718 Fritsch and Haines .— The Moisture-relations 
possibility of imbibition phenomena being also concerned. In the case of 
Prasiola , Pleurococcus , and Cystococcus such an explanation would appear to 
be the only feasible one to account for the absence of plasmolysis in a con¬ 
siderable number of the cells in solutions of a strength of 20 per cent, and 
upwards. 
(c) Observations with Dark-ground Illumination. 
For these investigations the illuminant used was an electric arc taking 
from fifteen to twenty-five amperes, in conjunction with a Watson paraboloid 
condenser. Fresh material of Spirogyra was always examined first with the 
object of obtaining an idea of the condition of the normal algal protoplast. 
This shows complete milkiness and vigorous Brownian movement through¬ 
out the cytoplasm. 
When fresh material of Zygogonium is examined, vibratory Brownian 
movement of minute particles in the protoplasm is distinct in all except the 
dead cells. Non-granular cells are seen to have but a thin cytoplasmic 
lining beneath the cell-wall, with a wide vacuole between it and the central 
chloroplast, whilst in granular cells there is a much wider cytoplasmic 
lining and a smaller vacuole. 
Examination of the drought material of Zygogonium from Experiment 
XII showed a general appearance of milkiness in the granular cells, but no 
Brownian movement was recognizable. Pififraction lines were observed 
around the granules, but were definitely limited to them, and the milky 
appearance of the protoplast throughout demonstrated that no coagulation 
had taken place. In none of the granular cells of drought material could 
a vacuole be recognized, an observation which was confirmed on other 
occasions using other material. Similarly in the case of Zygogonium that 
had been for two months in a solution of 5 per cent, sea-salt, the protoplasts 
of the healthy cells, though mostly contracted (cf. p. 706), were milky 
throughout, showed no Brownian movement, nor could vacuoles ever be 
distinguished. After shorter periods of exposure to drought or hypertonic 
solutions some of the cells still show Brownian movement. 
In the healthy cells of fresh Hormidium -material observation with dark- 
ground illumination again shows general milkiness of the protoplast, vigorous 
Brownian movement of the minute particles present, and a distinct vacuole. 
In the drought material from Experiment XIV and others, the milkiness 
was less marked, no Brownian movement could be recognized, and no vacuole 
was discerned. The cells showed progressive degrees of decreasing milki¬ 
ness, but in no case were they as milky as in the fresh material ; the slight 
diffraction discernible again proved to be due to the granules rather than to 
coagulation of the protoplast. The healthy cells of filaments that had been 
immersed in 5 per cent, sea-salt for two months presented just the same 
