60 A TEXTBOOK OF GENERAL BOTANY 
spaces. Again, the rate of transpiration is greatly influenced by 
the humidity of the air, that is, the amount of water vapor in the 
air. Dry air not only absorbs more moisture than wet air does 
but absorbs it more rapidly. Therefore the rate of transpiration 
decreases with rising humidity. 
Control of transpiration. The rate of transpiration is greatly 
influenced not only by external conditions but also by the 
plants themselves. This 
is brought about in two 
general ways: by perma- 
nent morphological changes 
which restrict or acceler- 
ate the rate of transpira- 
tion, and by physiological 
changes which control the 
rate temporarily. 
Restriction of transpi- 
ration. Among the struc- 
tural modifications which 
may influence the rate of 
transpiration are the num- 
ber and size of the leaves, 
the thickness of the leaves, 
the compactness of the 
The small figure on the right shows a piece 
: ; chlorenchyma, th - 
of a branch with whorls of scalelike leaves : Cay es the num 
at the nodes ber and arrangement of 
the stomata, the thickness 
and cutinization of the epidermal walls, and the development 
of trichomes (outgrowths from the epidermal cells). 
Number and size of leaves. Other things being equal, the 
amount of water transpired by a plant will tend to vary with 
the number and size of its leaves, as the more surface the plant 
has for transpiration the more water it can transpire. It is not | 
surprising, therefore, to find that most plants growing in moist 
regions have a large number of fair-sized leaves. When the 
leaves are few in number they are usually large. On the other 

Fic. 48. Branches of Casuarina with scale- 
like leaves. (x 4) 
