40 
ORGANOGRAPHY. 
BOOK I. 
cell is often constructed so exactly like its neighbour, that 
it is impossible to regard it as a mere accidental disten- 
sion of the tissue: on the contrary, air cells are, in those 
plants to the existence of which they are necessary, evidently 
formed upon a plan which is uniform in the species, and 
which has been wisely contrived by Providence in the man- 
ner best adapted to the purpose for which they are destined. 
They differ from receptacles of secretion in containing air 
only, and not the proper juice of the plant; a peculiarity 
which is provided for by a curious contrivance of Nature. 
In receptacles, the orifices of the intercellular passages through 
which the fluid that is to be deposited drains, are all open ; 
but, to prevent any discharge of fluid into the air cells, the 
orifices of all the intercellular passages that would otherwise 
open into them are closed up, except in the partitions that 
divide them from each other. 
Air cells are very variable in size, figure, and arrangement. 
In the stem of the Rush (Juncus articulatus), they consist of 
a number of tubular cavities placed one above the other, and 
separated by membranous partitions composed of a combina- 
tion of minute bladders ; in some aquatic plants they are very 
small, as in Butomus umbellatus. In form they are either 
cylindrical, or they assume the figure of the bladders by which 
they are formed, as in Limnocharis Plumieri (Plate III. 
fig. 1. and 2.), in which the structure of the air cells and 
their coats forms one of the most beautiful of microscopical 
objects. In the green parenchymatous parts of plants, such 
as the leaf, the cortical integument, &c., where they always 
abound, they are irregular spaces among the tissue, com- 
municating freely with each other. They are represented in 
Plate I. fig. 2. 
The inner surface of the air cells, when those parts are 
essential to the life of a plant, is smooth and uniform ; but 
in grasses, umbelliferous plants, and others where air cells 
are not essential, they seem to be caused by the growth of 
the stem being more rapid than the formation of the air cells ; 
so that the tissue is torn asunder into cavities of an irregular 
figure and surface. Kieser was the first to observe that in 
many plants in which the air cells of the stem are regularly 
