STOMATA. 135 



Epidermis treated in this way may be preserved in 

 absolute alcohol indetiniteiy, or it may be stained in a solu- 

 tion of congo-red in absolute alcohol, cleared In clove oil 

 and mounted in balsam. If, however, water is allowed to get 

 access to the stomata at any time, the cell walls will become 

 soft and the guard cells will act as if plasmolysed. The 

 stomata will then close and the preparation be valueless for 

 the purpose it was intended. 



With this method of observing the stomata available, 

 enabling one to measure and count under the microscope as 

 many as necessary at one's leisure, it remained to get hold of 

 a method of measuring rapidly and with sufficient accuracy 

 the rate of transpiration from the plant, the stomata of 

 which were to be examined by the above outlined method. 

 Such a method was had by a modification of the glass tube 

 potometer, in which was used an ordinary glass tube of, say 

 5 mm. bore, or better a chemical burette to which 'was con- 

 nected, by a piece of rubber tubing, a branch of the plant 

 studied, which was the ocotillo {Fouquierui splendens) . 

 Careful manipulation made it possible to observe the rate 

 of transpiration after the first hour or two, for a period 

 of 24 hours or, in some cases, for two or three days. A 

 whole experiment when set up and running consists simply 

 of the piece of the plant, a glass tube placed parallel to it 

 and tied, and the lower ends of both properly connected 

 with a short piece of rubber tubing, the whole being sus- 

 pended where desired. It will be seen that such a simple 

 device makes it possible to have several exactly similar ex- 

 periments going on at the same time, and, in such case, it 

 will be seen too, that each will act as a control on the 

 others. If one of them acts very much differently from the 

 •rest, it may be discarded. Uniformity of operation in a 

 number of experiments is a good guarantee that they are 

 all working well and normally. To be sure several objec- 

 tions may be raised to the use of the potometer as a physio- 

 logical instrument, but in this connection it can be made to 



