WATER. 101 



(1) chemical processes within the cells ; (2) the imbibition 

 of water by the protoplasm and cell-walls ; and (3) the 

 evaporation of a portion of the water. All these are in 

 almost constant operation ; and, therefore, there is a per- 

 petual movement of water in the growing plant. The 

 Chemical processes include the actual consumption of 

 water by breaking the molecule up into oxygen and hydro- 

 gen, the formation of substances which are more soluble 

 than those from which they were formed, and the formation 

 of others less soluble than the substances from which they 

 were formed. The protoplasm has imbibing power in a 

 marked degree. It imbibes more water than it can retain, 

 the surplus being separated in drops, the so-called vacuoles. 

 The protoplasm of rapidly growing tissue is more watery 

 than that in a dormant state, as in seeds. The cells are 

 kept turgid, and succulent parts made rigid. 



128. The evaporation (called also exhalation and 

 transpiration) of water from the surface of plants is a 

 more potent agency disturbing the equilibrium than the 

 two previously mentioned. No evaporation, however, can 

 take place when the air is saturated with ipoisture. The 

 further below the point of saturation the amount of moist- 

 ure falls, the greater is the amount of evaporaition. The 

 epidermis offers considerable hindrance to the process ; and 

 the thicker and less impermeable they are, the less the 

 amount of water which escapes. In such case the escape 

 is mainly through the stomates, which may be considered 

 special organs of exhalation (and breathing). They are 

 placed over intercellular spaces (Fig. 212), and these are 

 connected with intercellular passages, which, of course, 

 are filled with moist air and gases. Stomates open more 

 widely the greater the amount of light, and the greater 



