Terrestrial Form of Zygnema ( Zygogonium ) ericetorum. 147 
these special pigment-cells, being not uncommon at all times of the year 
(cf. West and Starkey, ’ 15 , p. 198). The cells in question generally appear 
deeply pigmented at first and, as indicated in Fig. 3 , B, constitute points at 
which the filaments readily give way. In many cases rupture is delayed 
until the contents have completely disappeared. Apart from such isolated 
cells, it occasionally happens that large numbers of the cells of a thread die 
away, leaving healthy akinetes only here and there, separated by more or 
less extensive stretches of dead filament. The akinetes in such threads are 
generally well rounded and provided with a very defined layer of the peri- 
pheral fat-globules. No doubt they ultimately become free and give rise 
to new threads. 
Mention may be made at this point of a peculiar form of akinete, 
rarely observed in the material from Wales (Fig. 3, a). In this case the 
greater part of the cell had swollen up very considerably, this distension 
however not affecting the last quarter of the cell, so that the resulting shape 
rather recalled the oogonia of some Oedogoniums. The wall was rather 
strongly thickened and stratified, and the cell-contents, though appearing 
perfectly healthy, were so dense as to make it impossible to decipher any 
details of structure. Nothing corresponding to these swollen cells has been 
observed in the remaining material. Apart from such elements, no akinete 
formation was observed in the material from Wales. 
D. Comparison of the Hindi-iead Form with the Aquatic and 
Ordinary Terrestrial Types. 
Apart from the chloroplasts which have already been fully considered 
(p. 136), the Hindhead Alga differs from the ordinary terrestrial form of 
Zygnema ericetorum in the length of the cells, the degree of thickening 
of the walls, and the abundance of the fat-globules. The following table 
shows the relative dimensions of the Hindhead form, the form from Wales, 
and the aquatic one : 
Length of cell 
Width of cell 
Thickness of longitudinal walls 
Terrestrial form , 
Hindhead. 
26-5 fx (24-30) 1 
25-5 e (24-27) 
5 r ( 4 - 6 ) 
Aquatic form , 
Frensham. 
4 ° r (34-4^) 
19 \x (16-21) 
i*8 r (1*5-2) 
Terrestrial form 
Wales. 
31.1 fx (21-48) 
24-2 [x (23-26) 
zr ( 2 - 3 - 5 ) 
It should be mentioned here that the form from Wales is perhaps not 
quite typical of the true terrestrial Alga, since it occurred in the immediate 
neighbourhood of numerous small streams which are practically never dry. 
As contrasted with the conditions obtaining at Hindhead, such a form 
would be very advantageously situated. It can scarcely be doubted that 
the extreme shortness of the cells of the Hindhead form is a result of the 
brief periods during which growth is alone possible. 
1 The figures in round brackets give the extreme dimensions. 
L 3 
