Terrestrial Form of Zygnema ( Zygogonium ) ericetorum. 143 
was produced from a mature cell with two chloroplasts, a formation of 
daughter-cells may take place almost immediately. If, on the other hand, 
the akinete was formed soon after a cell had divided, so that it contained 
but one chloroplast, division may be considerably delayed. The septa cut 
the akinetes into two approximately equal halves and, since the adjacent 
faces of the daughter-protoplasts are more or less flat, whilst their distal 
faces are often rounded off, the pairs of protoplasts produced from a single 
akinete, as the result of division, frequently recall the two halves of 
a Cosmarium - cell (Fig. 2, h). In this way the products of division of the 
individual akinetes of each filament are quite plainly distinguishable. 
The two daughter-cells may divide once more, this depending on how 
soon the first division sets in after the arrival of favourable conditions. 
Thus we may get groups of four cells (Fig. 2, H, 11) developed, before 
a fresh drought causes renewed rounding off and fresh akinete formation. 
I have never observed more than four cells thus produced from a single 
akinete, during the interval between one drought and another. 
The repeated formation of akinetes, involving each time the apposition 
of a new layer of membrane on the inner side of the cell-wall, leads to 
a gradual thickening of the latter, mainly noticeable in the transverse septa. 
In examining a given length of filament, strongly thickened septa are 
found at more or less remote intervals (Fig. 2, H, 1). Between these one 
meets with septa of varying thickness, some quite thin and recently formed, 
others already more or less thickened and of older date (Fig. 2, H, II, 
ill, iv). The region lying between two strongly thickened septa is the 
product of a single akinete of a remote generation ; the intervening cells 
can be grouped, on the same principle, into smaller and smaller sets formed 
from more and more recently produced akinetes. In this way, by a careful 
scrutiny, it is often possible to estimate roughly the number of periods of 
drought to which a given filament has been subjected (Fig. 2, H). The 
longitudinal walls do not appear to increase much in thickness beyond 
a certain stage, since the outermost layers gradually merge into mucilage 
(cf. above). 
In the cells of the filaments of the Hindhead form, numerous small 
globules, of irregular shape and varying size, are found distributed, more 
particularly in the peripheral part of the protoplast (Fig. 2, D). I have not 
observed such globules in the aquatic form, and in the terrestrial form from 
Wales they occur in much smaller numbers and are, on the average, much 
coarser (cf. also West and Starkey, T5, p. 197)* When threads of the 
Hindhead form are subjected to gradual desiccation, an exceedingly dense 
layer of small and rather uniform globules appears in immediate contact 
with, the inner layer of the cell-wall, whilst relatively few are found in the 
rest of the protoplast (Fig. 2, E, e'). The peripheral layer of globules is so 
dense and so closely apposed to the cell-wall that it looks like a second 
