22 THE STORY OF PLANT LIFE 



(b) Water. 



Water is the factor which divides plants into land- 

 plants and water-plants.^ Over and above this it 

 plays one of the most important parts in the vital 

 activities of both classes of plants, enabling them to 

 carry out the processes of absorption, transpiration, 

 and in part of assimilation. Mineral salts are con- 

 veyed in solution to the plant by water. The cell- 

 sap, in which the solutes are carried, once they have 

 entered the root-hairs, is largely w^ater. The propor- 

 tion of water in a plant is large. This is necessary 

 to maintain the plant in a state of turgescence. The 

 water current ascends and the food with it. It is 

 given off in transpiration as vapour, or in small drops 

 by hydathodes, as where the air is humid in the 

 tropics or wet bogs. 



The soil water, its amount, availability, and its 

 characteristics have been described under soil {ante). 

 In the case of land-plants the dry soil types or xero- 

 phytes are most dependent on the adjustment 

 between the soil and its water capacity. 



In order that a proper balance may be established 

 between the amount of water taken in and that given 

 off, or used, by the plant, absorption and transpiration 

 must be compensatory. In the case of xerophytes 

 absorption may be diminished, if the soil is cold, for 



1 Plants are divisible into land plants, including Hygrophytes, in 

 which case the conditions are moist, xerophytes, where they are dry, 

 mesophytes, where they are intermediate, and aquatic, where the 

 plant lives submerged, or floating on the water. 



