Piercy .* — The Structure and Mode of Life of a 
and refractive masses, described below, than those beneath the surface of 
the stratum. 
Under the varying influence of wetness and dryness, cold and heat, the 
Alga preserves its general features. It thrives at Woodford during the 
winter, when, as already described, the soil is continuously damp. Experi- 
ments have, moreover, shown that many cells can resist considerable 
desiccation during cold weather. The Hormidium is rarely subjected, in its 
native habitat, to abundant rainfall simultaneously with high summer tem- 
perature, but the combination of such conditions experimentally produced 
in a greenhouse tends to destroy it. The filaments, on the other hand, can 
resist a remarkably long period of desiccation during warm weather, though 
the resistance of individual cells varies considerably. The intense insolation 
which occurs in midsummer may, however, cause the death of a large 
FlG. 2. A, false plate of cells formed by cohering filaments of Hormidium after drought. 
B, cell of a narrower form of Hormidium Jlaccidum infected by a parasite, c, shows escape of 
the parasitic organisms from the Hormidium cell, p., parasite ; bl., enveloping vesicle, (a x 750 ; 
B x 1,200 ; C x 1,500.) 
proportion of cells even after a few weeks, the algal layer then assuming 
a general dark greyish colour. 
Dry filaments retain the same form as wet ones, but become narrower 
to the extent of about 2 1 u. On access of water turgor is instantaneously 
restored and the cell-cavities widen ; if desiccation has been prolonged, there 
is frequently a visible swelling of the longitudinal wall ; 1 this seems to 
indicate a partially mucilaginous consistency. Neighbouring filaments, 
which usually cohere as plates or cords when dry (Fig. 2, A), move apart, 
and any existing curvature in a filament increases, unless it be due to hinging 
about a dead cell, in which case it decreases. 
A close examination of filaments which have been subjected to desicca- 
tion shows that the latter produces definite changes in them. Some of the 
septa tend to become thickened so as to assume a biconcave form , 2 as described 
1 This was established by mounting the dry filaments first in alcohol and adding water subse- 
quently. 
2 A considerable amount of dew may be necessary during the period of drought to cause this 
development, as it is not produced by rigid desiccation. 
