Fritsch . — The Moisture Relation of Terrestrial Algae. I. 15 
hand, Zygnema and the Hormidium stage evidently part with moisture 
more readily, although the thick mucilaginous walls in the case of the 
former will oppose evaporation, and the process may be expected to stop 
sooner than in the Hormidium , in spite of the usually larger quantity of 
sap present. We thus arrive at an explanation of the relative rates of 
drying of these different algae noted on p. 4. Moreover, seeing that 
Pleurococcus exhibits little contraction on drying, it is not to be expected 
that it will have any great absorptive capacity for moisture, which should 
be much less than in the case of the Hormidium stage or the Zygnema 
(cf. Table II). 
The results above obtained also indicate the probability that the cells 
of these terrestrial algae still hold a larger or smaller amount of moisture 
in the air-dry condition. This problem can be attacked by determining 
the loss of moisture when such air-dry material is subjected to prolonged 
heating at temperatures sufficiently high to kill the protoplasts and to 
drive off the uncombined water. 
E. The Moisture-content of the Air-dry Alga. 
A considerable number of determinations of this kind were made with 
the Hormidium-si^go. of Prasiola , of which abundant material was available. 
In all cases the pieces of alga-mat were allowed to reach an air-dry 
condition in the laboratory, and were then subjected to a temperature 
slightly below ioo° C. for several hours until a constant weight was attained. 
After the dry weight had thus been determined the material was allowed 
to stand exposed to the air of the laboratory for several days in order to 
establish what percentage of the moisture lost could be reabsorbed from the 
atmosphere. Some of these results are given in Table VI. 
Table VI. 
Determination of dry weight and of amount of moisture permanently lost 
after heating in the case of the Hormidium stage of Prasiola crispa. 
I. 
II. 
III. 
IV. 
v. 
vi. 
Air- dry 
weight 
of alga . 
Dry weight 
of same . 
Moisture 
lost . 
Ditto in 
% of dry 
weight . 
Weight 
after 
exposure 
to air . 
Permanent 
loss . 
Ditto in 
%of 
total 
loss . 
grin. 
grm. 
gnn. 
gnn. 
grm. 
1.096 
1-009 
0-087 
8.6 
1.066 
0-030 
34-5 
1. 170 
1-076 
0-094 
8.7 
1-140 
0-030 
3 i *9 
1.896 
1.746 
0.150 
8-6 
1.825 
0.071 
47*3 
3.616 
3.420 
0.196 
5-7 
3*520 
0.096 
49.0 
2-479 
2.234 
0.245 
10.9 
2.422 
0-057 
23*3 
2 - 3^5 
2.190 
0.195 
8.9 
2 - 33 ^ 
0.049 
25.1 
The alga in all cases loses a considerable amount of moisture on 
heating, usually between 8 and 9 per cent, of the dry weight. On subse- 
quent exposure to air a part of this moisture is regained, but a quarter to 
