DIY. ii PHYSIOLOGY 231 



organs which react in a wonderfully purposive fashion. Opening 

 is caused by illumination and by a certain degree of humidity of 

 the air; on the other hand, darkness or dry air effect a closing 

 of the pore. 



The movements of the guard cells are movements of irritability 

 and are brought about by changes in turgescence. As a consequence 

 of the peculiar thickening of the elastic cell walls of the guard cells 

 (p. 52), an increase of the turgor pressure intensifies the curvature 

 of the cells and a diminution of turgor lessens the curvature. The 

 former change leads to the opening of the pore and the latter to its 

 being closed, as will be evident from Fig. 240 without further descrip- 

 tion (cf. also Figs. 47-49). 



The stomata are mainly present on the leaves, which are thus to 

 be regarded as organs of transpiration (and of assimilation, p. 249). 



FIG. 240. Storua of Hdleborus sp. in transverse section. The darker lines show the shape assumed 

 by the guard cells when the stoma is open, the lighter lines when the stoma is closed. (After 

 SCHWENDENER.) The cavities of the guard cells with the stoma closed are shaded, and are 

 distinctly smaller than when the stoma is open. 



The amount of water evaporated from the leaf surfaces is surprising ( 14 ). 

 For instance, a strong Sunflower plant, of about the height of a 

 man, evaporates in a warm day over a litre of water. It has been 

 estimated that an acre of Cabbage plants will give off two million 

 litres of water in four months, and an acre of Hops three to four 

 millions. For a Birch tree with about 200,000 leaves and standing 

 perfectly free, YON HOHNEL estimated that 300-400 litres of water 

 would be lost by evaporation on a hot dry day ; on an average the 

 amount would be 60-70 litres. A hectare of Beech wood gives off on 

 the average about 20,000 litres daily. It has been calculated that 

 during the period of vegetation the Beech requires 75 litres and the 

 Pine only 7 litres for every 100 grammes of leaf substance. For every 

 gramme of dry, solid matter produced, 250-900 grammes of water 

 are evaporated on the average. 



It is evident from these and similar experiments that more water is evaporated 

 in a given time from some plants than from others. These variations are due to 



