33 



The Stomatograph. 

 By W. Lawrence Balls, M.A., Fellow of St. John's College, Cambridge, 

 Membre de l'lnstitut Egyptien, Botanist to the Egyptian Government 

 Department of Agriculture. 



(Communicated by Francis Darwin, F.R.S. Received December 22, 1911, — - 

 Read February 22, 1912.) 



Introduction. 



The appliance here described was the result of an attempt to devise a self- 

 recording modification of Mr. Francis Darwin's " Porometer."* 



This modification was designated the " Stomatograph," since it auto- 

 matically records the cbanges which take place in the mean stomatal 

 aperture of a given leaf, by measuring the velocity at which air under 

 constant pressure can be passed through the tissues. Certain special 

 mechanical difficulties had to be overcome in order to employ it for 

 researches on stomatal movement in the open field, with particular regard 

 to the cotton plant in Egypt. 



The following article gives a brief account of the evolution of the 

 appliance ; a description of a " stomatograph " as made by the writer, with 

 specimens of records actually obtained from it ; a short discussion of the 

 sources of error, indicating the modifications which I have suggested with 

 the aim of making it into a precise and sensitive air-pump. 



§ 1. Development of the Appliance. 

 The alarming deterioration of the Egyptian cotton crop, produced mainlyf 

 by unsuitable soil-water conditions, made it imperative to obtain some 

 precise knowledge of the relationship of the plant to water. The stomata 

 being of pre-eminent importance^ in this respect, an investigation of their 

 behaviour in the field crop appeared to be the first line of attack. Such an 

 investigation was beyond the powers of our laboratory§ unless self-recording 

 apparatus could be charged with the task. There was no existing appliance 

 for the purpose, except the indirect method by means of leaf temperature,[| 

 and this was quite unsuitable for field work, owing to the much greater 



* Darwin, F., and Pertz, D. F. M., ' Eoy. Soc. Proc.,' 1911. 

 t 'Eeport of Egyptian Government Cotton Commission,' 1910. 

 X See, however, Lloyd, F. E., ' Caruegie Inst.,' 1908. 

 § Department of Agriculture, P.W.M., Cairo. 

 || Darwin, F., 'Bot. Gaz.,' 1904. 



VOL. LXXXV. — B. 



