716 
Fritsch and Haines .— The Moisture-relations 
(b) Microscopic Characters of Drought Material. 
The cells to be found in drought material can be broadly divided into 
three groups : (a) the dead cells which have discoloured, though not neces¬ 
sarily shrunken, contents ; (b) living cells with a slightly contracted and 
apparently rigid protoplast, since the contraction does not disappear m 
ordinary water and plasmolysis does not ensue even after treatment with 
very strong solutions (25 per cent, sea-salt); and (c) healthy pure green cells 
with a quite uncontracted protoplast and not affected by similar strong 
plasmolysing solutions. In Pleurococcus and Cystococcus , as far as our 
observations go, the second type of cell is not found and many of the cells 
remain capable of plasmolysis even after very prolonged drought. In the 
filamentous forms the relative numbers of the different types of cells present 
depend on the intensity and duration of the drought; at first there are 
relatively many of the third type, later the second type predominates, and 
under extreme drought the majority of the cells may die. What extent of 
drought may be necessary to lead to the death of all the cells in a given mass 
of terrestrial alga or moss protonema we are unable to say (cf., however, 
Schroeder, 1886 ), but we are inclined to think it would have to be very intense 
or very prolonged. 
The cells of the second type are not dead (cf. section (c)), although 
a large number of them may be moribund, if the drought has been intense 
or prolonged. These cells, when granular, invariably contain large coarse 
granules of unequal size and showing an irregular distribution, some parts of 
the periphery of the protoplast being crowded with them, others practically 
free. Non-granular cells, especially in Zygogonium, often have a peculiar 
whitish opaque look about the edge of the protoplast. Material that has 
been in a sealed slide presents similar features, but cells of either type are 
rarely found in material out of doors and then only in small numbers. In 
Hormidium during drought a large spherical granule is often to be seen in 
these cells in the clear area outside the chloroplast. 
It is evident, however, that this rigid state of the protoplast can arise 
also as a result of conditions other than drought. In the first place it would 
seem to develop after some weeks’ sojourn in the laboratory, even in material 
that has been kept continuously moist. Further, it has been observed to 
appear in a large number of the cells of Zygogonium as a result of exposure 
to bright light. Thus, in one of our experiments on the influence of dark¬ 
ness and illumination on the plasmolysing capacities of the cells of this alga, 
in which the experiment had been inadvertently allowed to continue for 
four weeks, the cells of the ‘ light ’ material were found to be mainly in this 
condition, whilst those of the ‘ dark * material showed it to a much lesser 
extent. It may be added that in the former the cells were nearly all purple 
