CHAPTER XXIII 



RESPONSE OF EPITHELIUM AND GLANDS. 



Epidermal, epithelial, and secreting membranes in plant tissues Natural 

 resting-current from epidermal to epithelial or secretory surfaces Current of 

 response from epithelial or secretory to epidermal surfaces Response of 

 Dillenia Response of water-melon Response of foot of snail The so- 

 called current of rest from glandular surface really due to injury 

 Misinterpretation arising from response by so-called ' positive variation ' 

 Natural current in intact foot of snail, and its variation on section 

 Response of intact human armpit Response of intact human lip Lingual 

 response in man Reversal of normal response under sub-minimal or super- 

 maximal stimulation Differential excitations of two surfaces under different 

 intensities of stimulus, with consequent changes in direction of responsive 

 currents, diagrammatically represented in characteristic curves Records ex- 

 hibiting responsive reversals. 



HAVING now seen how the responsive peculiarities of the 

 epidermis may be elucidated by the responses of similar 

 tissues in the plant, we shall next take up an inquiry as to 

 the parallelism between the responses of epithelium and 

 glands in animal and in vegetable tissues. And here, as in 

 the last case, we shall find the obscurities of the one made 

 clear by the study of the other. 



If we take the hollow peduncle of a Uriclis lily, and, 

 cutting this into longitudinal halves, take a portion from the 

 upper end of one, we shall observe noticeable differences 

 between the investing membranes of the outer and inner 

 surfaces. On the outer, as we have seen elsewhere, the cells 

 are dry, thick-walled, and cuticularised. This surface then 

 is naturally distinguished as epidermal. The internal mem- 

 brane of the hollow tube, however, is very thin, and its cells 

 very little differentiated (fig. 192). The internal membrane 

 may thus be distinguished as epithelial. 



If now we examine this inner membrane continuously 



