ANATOMY OF LEAVES. 311 



gum-arabic and a small proportion of tragacanth ; 

 and when this was dry, each twig was placed under a 

 cylindrical jar containing air, and immersed in a sau- 

 cer of water. In a short time the sides of the jar 

 containing the twig in its natural state, were covered 

 with drops of water ; but, at the end of two days, not 

 the smallest quantity of moisture appeared on the sides 

 of the jar containing the twig, the apertures of the 

 leaves of which had been obstructed by the mucilage. 

 The conclusion therefore is, from this experiment, 

 that that surface only on which apertures exist ex- 

 hales, and consequently that these apertures are the 

 exhaling organs. 



All animals that require the presence of air for their 

 existence, have some peculiar apparatus for producing 

 that change in the blood which has been termed its 

 oxygenizemeut ; and the change is said to be the re- 

 sult of respiration, whether it be performed by ungs 

 or by spiracula. Plants, also, require the presence of 

 air; vitiating it, under certain circumstances, in the 

 same manner as animals, but, under others, increasing 

 the proportion of its oxygen : hence plants may prop- 

 er'v be said to respire, and the question arises, by 

 what organs is this function performed . ? Phytologists 

 have generally agreed, that the leaves are the lungs 

 of plants, but in what part of it are the respiratory or- 

 gans situated ? The foliar apertures appear to be the 

 actual breathing organs of the plant. In support of 

 which position it may be advanced, that these aper- 

 tures are never seen on leaves that are not expos- 

 ed to the air ; for the leaves of submersed aquatics 

 are devoid of them ; even the leaves of plants which 

 are not naturally aquatic, if kept submersed, soon lose 

 them ; and although some plants of the higher classes, 

 which mow in the air, nave no leaves, yet, these have 

 apertures on the stem, which, in such instances, per- 



