Terrestrial Form of Zygnema (Zygogoniu m) ericetorum. 141 
a gradual thickening of the intermediate layer pushing in the inner layer 
at this point. The invagination slowly progresses towards the centre 
(Fig. 1, C, D), thus gradually constricting the protoplast in the manner 
customary among the simpler Algae. It does not seem, however, that 
a complete septum is at once, formed, the two daughter-cells remaining for 
some time connected by a more or less narrow pore (Fig. 1, D, h), which 
probably ultimately closes. In optical section the continuity of the two 
protoplasts is often readily recognized. The septum at its first initiation is 
not a uniform structure, but exhibits three layers meeting in a point at the 
inner edge of the annular invagination. Two of these layers, forming the 
limbs of a V, are continuous with the innermost layer of the longitudinal 
wall, whilst the third lies between them and appears to be similar in nature 
to the intermediate layer (cf. especially Fig. 1, H). 
In many of the filaments of the Hindhead form one observes pairs of 
daughter-cells in which the one protoplast is prominently beaked towards 
that of the other cell, even when seen in surface view (Fig. 1, G; Fig. 2, G). 
This phenomenon is a result of the frequent asymmetry of the ingrowing 
septum. The latter, in optical section, either appears symmetrical and 
V-shaped (Fig. 1, B, c) or its two edges are asymmetrical (as in Fig. 1, D, G), 
the one being generally placed at right angles to, the other at an obtuse 
angle to, the longitudinal wall. It has not been possible to find any under- 
lying reason for the development of these asymmetrical septa, which are 
very commonly observed. 
C. The Reaction to Drought on the Part of the 
Hindhead Form (Fig. 2). 
The appearance of a mature filament of the Hindhead form is a definite 
expression of the alternating periods of drought and rainfall which follow 
one another in the habitat. On the arrival of a dry period the protoplast, 
probably shrinking slightly and mostly rounding off to a more or less oval 
or spherical shape, according to the previous form of the cell, generally 
excretes a new layer of membrane on its outer .surface and contiguous to 
the inner layer above described (p. 139). The latter often seems at the same 
time to lose in definition and gradually to become merged in the inter- 
mediate layer, but this does not necessarily occur ; particularly in thick- 
walled filaments the former inner layer may persist for some time as 
a definite stratum in the intermediate layer. 
The septa also generally lose their sharp definition and, as the new 
membrane develops on either side, the substance composing them becomes 
gradually indistinguishable from the intermediate layer of the longitudinal 
walls, which now therefore extends uniformly all round the cell (Fig. 1, A ; 
Fig. 2, F). Except for the development of the new layer of membrane and 
