INFLUENCE OF WATER ON PLANT-LIFE 27 
and close the opening. This controlled or regulated diffusion 
of water-vapour through the stomata is called trans- 
piration. 
In a few cases water is actually expelled in liquid drops 
from the leaves. These emergency - exits are special 
openings (hydathodes ; Gr. hydatos, water ; hodos, way, 
channel), usually in the form of large stomata, always 
open. 
Plants which grow in an atmosphere constantly moist, 
such as exists in a tropical rain-forest, and to a lesser 
degree in damp lowland valleys in this country, must get 
Fic. 4.—VERTICAL SECTION OF Part or A LEAF CUT THROUGH A STOMA. 
(HicHuy MacGniriep.) 
a, stoma ; b, guard-cells ; c, epidermis ; d, cuticle ; e, air-cavity ; /, chloro- 
phyll-tissue. 
rid of their surplus water in this way, for the amount 
transpired is very small. The young leaves of most 
rapidly - growing herbaceous perennials also possess 
hydathodes. In these cases large quantities of water 
must be absorbed in order to supply adequate nourish- 
ment, but, at the same time, the surface through which 
water can be transpired is small. The only method by 
which the whole of the excess can be got rid of is by excret- 
ing it in aliquid form. The small white spots on the teeth 
of such leaves indicate the position of these hydathodes, 
and if they are examined in the early morning of a moist 
spring day, drops of water can be seen hanging from 
