THE LEAF 61 
hand, plants growing in an arid region will usually have few 
and small leaves, so that the rate of transpiration is restricted 
by a reduction in the transpiring surface. 
Gardeners, when transplanting large plants, very commonly 
reduce the number of leaves, to restrict transpiration. In remoy- 
ing a plant from the soil the parts of the roots which absorb 
water are destroyed to a very considerable extent. The plant 
can therefore absorb less water and so has less water for tran- 
spiration. In order to balance this effect it is a usual practice 
to remove a considerable portion 
of its leaves. 
Thickness of leaves. If the 
leaves of a plant are thick, they 
will have a smaller outside area 
per unit of volume than they 
would have if they were thin. 
Therefore thin leaves will tran- 
spire more per unit of volume 
than will thick leaves. In keep- 
ing with this fact we find that 
plants growing in very moist 
places usually have thin leaves, 
while those which are exposed 
to dry conditions have thicker 
leaves. 
Compactness of chlorenchyma. If the cells of chlorenchyma 
are arranged close together, there will be a less free circulation 
of gases inside the leaf than there would be if the cells were 
loosely arranged, with large intercellular spaces; consequently, 
the more the spongy chlorenchyma is replaced by palisade the 
more the rate of transpiration will be restricted. The relative 
development of the palisade and the spongy chlorenchyma cells 
varies greatly under different environmental conditions. In 
very moist places, as in damp, shaded ravines, the spongy chlo- 
renchyma usually occupies a larger portion of the leaf than 
does the palisade, and not infrequently the palisade disappears 

Fig. 44. A branch of an Australian 
desert plant, Hakea, with the leaves 
modified as spines. (X 3) 
