630 Htmter .— The Aerating System of Vicia Faba. 
a different manner from the smaller cavities above it, and is clearly of 
lysigenic origin. Here, therefore, is an example of a number of schizogenic 
intercellular spaces enlarging into air-cavities which are converted into one 
lysigenic cavity. The transformation might be described in the terms 
employed by Frank, 1 as of protogenetic origin, because the cavities are 
really produced in the earliest differentiation of the tissues, and of subse¬ 
quent hysterogenetic development, since the final form appears only in the 
older mature stem. In the mature plant this central cavity extends for 
the greater part of the stem, but in the lower internodes it decreases in 
size, and ultimately disappears completely. This central air-cavity may 
be described as a very fine, tube, gradually tapering in the oldest internodes 
and finally disappearing completely, whilst in the region of the stem apex 
it branches into a number of still finer tubes. It occupies relatively the 
greatest portion of the area of the cross section of the stem of a young 
plant in the youngest internodes. 
In the region shown in Fig. 2, A 
and B, it occupies about 7 per cent, 
of the total area of the cross section 
of the stem. In that part of the 
stem of a young plant where it 
forms a fairly wide tube (Fig. 3), it 
occupies about 4 per cent, of the 
cross section area. In the lowest 
internodes it may be reduced to 
0-5 per cent., and ultimately dies 
out. In the full-grown plant the 
central cavity may occupy 50 per 
cent, of the cross section of the stem. 
The cells at, and near to, the growing point naturally require a large 
supply of oxygen to assist in the processes necessary for their growth 
and division. A longitudinal section of the growing point shows that 
intercellular spaces occur amongst the very youngest cells, and are only 
absent amongst those cells which have just been formed. It seems quite 
reasonable to suggest that the large relative area occupied by the cavities 
in the young internodes is a device to secure a sufficient aerating system 
for this active region, and that the division of the cavity will assist in 
the even distribution of the oxygen for respiration, and the removal of the 
resulting carbon-dioxide. 
In the lowest and oldest internodes no central cavity is present, but an 
interrupted ring of lysigenic cavities, occupying about 2 per cent, of the 
area of a transverse section, occurs (Fig. 4). It is rather difficult to 
understand what is the function of this arrangement of cavities. In the 
1 Frank : Beitr. zur Pflanzenphysiologie, p. ioi. 
Fig. 3. Transverse section of pith from some¬ 
what older internode than that of Fig. 2. Central 
cavity developed, x 100. 
