Terrestrial Form of Zygnema ( Zygogonium ) ericetorum . 145 
occurs to a slight extent when the threads are exposed to drought, the 
smaller surface area of the protoplasm must lead to a still closer crowding 
of the peripheral layer of fat-globules. If we could imagine that this 
contraction brought about a coalescence of the individual fat-bodies, a con- 
tinuous layer of fat would be formed on the surface of the protoplast and 
this would act as a kind of macintosh to the latter, tending to prevent 
evaporation from the cell. But such a coalescence is unlikely on physical 
grounds, and moreover, if it occurred, it would be difficult to understand 
how the peripheral layer came to consist of numerous minute separate 
globules the moment the dry filament was placed in water. For the 
present, therefore, the function of this peripheral fat-layer must remain 
unsolved, although its extreme development, especially in the cells of the 
dry threads, will dispose one to associate it in one way or another with 
the great power of resistance to drought possessed by the form under 
discussion. 
This power is displayed not only by the instantaneous recovery of the 
filaments, when placed in water, even after many months of desiccation, but 
also by the fact that there does not appear to be any appreciable difference 
in osmotic pressure between the cells of dry filaments and of those growing 
under moist conditions. Material collected at Hindhead was kept for five 
months in part dry, in part moist. At the end of this time it was found 
that there was no difference between the two sets as regards the number of 
living cells in the threads. 
According to Lagerheim (’ 95 , p. 24) the cells of the filaments of 
Z. ericetorum , on being subjected to desiccation, become filled with reserve 
substances and thicken their walls, whilst the phycoporphyrin disappears ; 
as a consequence the resting-cells are almost colourless. In the case of the 
Hindhead form, although it has been collected at many different times of 
the year, such akinetes have never been observed ; in fact, the cells of the 
resting threads are coloured just as deep a purple as those of the active 
filaments. It is plain therefore that the state of affairs described by 
Lagerheim is not an essential consequence of drying up. 
On the other hand, a modification of the ordinary response to desicca- 
tion has been observed in material of the Hindhead form collected in the 
early part of the year, and here a certain reduction in the amount of pig- 
ment may possibly take place. In this case, the contents of most of the 
cells had divided unequally into a large akinete and a small cell, apparently 
provided with relatively little protoplasm, but filled with deep purple sap ; 
the pigment-cell formed a kind of cap over the end of the adjacent akinete 
and was cut off from it by a delicate wall (Fig. 2, a). The pigment-cells 
were either produced on the same side over a considerable length of filament 
or no such regularity was apparent (Fig. 2 , a) (cf. West and Starkey, T 5 , 
p. 199, Fig. 2, c). As a general rule the akinetes were more or less pointed 
L 
