covering of 
hairs on their leaves, others, e.g., Crassula and Coty- 
ledon, have thick fleshy leaves which store up water. 
Pinus has needle-shaped leaves which do not expose so 
much surface for transpiration ; in Erica the leaves are 
small, narrow, and closely crowded. In many species 
of Acacia the leaves are so finely divided that the 
leaflets are quite minute, so that again the surface 
exposed is diminished. 
One species of Acacia bears phyllodes, instead of 
compound leaves, and this makes the flat surface lie in a 
vertical plane, and so checks transpiration. The same 
result is obtained in Eucalyptus, by twisting the leaf on 
its petiole, so that it is edgewise to the mid-day sun. 
In Euphorbia, the stem is thick, fleshy and green, and 
the leaves are altered to thorns; in Asparagus, the 
stems are green and needle-like, and the leaves are 
scales or thorns. Such leaf-like stems are called 
cladodes; and they too have vertical assimilating sur- 
faces, thus checking transpiration. 
A few plants, like Psamma, and some grasses, roll up 
their leaves, thus protecting the stomata and check- 
ing transpiration. Many Xerophytes are thorny, e.g. 
Prickly Pear, Aloe. This is probably because growing 
as they do in places where moisture is scarce, they are 
very liable to be eaten by animals, and must therefore 
protect themselves against them. 
