262 PLANT-LIFE 



bear its precious nutritive burden to the protoplasts, so 

 that their demands may be met, and their chemical 

 activities may be successful. It is well Ifnown that the 

 aerial part of a plant gives off a good deal of water 

 vapour. We can show this by covering a growing plant 

 with a bell-glass, which will become clouded by the 

 condensation of the water vapour. In some instances 

 water is actually . exuded, but the great bulk of water 

 given off by plants passes out as water vapour. The 

 water which is given off by a plant is replaced from the 

 soil by means of the absorptive root-hairs. Evapora- 

 tion and replacement naturally bring about circulation, 

 and the process is denominated transpiration. The 

 vapour escapes through stomata, and perhaps to a very 

 minor extent through the cuticle. In typical land 

 plants transpiration through stomata is very pronounced, 

 and cuticular transpiration is negligible, yet the latter 

 takes place to a marked extent in plants which flourish 

 in damp places. Seeing that stomata are exits for 

 water vapour, it is not remarkable that in normal land 

 plants they are exceedingly numerous on the under 

 sides of their leaves. In such a situation they cannot 

 be inundated with rain-water, and so be prevented from 

 playing their important part in transpiration. The 

 openings of the stomata are regulated by " guard cells," 

 which close the passages when evaporation is calculated 

 to deplete the plant of its due meed of water, and open 

 them when transpiration is essential. These guard cells 

 perform this important duty quite automatically. 

 When water is plentiful, they swell with turgidity, 

 which so changes their size and form that the stomatal 

 pores are opened to allow of the escape of water vapour; 



