Frit sch. — The Moisture Relations of Terrestrial Algae . /. 13 
Cladophoraceae on certain South African marshes which are dry land for 
about half the year. 
The usually close contact between the membrane and the protoplast 
in the cells of the above-discussed terrestrial algae during drought renders 
it highly probable that atmospheric moisture imbibed into the cell-wall 
will pass readily into the adjacent protoplast. And it is scarcely likely that 
the latter will not profit by any atmospheric moisture thus absorbed, even if 
the amount is not nearly sufficient to distend the cell to its normal ‘ wet ’ 
dimensions. Even should a very slight space exist between the protoplast 
and the shrunken membrane, the fact that the inner parts of the latter are 
mucilaginous will involve their swelling and the filling out of this space as 
soon as any appreciable quantity of moisture has been absorbed. The 
least well-equipped form as regards maintenance of contact between wall 
and protoplast is the Zygnema, which, however, has thicker mucilaginous 
walls than the other two algae. 
In this connexion attention may be drawn to the fact that liquid water 
is absorbed rapidly by the dry mats both of Zygnema ericetorzim (Fritsch, 
1916, p. 139 ) and of the Hormidium stage of Prasiola. In the former case 
this is due to the abundant mucilage, in the latter the longitudinal folds 
probably act as capillary channels for the absorption of liquid water. In 
this way, in the case of such soil-inhabiting algae, a certain amount of 
moisture may possibly also be absorbed with the help of threads penetrating 
into the substratum. Masses of dry Pleiirococcus , on the other hand, are 
not easily wetted, the difficulty lying in the displacement of the air-films 
around the cell-groups. In view of the slight contraction exhibited by the 
protoplasts of this alga in drying, however, very small amounts of moisture 
are probably adequate to start the full vital machinery. 
It is thus rendered probable that moisture imbibed from the air by the 
cell-walls of these algae reaches the protoplast, and that at times when the 
air remains near saturation point for long periods sufficient moisture may 
thus be absorbed to admit of a certain amount of growth taking place. 
Attempts to establish the occurrence of such growth by direct measurement 
of material kept in a saturated atmosphere have, however, so far proved 
quite inconclusive. 
During prolonged drought, with a low percentage of air humidity, the 
protoplast will part with moisture to the adjacent membrane and the latter 
will part with it to the air until the sap in the cells of the alga reaches 
such a concentration that the imbibition forces in the membrane are 
balanced by the osmotic suction of the sap. When this condition of 
equilibrium is reached the algal material will neither gain nor lose moisture 
and will exhibit a constant average weight. That this stage is attained 
soon after drying sets in has been shown in Section B, and it remains now 
to consider the condition of the protoplast at this time. 
