Terrestrial Form of Zygnema ( Zygogonium ) ericetorum. 139 
placed in the laboratory under a screen formed by a thick pad of cotton- 
wool ; after three days a considerable number of the threads showed 
a similar shifting of the chloroplasts to the upper sides of the cells, so that 
this movement is evidently readily accomplished. 
The possibility of the pigment being related to the low temperature of 
the habitat was also considered by Lagerheim, but, in view of the abundant 
occurrence of Zygnema ericetorum in the Tropics, this explanation does not 
seem very plausible. On the other hand, as Bohlin (’ 01 , p. 51) suggests, its 
appearance may be connected with the extremes of temperature to which 
such a terrestrial Alga would be subjected. The phycoporphyrin may also 
be related in some way to the remarkable power of resistance to desiccation 
possessed by this Alga (cf. p. 145). 
B. The Cell- wall. 
The cell-wall of the Hindhead form is thick and stratified, these features 
being more pronounced than is usual in the terrestrial form. Two or three 
regions are generally distinguishable in the longitudinal walls. The inner- 
most layer is well defined, appearing in optical section as a bright line of 
varying thickness. Beyond this come successive strata, making up the greater 
part of the wall and forming what I will call the intermediate layer. These 
strata, which are not always easily recognized individually (although plainly 
seen in filaments placed in strong potash), appear progressively duller and 
duller the further out they lie. The manner of refraction of the light shows 
that the innermost layer is the densest, and that from there outwards the 
layers decrease in density, probably owing to gradual gelatinization (see 
below). In some cases the surface of the filament is quite smooth, but as a 
general rule the latter is bounded by a much interrupted dark line, composed 
of numerous adhering foreign particles (Fig. 1). There is often more or less 
obvious constriction between the cells. On treatment with dilute sulphuric 
acid, the surface and intermediate layers swell and become lost to view, 
whilst the inner layer remains distinct for some little time longer. Addition 
of iodine, subsequent to the acid, gives the blue coloration of cellulose 
(cf. West and Starkey, T 5 , p. 194). 
The same three layers are recognizable in the aquatic and ordinary 
terrestrial forms, but in both the intermediate layer is much thinner and, 
even in the latter, stratification is often unrecognizable without the applica- 
tion of special means. The surface is quite smooth. 
The Hindhead form, if collected during a period of drought, consists of 
a hard brittle mass having an almost horny consistency. If a small portion 
be placed in a drop of water, it instantaneously softens and becomes 
exceedingly pliable. When a larger piece is brought into contact with 
water, swelling of the Alga is macroscopically visible, and if the quantity of 
water be small, the bulk of it is immediately absorbed, far more rapidly 
