Fritsch. — The Moisture Relation of Terrestrial Algae. /. 17 
amount. Support is thus given to the conclusion that in the air-dry 
condition the protoplasts of Pleurococcus still retain a relatively large 
percentage of the moisture that this alga can absorb, although the total 
amount held is smaller than that observed in the other forms. This 
probably in great part accounts for the lack of any marked contraction of 
the cells of this alga on drying. 
Zygnema ericetorum and the Hormidium stage of Prasiola show about 
the same percentage loss of moisture on heating the air-dry material, the 
variation noted in the case of the latter (cf. also Table VI) possibly 
according with the varying degree of contraction exhibited by the threads 
(p. 11) on exposure to drought. But these two algae differ altogether in 
the percentage of moisture that is permanently lost after heating, the 
amount being much smaller in the case of Zygnema. This may be taken 
to mean that a smaller quantity of moisture is retained within the proto- 
plasts of this alga than in those of the Hormidium stage, which is again 
in agreement with the fact that those of Zygnema show a much more 
marked contraction on drying than do those of the other alga. The fact 
that, nevertheless, the total amount of moisture lost b y Zygnema on heating 
is about the same as that lost by the Hormidium stage is no doubt due to 
the high mucilage-content of the walls of the former. 
By contrast with these terrestrial algae the two aquatic forms studied 
give quite different results. In the first place, the quantity of moisture lost 
on heating is higher in proportion to the dry weight, which is probably 
due to the water-retentive power of the mucilage of the Spirogyra and the 
thick membranes of the Cladophora. In the second place, the percentage 
of permanent loss to total loss is low, even very low in Spirogyra , indicating 
that the air-dry alga contains only very small amounts of moisture that 
cannot subsequently be reabsorbed from the air. In this respect the 
difference as compared with the Hormidium stage and with Pleurococcus 
is very striking. It is noticeable that the amount of permanent loss is 
greater for Cladophora than for Spirogyra (the difference would perhaps 
be even more pronounced if Oedogonium had not been intermixed with the 
Cladophora ), and in this connexion the fact that a great part of the 
Cladophora was in the winter condition should be noted, since such threads 
(cf. above) exhibit less contraction on drying and presumably retain some 
moisture in their protoplasts. 
Comparison of cotton-wool with the algae investigated again shows 
a relatively low percentage of permanent loss, although the results are 
somewhat variable and not altogether clear The fact that there is never- 
theless an appreciable permanent loss in the case of the cotton-wool 
indicates some alteration of the fibres as a result of heating, whereby their 
absorptive capacities are reduced. The possibility of such alteration in 
the case of the algae experimented upon may not be overlooked, but the 
c 
