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 (Jiydaihodes ; 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 



FIG. 4. VERTICAL SECTION OF PART OF A LEAF CUT THROUGH A STOMA. 

 L. (HIGHLY MAGNIFIED.) 



a, stoma ; 6, 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 a liquid 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 



