42 



And that the exception, where it exists, i. e. where the stomata are 

 closed, proves the existence of the overlying membrane discovered 

 and described by Dr. Brown. 



This last fact, indeed, is very satisfactorily established. The mem- 

 brane alluded to, though in some cases wholly absent, yet in others it 

 slightly projects beyond and along the margin of the apertures, it is 

 sometimes partially extended quite across, and again, as a delicate 

 tympanum, it wholly overlies the stomata. 



I venture to submit these few observations, under the impression 

 that a successful investigation of the functions of elementary organs 

 must be preceded by an accurate knowledge of their structure. 



Stone Vicarage, 



Aylesbury, May 25, 1841. 



Postscript to The Rev. J. B. Readers paper " On the process of char- 

 ring Vegetable Tissue, as applied to the Stomata in the Epi- 

 dermis of Garden Rhubarb." By the Rev. J. B. Reade. 



Some ingenious experiments by Dr. Williams, communicated to the 

 Microscopical Society last August, appear to lead to the conclusion 

 that the process of charring vegetable tissue is of very doubtful effi- 

 cacy in determining delicate structure. With respect to the stomata 

 in question, he is led to infer that their overlying membrane, though 

 having, under the application of this process, every appearance of a 

 continuation of the epidermis, is really nothing more than an inspis- 

 sation of gelatinous mucus, raised by the heat into contact w r ith the 

 glass, and, by the mere accident of pressure, extending as a carbon- 

 ized pellicle from one edge of the aperture to the other. 



That much caution is necessary in the application of red heat to a 

 delicate and almost imperceptible tissue must be conceded at once, 

 and it is certainly far from unreasonable to call in question the results 

 of such agency. In the present case, however, a very few words of 

 explanation will convince those who are interested in this enquiry, 

 that the process of charring not only enables us to say, in every in- 

 stance, whether a given stoma is open or closed ; but, with reference 

 to the point in dispute, it assists us in detecting the structure which 

 determines the character of the membrane. 



The charred specimen which illustrates these few remarks was first 

 of all immersed in alcohol, distilled water, and dilute hydrochloric 



