Frit sch . — The Moisture Relations of Terrestrial Algae . /. 5 
a mass of soaked freshwater alga, uncombined water makes up a far larger 
percentage of the total weight than it does in the case of any of the 
terrestrial algae studied. This implies that in proportion to the air-dry 
weight the terrestrial forms have a smaller water-absorbing capacity than 
the others. Attention may be drawn to the fact that the Hormidium stage 
of the Prasiola is able to take up most moisture among the terrestrial forms 
investigated. 
C. The Relation to Air Humidity. 
During August and September of 1919 a large number of weighings 
were made of algae, mosses, and lichens, growing on different substrata, 
with the object of ascertaining the extent to which their moisture-content 
varied with that of the air ; the materials were in all cases allowed to 
attain an air-dry condition before any weighings were undertaken. The 
method of experimentation proved unsuitable for detailed work, but a few 
of the general results are summarized in Table III in order to give some 
idea of the values involved. It will be noticed that there is in all cases 
Table III. 
Range of moisture-content of terrestrial Cryptogams and various substrata 
for period of experiment (4 -6 weeks), August-September, 1919. 
Nature of material. 
Dense covering of Pleurococcus 
on thin layer of soil 
Holly bark with covering of 
Pleu.rococcus 
Holly barfc alone 
Pinus bark with Parmelia phy- 
sodes 
Pinus bark alone 
Pinus bark with dense Pleuro- 
coccus 
Moss 1 
Powdered soil 
Pure stratum of Pleurococcus 
Approx. 
Minimum 
Maximum 
Range . 
area. 
weight. 
weight. 
sq. cm. 
grm. 
grm. 
grm. 
38*5 
5.686 
5 - 85 2 
0.166 
46.8 
4-753 
4.908 
o-i 55 
44-5 
1-574 
1-670 
0.096 
large 
6.118 
6.640 
0-522 
42.8 
6.040 
6 - 47 i 
o- 43 i 
35 - 1 
4 - 7 11 
5.082 
Q- 37 1 
large 
1.600 
1.822 
0.222 
7 8 '5 
18-141 
18.384 
0.243 
7 8 -5 
2.003 
2.167 
0.164 
a considerable range in moisture-content which is particularly striking in 
relation to the total weight in the case of the moss and Pleurococcus . The 
various substrata, however, also take up a considerable amount of moisture 
from the air, and there would appear to be differences in this respect between 
the bark of different trees. I hope to deal more fully with this aspect of 
the matter in a later communication. 
At the time at which these experiments werehindertaken, the maximum 
moisture-content of the air (usually 75-85 per cent, of saturation) was 
1 Hypnum cupressiforme , L., var .filiforme, Brid. 
