THE MOLECULAR FORCES IN PLANTS. 173 



the stomata in a short time close under the eye of the ob- 

 server.* 



It is very instructive to determine experimentally that the 

 stomata are open and form free outlets from the intercellular 

 spaces. We may first make the following experiments, using an 

 uninjured leaf of Primula sinensis. The apparatus depicted in 

 Fig. 61 is employed. The glass G, fitted with a two-holed rubber 

 stopper, is half full of water. One limb of the tube JR, which is 

 bent at right angles, passes just through the stopper. The leaf- 

 stalk P dips into the water. If we rarefy the air in the appar- 

 atus by sucking with the mouth, fresh air passes into the stomata 

 of the leaf and emerges in a stream of bubbles at the cut surface 

 of the leaf-stalk situated below the surface of the water. If we 

 experiment with other leaves, we shall very often find that the re- 

 duction of pressure effected by mere mouth suction is not sufficient 

 to cause an escape of air from the cut surface of the leaf-stalk. 

 We must then connect the end of the tube R with an air-pump 

 in order to get better exhaustion. 



We can, however, show conversely that air forced into the leaf- 

 stalk passes into the intercellular spaces, and escapes from the 

 stomata. I also used leaves of Primula sinensis for experiments 

 of this kind. If we dip the blade of the leaf under water, and then, 

 taking the leaf-stalk into the mouth, blow vigorously into it, bub- 

 bles of various sizes will be seen to detach themselves from the 

 leaf. There may not be, as we might perhaps expect, a fine stream 

 of bubbles from each storna, but the air forced out collects to form 

 larger bubbles, which then here and there rise from the surface 

 of the leaf. If the blade of the leaf is left for some time under 

 water, it becomes impossible to drive air through it by mere blow- 

 ing. The stomata, in fact, gradually become blocked up with 

 water retained by capillarity, and this state of affairs may be very 

 rapidly brought about in the leaf of Primula sinensis by applying 

 the lips to the cut end of the leaf-stalk, and exerting suction 

 while the blade is under water. The stomata and the inter- 

 cellular spaces are thus injected with water, and the blade con- 

 sequently changes in colour, becoming also translucent. We are 

 now unable to drive air through the leaf by vigorous blowing. 



* This closure of the stomata is due to the death of the guard cells under the 

 influence of the current. The stomata, therefore, do not subsequently open 

 again, and the contents of the guard cells often rapidly break up. 



