
ECOLOGICAL CLASSIFICATION OF PLANTS al 
surface, and further, the leaf may be rolled in so that the 
stomata are doubly protected, for they are on the side of the 
leaf which is inrolled (p. 74). - : 
In all these ways transpiration is lessened, but the means of 
protection just described against too rapid transpiration may 
be insufficient. Some xerophytic plants have adapted them- 
selves to the dryness of the situation by a reduction of their 
leaves, in order to lessen the transpiring surface. This is very 
noticeable in seashore plants; thus Saltwort (Salsola) has 
narrow, succulent leaves ending in thorns. Some seashore 
plants have practically no leaves at all—eg. Glasswort 
_(Salicornia). Plants in which the transpiring surface is very 
much reduced have to make up for the assimilating surface 
lost. by the reduction of the leaf. In some. cases, as in the 
evergreen Butcher's Broom, the true leaves are reduced to 
mere scales, whilst short lateral branches, very much re- 
sembling leaves, are produced. These present their edge to 
the light, and transpire less than a leaf of the same size with 
its full surface exposed to the sun would do. Some Xero- 
phytes, in which the leaves are very much reduced or. absent, 
as in Glasswort, have special assimilating tissue in the 
stem (p. 62). As a provision against the irregular supply of 
water many Xerophytes have a water-storage tissue, which 
accounts for the succulency of the leaf. 
In connection with the irregular supply of water, it should 
be noted that xerophytic plants usually have long tap-roots, or 
long underground stems. The surface of the soil is the part 
most affected by extremes of temperature, or dryness of the 
atmosphere, so that the longer the root is, the more likely it 
is to come in contact with water, which may exist some depth 
below the surface. Probably it is the presence of water at 
some depth below the surface that leads to the growth in length 
of the root, for roots, especially at their tips, are peculiarly 
sensitive to the presence of moisture. 
The thorniness of Xerophytes is very striking, and is 
probably a means of protection against grazing animals. In 
Cactus, for instance, the leaves are reduced to spines. 
