10 Researclies respecting the 



v/hich I can ascribe only to an ambiguity occasioned bv the 

 meaning of the word, since the pores of Hedwig; are only 

 the cortical glands of Saussure, with which the illustrious 

 author of that physiology seems to have been very well 

 acquainted. 



Mirbcl calls these pores exterior pores, in contra-distinc- 

 tion to those with which the interior cells are perforated*: 

 *' Each pore f," savs he, " is an oblong fissure surrounded 

 by an cliiptical area at which the neighbouring cells termi- 

 nate. In these pores I can see only cells having a parti- 

 cular disposition, the exterior side of which is rent in a 

 longitudinal dnection. Wlien the vegetable is exposed to 

 tile air and to light, circumstances necessary for transpira- 

 tion, the fluids proceed in abundance towards the surface, 

 the cells become elongated to receive them, and pierced to 

 afford them a passage. Hence the formation of the cortical 

 pores : but this elongation of some cells is, in a certain 

 measure, only accidental ; it is not invariably determined by 

 the organic plan of the vegetable." 



The diversity of opinions which prevails among the au- 

 thors above mentioned induced me to study these origans 

 with great attention, that I might endeavour to discover 

 their real structure, and in this manner facilitate the ex- 

 planation of their uses. 



I have found that the surfaces of almost all leaves are 

 perforated with a great number of small apertures, to which 

 I shall retain the name oi' pores. ' 



The size of these pores varies very much, according to 

 the plants. In the orchis and lily kind they are very large, 

 while in the sow -bread, the jessamine, the oak. See. they are 

 very small. 



The number of them varies also according to the leaves. 

 Hedwig counted 577 in a square line of the liliiun hull'i- 

 fenim. I counted 1 10 in the same extent of a leaf of tlie 

 f'ritillaria, and 150 in that of the aloe; but in leaves where 

 they are very small I was not able to count them very ex- 

 actly, because they are too numerous. 



In most plants the pores have an oval form, the direction 

 of which seems to be subordinate to the form of the exterior 

 utriculse. Thus in the lily kind, the utriculje of which are 

 elongated, the large diameter of the pore proceeds in the 

 direction of the length of the utriculse, fig. 1 and 3 ; and 

 in the leaves, the utriculse of which are festooned, the ports 

 •extend in every direction, fig. 14. This rule, however, is 



» Jouma' df Physique, Fnictidor. an. 9, p. 2:1. 

 ' f ILid. Praiiial, an. 9. p. 444 — 448. 



not 



