Fritsck . — The Moisture Relations of Terrestrial Algae. /. 9 
humidity of the air than does the soil (cf. especially the afternoon of each 
day). The alga also absorbs in general a greater percentage of the possible 
amount of moisture when the air is damp and parts with it more rapidly 
as the humidity of the air falls (Table V). Moreover, the rate of loss in 
the morning falls off more rapidly in the case of the alga than it does in 
the case of the soil. There is some evidence, therefore, to indicate that the 
alga constitutes a transpiring mechanism which in dry air rapidly parts with 
moisture, if it has been previously absorbed in quantity, until an osmotic 
check comes into play due to increasing concentration of the sap. It is no 
doubt the concentrated sap that is in great part responsible for the rapid 
uptake of moisture when the humidity of the air again increases. 
Similar results were obtained by comparing a mass of air-dry Clado- 
phora with soil. It seems that, if different kinds of materials are compared 
in this way, there is great correspondence in the variations in moisture- 
content for small differences in atmospheric humidity, but that with larger 
differences the correspondence is not so marked. 
In the experiments referred to in this section, as already stated, the 
materials were placed under shelter. In a more exposed position, especially 
if reached at certain hours of the day by the sun, the diurnal range would 
of course be greater and the absorption of moisture from dew on many 
a night appreciable. 
D. The Condition during Drought. 
The data given in the preceding section are in large part a natural 
outcome of the hygroscopic character of the materials employed, but I may 
be pardoned for dealing with them in some detail, since in relation to these 
lowly terrestrial plants they may be of considerable significance. If a mass 
of Pleurococcus , weighing 3*003 grm. and with a superficial area of about 
78*5 square centimetres, can, under the conditions of the experiment, 
increase in weight by 0*164 grm. (cf. Table III), i. e. by an amount rather 
over 8 per cent., this must mean the intake of a considerable quantity of 
atmospheric moisture. This, being retained for a number of hours at least, 
may be sufficient to admit of a certain amount of growth. It is to be 
noticed that the maximum moisture-content of the air and of the alga was 
realized at about 8 a.m. summer-time (p. 6), i. e. some hours after daybreak, 
so that a certain amount of photosynthesis would be possible, if the 
quantity of moisture present were adequate. It is not unlikely, moreover, 
that the above-mentioned granules constitute a form of food-reserve (cf. 
Piercy, 1917, p. 533), laid up during periods of active metabolism, to be 
used for growth overnight at times of high moisture-content during 
subsequent dry periods. 
The question arises whether moisture adsorbed on the surface and 
imbibed into the cell-wall is able to obtain entry into the protoplast, a point 
