of Terrestrial Algae. II. 
691 
C. The Behaviour when exposed to Drought. 
An appreciable alteration in the behaviour towards plasmolysing solu¬ 
tions, in most of the forms investigated, is to be observed during a spell of 
drought. Using the same strength of sea-salt solution, the number of 
plasmolysed cells and the extent of plasmolysis in the affected cells 
gradually diminish day by day till ultimately very little plasmolysis is 
to be observed. In the experiments in question, a large number of which 
were performed, the initial material was usually obtained during damp 
weather and a strength of salt solution selected sufficiently concentrated 
to bring about marked plasmolysis in a large majority of the cells. The 
material was then allowed to dry exposed to the air of the laboratory 
in the earlier experiments, but in the later ones was dried in a desiccator 
over suitable strengths of sulphuric acid. Except in a few cases, in which 
a constant temperature was maintained, the experiments were exposed to 
ordinary room temperature. Relatively small quantities of material were 
employed, all collected under the same conditions, on the same day, and 
from the same habitat—usually not more than could be placed in an 
ordinary watch-glass. The material was left on the patch of soil as 
collected. The experiments were almost always started on the same day or 
on the day after collection. As far as possible daily estimations were made 
of the drying material until it appeared that no further appreciable change 
was taking place. 
The results of a number of these experiments are epitomized in Table V. 
They are essentially uniform as regards the three filamentous algae, and fall 
far outside the limits of our experimental error. Only in the case of 
Experiment VI with Hormidium flaccidum was there any appreciable 
percentage of strongly plasmolysed cells at the end of the period of 
drought, and even in this case the very considerable reduction as compared 
with the original material is quite obvious. 1 It will be noticed that, whilst 
in all these cases there is a great increase in the percentage of unaffected 
cells after drought, there is no such uniformity as regards the slightly plas¬ 
molysed ones, which sometimes show a decided decrease and sometimes an 
increase. We are inclined to think that this depends partly on the condition 
of the original material, and on the duration and intensity of the drought. 
Attention must also be drawn to the fact that, after prolonged drought, 
a certain number of the cells of all the filamentous forms are found to show 
a permanent slight contraction of the protoplasts, visible even when material 
is mounted in ordinary tap-water (cf. below, p. 716). This fact was not 
realized until a considerable number of the estimations had been carried out, 
1 The presence of strongly plasmolysing cells in this case is probably due to the high humidity 
of the air during this experiment (cf. p. 696). 
