1 8 Fritsch . — The Moisture Relation of Terrestrial Algae. I. 
results obtained with freshwater algae, where the membranes are pre- 
sumably of much the same nature as in the terrestrial forms, indicate that 
here no great change of this kind can obtain. The appreciable permanent 
loss shown by the clay soil is probably the result of alteration of the 
imbibitional powers of some, of the soil constituents. 
Examination of cells of Pleurococcus which have been subjected to 
such prolonged heating shows that the protoplasts in all of the cells have 
contracted away from the wall, presenting a very different appearance to 
that of the air-dry alga. The degree of contraction, however, varies 
considerably, and appears in general to be least in ceils with plenty ot 
granules. This observation supports the view that a considerable amount 
of moisture has been given up from the protoplasts. In the case of the 
heated threads of the Hormidium stage contraction of the protoplasts is 
likewise observed, but it is scarcely as pronounced as in Pleurococcus , 
implying a smaller moisture-content in the air-dry condition. In both 
cases, however, the protoplasts still fill a large part of the cell-cavity and 
are not as strongly contracted as in most plant-cells after heating. The 
cells in the heated threads of the Zygnema do not differ much in appearance 
from the air-dry ones, although contraction of the protoplasts is still more 
pronounced. 
F. Summary and General Conclusions. 
The following are the principal conclusions that emerge from the 
foregoing considerations : 
3. The protoplasts of the various terrestrial algae examined exhibit 
either a complete absence (. Pleurococcus , Hormidium stage of Prasiola :), or 
a paucity ( Zygnema ericetorum ), of large vacuoles, most of the sap apparently 
being dispersed through the cytoplasm. 
2. In the air-dry condition a considerable proportion of this sap is 
retained, more in the case of Hormidium and Pleurococcus than in the case 
of Zygnema. 
3. When drying takes place contraction occurs in such a way that the 
cell- wall remains closely investing the protoplast (. Pleurococcus , Hormidium 
stage) or in contact with it at certain points {Zygnema). 
4. Moisture imbibed into the walls from the atmosphere will therefore 
reach the protoplast, especially in the case of Pleurococcus and the Hormidium 
stage of Prasiola. 
5. The cells of Pleurococcus exhibit no appreciable amount of con- 
traction on drying, whilst in the Hormidium stage of Prasiola there is 
evident contraction accompanied by longitudinal folding of the walls along 
special lines of weakness. In Zygnema the amount of contraction is variable. 
The capacity for absorbing moisture is least in Pleurococcus and greatest 
