136 THE PHYSIOLOGY OF STOMATA. 



GENERAL SUMMARY. 



The material chiefly studied for the purpose of the foregoing paper was 

 derived from two distinct types of desert plants, Fouquieria splendens and 

 Verbena ciliata. The former is possessed of a water-storage tissue, while the 

 latter has no obvious adaptive character related to desert conditions. Both 

 plants have leaves of rather unspecialized character, especially as regard 

 the stomatal apparatus, which conforms to a widely generalized type. There 

 are no pits, plugs, or other contrivances which might be regarded as having 

 an accessory modification on the rate of water-vapor diffusion through the 

 stomatal pore. The problem of the regulation of transpiration by the stomata 

 has therefore not been complicated by such "adaptations," but is confined 

 wholly to the possible degree of correlation of the transpiration rate and the 

 movements of the stomata which result in the change in the size of the pores. 

 It has also occurred that the stomata of Verbena ciliata have proved of especial 

 value in the study of the internal economy of the guard-cell, and the experi- 

 ments carried out on this plant have, it is believed, thrown light on the cause 

 and regulation of stomatal movement. The investigation has then been 

 directed along these two lines of thought, namely, the alleged regulation of 

 water loss by the stomata and the physiological causes of stomatal movement. 



Methods of probable wide application, to which, however, exceptions have 

 been noted, have been devised for the more accurate estimation of the phys- 

 ical condition of the stomata at any given moment, by direct examination 

 of these organs, not in situ upon the plants. By means of absolute alcohol 

 the walls of the guard-cells are rendered rigid by the rapid extraction of the 

 imbibed water. With the exercise of due precaution the stomata may then 

 be made into permanent preparations if that is desirable. For the quan- 

 titative determination of changes in transpiration rate in successive periods 

 of time, potometers, in a modified form and arranged in batteries so as to 

 become mutual controls, have been used. By weighing, the probable error 

 for the absorption of water by the tissues, or its loss by wilting, may be 

 discovered. 



TRANSPIRATION AND STOMATAL MOVEMENT. 



On the theory of stomatal regulation of water loss by the plant we should 

 expect a close correlation between the daily periodicities of transpiration 

 and of stomatal movement. The rate of transpiration, however, continues to 

 increase for a considerable period of time after the maximum stomatal opening 

 has been reached. The diminution in the rate of transpiration during the 

 latter part of the day may be interrupted by intervals of increase, without 



