726 
Fritsch and Haines.—The Moisture-relations 
moribund really die, the few healthy surviving cells would probably serve 
as a sufficient basis for new growth when conditions again became favour¬ 
able. During the prolonged drought of 1921 many of the filamentous forms 
disappeared altogether (as far as macroscopic observation went) from their 
normal habitats. Yet, in the autumn or later, they appeared again in 
quantity in the old situations. 
Whatever the treatment may have been, the surviving cells, whether 
healthy or unhealthy, are characterized by the possession of a more or less 
rigid, highly viscous protoplast, absence of vacuoles, and in the algae 
usually abundant granular contents. Most of these features, distinctive 
only of drought material in other cases, are exhibited already by the fresh 
cells of Ptenrococcus and, to a less extent, of Prasiola. The very marked 
powers of resistance of the unicellular alga, a familiar fact of observation in 
nature and strikingly exemplified in our experiments, is no doubt to be 
related to the peculiar characteristics of its protoplasts. This form is, so to 
say, permanently in the drought condition. It is probable that Cystococcus 
altogether resembles it in these respects. 
The problem as to the mechanism of water retention, which we set out 
to investigate, can be regarded as only partially solved. It is plain that 
a high osmotic concentration of the sap, if it plays any role at all, is not 
a fundamental part of the mechanism. 1 The gel-condition of the protoplast, 
with absence of all large vacuoles, indicates the possibility of imbibition 
phenomena being principally concerned. That such a gel would retain 
a certain quantity of moisture on drying under any ordinary atmospheric 
conditions is undoubted, but it is open to question whether the amount 
would be as large as has been found to be the case. Since the protoplasts 
of Ptenrococcus are apparently always in this state, the behaviour of this 
form is of particular interest, for we know that its cells will resist drought for 
months without apparently losing any of their normal vital properties. This 
alga, however, exhibits a very small moisture-content (cf. Fritsch, 1922 , p. 16) 
and a low capacity for absorbing moisture when dry (loc. cit., p. 4). More¬ 
over, it shows a higher permanent loss of moisture after heating to ioo° C. 
than either Zygogonium or Prasiola. All these facts tend to show that the 
moisture is imbibed within the gel-like protoplast, that it is held tenaciously 
against dry air, and that it is only after heating that its imbibitional 
efficiency is destroyed. On the other hand, Prasiola , whose protoplast 
seems to be in much the same condition, does not appear to possess as 
efficient a mechanism. 
In all the other forms, where the gel-condition is only reached during 
drought, a majority of the cells at these times are probably in a far less 
healthy state than in Ptenrococcus and their imbibitional efficiency may 
1 1 he observations made since the writing of the article on terrestrial algae (in Journal of 
Ecology, vol. x, 1922) have led us to modify the view expressed on p. 230 of that paper. 
