96 AIR CELLS INTERNAL HAIRS. [BOOK i. 



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 com- 

 bination of minute cells; 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 cells 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. Such as these are repre- 

 sented 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 whose air cells 

 are apparently not essential, they seem to be caused by the 

 growth of the outside of the stem being more rapid than the 

 formation of the central cells ; so that the central 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 cells forming the partitions 

 are sometimes open, so that a free communication is main- 

 tained between all the tiers of air cells. 



M. S. F. Hoffmann has ascertained that processes analogous 

 to the hairs of epidermis frequently exist in air passages. Al- 

 though he found them in various species of Limnanthemum, 

 he was unable to discover them in Villarsia. Yet in other orders 

 they would seem to be more constant, for among the Water- 

 lilies (Nymphgeacese), the genus Eury ale (ferox) exhibited the 

 same kind of dotted hairs as are found in the air-cavities of 

 the different organs of the genera Nymphsea and Nuphar. 



