BOTANY OF THB LIVING PLANT 



, • w iU , a movement oi water from the small 



the right into the bent an ! ' lar y tube - 



\ n luced into this tube by raising its end out of 



the beak moments. The number of graduations 



luently t r.i I by the bubbl( tin time gives an in- 



rption, and since normally absorption is 



approodmati [ual to transpiration, we have here an indirect 



metho transpiration. 



With the potometer it can be shown that exposure of the shoot 



current oi air from an electric fan increases transpiration. 



1 xposure of the shoot to specially 



B 



dried air also increases transpira- 

 tion, while exposure to a very 

 moist atmosphere decreases it. 

 These results we should expect 

 from experience of ordinary 

 evaporation; but the further 

 observation that transpiration 

 at night is very much less than 

 in the daytime would not have 

 been entirely expected. This 

 ilt may be in part due to 

 differences of temperature : but 

 it is also referable to the influ- 

 =a ence of the stomata. Some in- 

 70. formation concerning these struc- 



Stocna v A - in the tiirp«; was aiven in Chanter V 



oper 150.) 1 on rures was given in ^napter v., 



where it was pointed out that 



the stomata] guard-cells are adjustable, so that the pore can 



her open or closed. Generally speaking the stomata open 



in light and close in darkness, the difference being due to a 



or prevailing in the guard-cells in the presence of light 



a in When turgor is high the guard-cells tend to 



nd the pore opens : but when turgor falls the cells 



tend 1 traight position, and the pore closes (Fig. 70). 



re due to a'terations in the amount of osmo- 



in the sap of the guard-cells. These, unlike 



other epidermal cells, regularly contain chloroplasts, and in the 



light irs will presumably be produced by photo- 



: in saying this we anticipate information to be given in 



tpter YIII. It 1- now believed that a production of sugars 



