OF VEGETABLE TISSUE. 17 



of cellular vegetables, with the exception of 

 certain mosses, in which recent observers have 

 detected them, and are rarely found in seeds. 

 These stomata are distributed at nearly equal 

 distances ; their principal use appears to be that 

 of effecting the aqueous transpiration, a view of 

 their office which is strongly confirmed by the 

 facts that they are very abundant in those plants 

 with membranous leaves which transpire freely, 

 and wanting in those which transpire little ; and 

 that they are closed during darkness, when no 

 transpiration takes place, and open in sunshine, 

 when it is most copious. It is probable that in 

 addition to these visible stomata, the superficies 

 of plants may be studded with other pores, too 

 small to be detected by the highest powers of 

 the microscope, and whose existence is only sus- 

 pected in consequence of physiological pheno- 

 mena for instance, if a portion of a plant, known 

 to be devoid of visible stomata, is exposed to the 

 air, it gradually loses weight ; and consequently 

 the liquid it contained must have found some 

 exit. 



8. Spongioles are certain exterior portions of 



vegetable tissue, which, without offering under 



the microscope any appearance of a peculiar 



organization, have a very strong disposition to 



c 



