CHAP. I. 
AIR CELLS. 
33 
ner which is most suitable to the purpose for which they are 
destined. 
They diiFer 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 flViid 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 veiy 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 combin- 
ation 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. 
The inner surface of the air cells, when they are essential 
to the life of a plant, is smooth and uniform ; but in grasses, 
umbelliferous plants, and others where they 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 separated by 
partitions, the intercellular passages of the bladders forming 
the partitions are sometimes left open, so that a free commu- 
nication is maintained between all the tiers of air cells. 
D 
