100 



MOKl'HOLOGY OF THE CELL. 



nected thvougliout the plant, and communicate so directly with 

 the slomata, that the3- 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 verj' large 

 in proportion to the volume occupied b}- the ceils themselves.^ 



In composition, the air within the plant usually 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, thej' are termed interstices ; if about as 

 large as the cells, lacunw ; if conspicuously larger, air-passages 

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

 b}' de Bary ; namel3', cavities suri'ounded b}' cells which are 

 more or less branched, and those surrounded hy 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 man^' Araceae, 

 for instance, Acorus Calamus. 



301. The continuitj' of the larger air-passages maj- be inter- 

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

 Such dividing plates, termed diaphragms, are frequentl3- com- 

 plicated in their structure. 



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

 air-passages of nian^- plants. Tliese form the stellate structures 

 in the Nymphaeacese, and the " H-likc" cells in some Aracese. 



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). 



