100 MORPHOLOGY OF THE CELL. 



nected throughout the plant, and communicate so directly with 

 the stomata, that they constitute an apparatus for bringing the 

 interior of the structure into close relations with the outer air. 

 Sometimes the aggregate volume of the air-spaces is very large 

 in proportion to the volume occupied by the cells themselves. 1 



In composition, the air within the plant usualty differs from 

 that of the atmosphere in containing a larger proportion of 

 nitrogen. If the air-spaces are much smaller than the cells 

 which surround them, they are termed interstices ; if about as 

 large as the colls, lacunae ; if conspicuously larger, air-passages 

 or air-chambers. Two chief forms of lacunae are distinguished 

 by de Bary ; namely, cavities surrounded by cells which are 

 more or less branched, and those surrounded by plates of cells. 

 Good examples of the former are afforded by many water-plants, 

 rushes and the like ; of the latter, by the stems of man3 T Araceae, 

 for instance, Acorus Calamus. 



301. The continuity of the larger air-passages may be inter- 

 rupted by plates crossing at an angle (generally slightly oblique). 

 Such dividing plates, termed diaphragms, are frequently com- 

 plicated in their structure. 



302. Hairs, sometimes much branched, are found in the larger 

 air-passages of many plants. These form the stellate structures 

 in the N3 - mpha?acea?, and the " H-like" cells in some Araceae. 



303. Intercellular spaces, usually those of small size, may 

 contain water together with air. This is the case in the cavities 

 under the water-pores of Fuchsia, etc. 



304. When intercellular spaces contain resins, oils, and the 

 like, they constitute, together with the simple cells described in 

 295, the structures loosely called internal glands. Often these 

 are merely irregular spaces left by the breaking down of one or 



1 The following measurements are taken from Unger (Sitzungsb. d. Wiener 

 Akad., xii. 373). 



