CHAP. I. 
ELEMENTARY ORGANS. 
251 
perly so called, are concerned. Whether or not ducts have a 
different function is uncertain ; it is probable, however, from 
the extreme thinness of their sides, that they are really filled 
with fluid when full grown, whatever may have been the case 
when they were first generated. 
In regard to the functions of air-cells and lacunae, it may 
be sufficient to remark, that in all cases in which they form a 
part of the vital system, as in water plants, they are cavities 
regularly built up of cellular tissue, and uniform in figure in 
the same species ; while, on the other hand, where they are 
not essential to vitality, as in the pith of the walnut, the rice- 
paper plant, the stems of Umbelliferae, and the like, they are 
ragged, irregular distensions of the tissue. 
In the former case they are intended to enable plants, to 
float in water ; in the latter, they are caused by the growth of 
one part more rapidly than another. 
