2 Fritsch . — The Moisture Relations of Terrestrial Algae . 1. 
ensue in a very short space of time. There are abundant data on the 
power of these forms to resist prolonged and intense desiccation (cf. Bristol, 
1919 ; Gay, 1891 ; Petersen, 1915 ; Piercy, 1917 ; Schroeder, 1886 ; West 
and Starkey, 1915). 
A peculiar feature of most, if not of all, of these algae is the presence 
in many, though by no means in all, of the cells of large numbers of highly 
refractive granules of diverse shape and size (cf. Fritsch, 1916, p. 143 ; 
Piercy, 1917); these are apparently fatty in character, though their actual 
chemical nature is scarcely yet established. The more liable the habitat 
to drought, the more marked as a general rule the granular character of the 
cells-— at least in the forms enumerated below which have been more 
particularly studied. Moreover, it seems that, as a spell of drought 
continues, the percentage of granular cells increases up to a certain point 
which is apparently the result of the dying away of a considerable number 
of the non-granular cells (Piercy, 1917, p. 532). In view of these facts it 
can hardly be doubted that the development of these granules is part of the 
drought-resisting equipment, but the nature of the mechanism is altogether 
obscure at present. 
The frequently observed astonishing power of these algae to withstand 
prolonged desiccation practically unharmed leads one to inquire (a) as to 
possible means of obtaining moisture during periods of drought, (b) as to 
the moisture-content of the alga during such periods. The experiments 
and observations detailed below in relation to these points were partly 
conducted in the open at my home in Surrey, and partly in the laboratory 
and greenhouse at East London College. The material employed consisted 
of : Pleurococcus naegelii , Chod., forming a practically pure, dense growth 
on some wooden palings just outside Dorking; the Hormidium (i. e. fila- 
mentous) stage of Prasiola crispa , which fofms extensive mats on the soil 
beneath Plane trees at East London College ; and Zygnema ericetorum 
(the ordinary terrestrial form) from Hindhead Common and the Redlands 
Woods in Surrey. A few observations were also made on other forms. 
In all the experiments the material was sheltered both from direct 
sunlight and atmospheric precipitations. For observing gain and loss of 
moisture weighings were made at definite intervals ; no attempt was made 
to obtain greater accuracy than the nearest milligram, since this was 
more than adequate for the purpose of the experiments. The materials 
were exposed partly in open glass Petri dishes, partly in glazed porcelain 
evaporating dishes, neither of which exhibit any appreciable condensation 
of moisture, even on a damp day. The moisture-content of the air was 
determined by means of a self-recording hair hygrometer, and such 
temperature readings as were required were furnished by a thermograph. 
