io Fritsch . — The Moisture Relations of Terrestrial Algae . /. 
which is very difficult to settle directly. A number of facts, however, point 
to this being the case, amongst others those mentioned above (p. 9). 
When cells of Pleurococcus naegelii are mounted in water and examined 
under the microscope, they appear more or less rounded off (Fig. 2, a). 
The cell-wall is distinguishable into two regions, a denser outer portion and 
a less dense, presumably mucilaginous, inner portion which appears as 
a slight space separating the protoplast from the outer layer of the 
membrane (Fig. 2, a , and Chodat, 1902 , Fig. 195, 196). If such cells be 
allowed to dry gradually on the slide, they retain their normal appearance 
until air-films commence to arise around them. At this stage the cells 
Fig. 2. a-b , Pleurococcus naegelii. a, cells mounted in water; b, ‘dry"' cells, mounted 
in cedar-wood oil. c-g, Hormidium stage of Prasiola crispa. c, portion of a thread, on part of 
which the longitudinal striation is shown ; d, part of a thread with a ‘ concave cell <?, ‘dry’ thread 
(cell-contents shaded) ; f, ditto, mounted in cedar-wood oil; g, ‘dry thread’, showing dilation due 
to a pair of ‘ concave cells ’. (All figures x 800.) 
become angular in outline (Fig. 2, b), but there is no considerable con- 
traction, though the mucilage layer of the wall appears thinner in such dry 
cells. 1 No further change can be detected, even in cells unsupplied with 
liquid moisture for several days. There is no marked contraction of the 
protoplast, which appears to be as closely invested by the cell-wall as in 
the wet cell (cf. below). 2 
In the case of the Hormidium stage of Prasiola crispa the effects of 
drying are slightly different. Even in the wet condition the filaments 
of this alga exhibit a delicate longitudinal striation of the cell-walls which 
1 Examined in cedar-wood oil. 
2 The slight contraction of the cells of Pleurococcus on drying appears of advantage in con- 
nexion with the vertical habitat frequented by this alga. It is difficult to conceive how adhesion to 
the substratum could be maintained if the cells were subject to frequent violent contractions. 
