632 Hunter .— The Aerating System of Vida Faba. 
case of Vida Faba : it will be seen that the cortical air-cavities of the stem 
would form an excellent means of supplying air to the parenchymatous 
cells in the cortex of the old root These cells are a considerable distance 
from the nearest root-hairs, and the lysigenic cavities in the oldest inter¬ 
nodes of the stem may be a device for bringing about a sufficient gaseous 
exchange in this region. 
The intercellular spaces in the ground tissue of the stem are very 
varied in shape. In young ground tissue they are all triangular in transverse 
section, but in the mature ground tissue this is not the case. Owing to the 
fact that the cells vary a great deal in size, the original triangular inter¬ 
cellular spaces give place to more complicated shapes (Fig. 5). These 
result from the irregular development of the parenchymatous cells, and the 
subsequent fusing of two or more intercellular spaces. The intercellular 
space system of the ground tissue of the stem is brought into communi¬ 
cation with the external air by means of 
the stem stomata. The stomatal cavities 
are small and quite distinct from those of 
the leaf. In addition to the cavities and 
intercellular spaces of the stem which have 
been described, there are present curious 
cavities one layer of cells below the epi¬ 
dermis. These take the form of long narrow 
splits, elongated in a plane parallel to the 
epidermis, and are similar to those which 
are present in the stem of Lamium. 
The palisade cells of the leaf of Vida 
Faba are not arranged regularly. Each 
palisade cell borders upon an intercellular space. These intercellular 
spaces are irregular in size—some are small and triangular in a vertical 
section, whilst others are considerably larger than the cells which they 
separate. The intercellular spaces of the spongy mesophyll are large and 
irregular. In a transverse section of the petiole, on the other hand, the 
intercellular spaces are small and triangular. 
A longitudinal section of the root-tip shows that the intercellular 
spaces are just as important here as at the stem-apex (see Fig. 6, at the 
bottom). Intercellular spaces are not very marked in the root-cap, but 
they are present very extensively in the youngest tissues of the root. 
Probably the air which they contain is obtained from that portion of the 
root which bears the root hairs. In transverse section these intercellular 
spaces appear triangular in form. With reference to the older roots, it is 
worthy of notice that splits often take place in the cortex. How far these 
are the result of the boring operations of the lateral roots, and to what 
extent they are due to active growth, has not been determined, but either 
Fig. 5. Ground tissue showing 
various forms of intercellular spaces 
resulting from the fusion of two or 
more spaces. 
