CHAP. I. 



AIR CELLS. 33 



ner which is most suitable to the piu-pose 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 combin- 

 ation of minute bladders ; in some aquatic plants they are very 

 small, as in Sutomus um-bellatus. 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 innei- surface of the air cells, when they are essential 

 to the life of a plant, is smooth and unifo)*m ; 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. 



