Hunter .— The Aerating System of Vicia Faba. 629 
of continuous intercellular spaces which are capable of bringing about a most 
active exchange of gases. The gaseous exchange between the living 
contents of the cells and the air in the intercellular spaces can only take 
place through the cellulose cell-walls when they are in a moist condition. 1 
It has been observed that bands of some material less refringent than, air 
occur in these canal-like intercellular spaces. It seems highly probable 
that these are masses of water, which assist in the active exchange of gases 
by keeping the cell-walls in the necessary moist condition. Similar masses 
have been noted in the intercellular spaces of the cortex in longitudinal 
sections of the root of Lupinus . 
The arrangement of the large air-cavities which are present in the stem 
of Vicia Faba is very instructive. 
3 . 
Fig. 2, A and b. Transverse and longitudinal sections of pith of young internodes, showing 
air-cavities (diagonally shaded) separated by sheets of active cells, x ioo. 
In the youngest internodes the centre of the pith is traversed by strands 
of active cells separated by well-marked air-cavities, as is shown in 
Fig. 2, A and B. The structure of the pith in this region is comparable 
with the arrangement of the ground-tissue in the stems of many aquatic 
plants. These cavities in the youngest internodes of Vicia Faba gradually 
become smaller as the growing point is approached, until they are reduced 
to mere chinks in the meristematic cells of the embryonic tissues. It is 
evident that the cavities are of schizogenic origin, as they result from the 
unequal development of the plerome in a radial direction. Lower down 
the stem these strands of active cells disappear, and one large central 
cavity replaces the much divided cavity of the younger internodes—see 
Fig. 3. This cavity is produced by the disorganization of the separating 
strands of cells. The remains of the cell-walls of these strands are to be 
found along the edges of the cavity. This cavity is therefore produced in 
1 Livingston : The Role of Osmotic Pressure and Diffusion in Plants, p. 116. 
