14 Fritsch. — -The Moisture Relation of Terrestrial Algae. /. 
Neither in the cells of Plenrococcns naegelii nor in those of the threads 
of the Hormidium stage of Prasiola crispa are large vacuoles distinguish- 
able. For the latter alga this fact has already been established by 
Imhauser (1889, p. 5), whilst none of the published figures of Pleurococcus 
show such vacuoles (cf. Chodat, 1902, pp. 279-83 ; Gay, 1891, PI. XIV, 
Figs. 136, 137; Nageli, 1849, Tab. IV, E, Fig. 2; Snow, 1899, PI. XI). 
Their absence is also evidenced by the behaviour of these two algae when 
immersed in strong solutions of sea-salt. Thus the protoplast of Pleurococcus 
exhibits no evident contraction when the cells are mounted in a 10 percent, 
solution of Tidman’s sea-salt, so that the amount of water which such 
a solution (with an osmotic pressure of some seventy-five atmospheres) can 
withdraw from the protoplast is negligible. There are two possible 
explanations : either the quantity of moisture present in the cells is very 
small or the sap which is dispersed through the cytoplasm has so high 
a concentration as to exceed that of the strong solution employed. 
The cells of the Hormidium stage are markedly plasmolysed by a 
10 per cent., very slightly by a 5 per cent., solution of sea-salt ; a 4 per cent, 
solution (osmotic pressure about fchirty atmospheres) has no effect. It is 
noticeable that the protoplasts of most of the cells do not show any very 
considerable contraction, even in the strongest solution employed. It 
appears therefore that, by contrast with Pleurococcus , the cells contain 
more moisture and the sap is not as concentrated, but the amount is small 
as compared with a freshwater alga. 
In the case of Zygnema ericetorum some of the cells show obvious 
vacuoles, whilst in others the cytoplasm fills practically the whole cavity 1 
(West and Starkey, 1915 , cf. Fig. 1 and 2 with Fig. 4). Immersion in 
a 10 per cent, solution of sea-salt causes pronounced plasmolysis of all the 
cells, whilst a 3 per cent, solution only produces slight plasmolysis and is 
evidently not much stronger than the cell-sap. It is noticeable that the 
degree of plasmolysis varies greatly in different cells and different filaments, 2 
which accords with the varying amount of contraction noted on drying. 
It will be realized that these facts agree well with the observed 
behaviour of the different algae during drying. As regards the influence 
of the different strengths of plasmolysing solution used, the three algae can 
be placed in the order : Pleurococcus , Prasiola , Zygnema. The same order 
expresses the relative degree of contraction during drying. If no appreciable 
amount of liquid can be abstracted from the . cells of Pleurococcus by a 
10 per cent, sea-salt solution, then it is hardly likely that there will be any 
sensible loss of moisture from them on exposure to dry air. On the other 
1 In the extreme terrestrial form found at Hindhead (Fritsch, 1916) vacuoles were never observed 
in the cells. 
2 It is probable that Hormidium flaccidttm would show similar relations. Accurate determina- 
tions of osmotic pressures were felt to be unnecessary in the present connexion, the more so as it is 
proposed to deal with this aspect of the matter in a subsequent communication. 
