Sunlight upon Aquatic Plants, 371 
powers from those of the vegetative filament which produced 
it. It contains much less water than the cells of the 
vegetative filament do, and its powers of resistance to desicca- 
tion and freezing are correspondingly increased. As is well 
known, unfavourable conditions generally tend to induce 
conjugation. Thus, plants of Spirogyra may be caused to 
conjugate by allowing the water in which they are kept 
to very gradually evaporate. Hence conjugation may, in 
many cases at any rate, be regarded as a sign that the 
external conditions have become unfavourable in some 
respect, and that the plant is endeavouring by those means 
to tide over the unfavourable period. This action of 
unfavourable external conditions in inducing a formation 
of zygospores is strictly comparable with the effects which 
partial starvation or an unfavourable habitat may exert upon 
many higher plants, causing them to run more rapidly to seed, 
and thus hastening their own exhaustion. 
No haphazard observations in which the period and con- 
ditions of exposure can only be approximately estimated can 
afford any decisive evidence in dealing with even the simplest 
problems of this kind. When a perennial land-plant is 
subjected to gradually increasing cold, as, for example, at 
the onset of winter, it commonly prepares itself in a variety 
of ways for the increased cold which may be expected to 
follow. In addition to the immediately obvious external 
changes, certain important internal changes also take place. 
Thus the starch in the bark, &c., of Phanerogamic trees and 
shrubs, as Fischer and others 1 have shown, and in the leaves 
of evergreens as well, according to Lidforss, may be converted 
into sugar in cold winters, and be retained as such, dissolved 
in the cell-sap, while starch-grains reappear, after a more 
or less prolonged latent period, when the plant is brought 
into a warm room. By the above means the concentration 
of the cell-sap is markedly increased, so that the freezing- 
1 Fischer, Beitrage zur Physiologie der Holzgewachse, Pringsh. Jahrbiicher, Bd. 
xxii., Heft i, pp. 73-160 : Bengt Lidforss, Zur Physiologie und Biologie der 
wintergriinen Flora, Bot. Ct.-Bl., Bd. Ixviii, 1896, p. 34. 
