302 ANATOMY OF LEAVES. 



croscopic examination of the cellular substance of 

 some leaves than of others ; thus, in a minute portion 

 taken from the leaf of Iris germanica (a common 

 garden species), we perceive that not only the cut 

 edges of the cells appear double ; but that where some 

 of the cells deviate from the hexagonal figure, there 

 are evident interstitial spaces between them, which, if 

 the cells were not distinct vesicles, would not occur. 



A question arises in consequence of the supposition 

 that each vesicle is a distinct sac : — in what manner 

 do the cells communicate with each other, and with 

 the vessels which they surround ? Malpighi main- 

 tained that a small tubular production issues from 

 each cell or vescicle, by which it communicates with 

 the contiguous cells, and with the vascular system of 

 the leaf. A similar idea was entertained, also, by 

 M. de Saussure. Mr. A. T. Thompson, however, 

 was unable to detect those communications. Even in 

 that peculiar modification of the cellular structure, 

 which is found immediately within the cutis of the in- 

 ferior disk of some leaves ; and in which the cells as- 

 sume the appearance of anastomosing tubes, none of 

 the tubular connecting processes, described by Mal- 

 pighi, are perceptible ; nor do these cells appear to 

 communicate directly with the vessels which they sur- 

 round. It may be asked then, in what manner do 

 the cells communicate ? To answer this question, 

 we ought to understand the structure of the intercel- 

 lular membrane. But here our instruments fail, if 

 they do not mislead us ; and, under glasses of the 

 highest power, this membrane appears different under 

 different circumstances ; by transmitted light, it seems 

 a simple, unorgauized, transparent pellicle ; but, by 

 reflected light, is evidently porous. As the cells of 

 the stem appear to communicate by pores, so may 

 also those of the leaf. An opinion has been advan 



