372 Ewart.— The Action of Cold and of 
point of the plant as a whole is lowered, and it is enabled 
to withstand cold, to which, if suddenly exposed in summer, 
it would have been more sensitive, and by which it might 
have been killed. 
Similarly water-plants may undergo preparatory modifica- 
tions when winter is approaching. Thus in Elodea the 
vegetative shoots die down, while from the rhizomic stems 
buried in mud arise condensed etiolated shoots, the cells of 
which are packed with starch-grains in November. Later 
still this starch may partially or entirely disappear, being 
gradually converted into sugar as the cold becomes more 
intense. In such mud the plants are well protected, for 
the water it contains may not freeze until the temperature 
falls several degrees below zero ; and moreover the chemical 
changes and decompositions which continue to go on to 
a certain extent in organic mud, even at low temperatures, 
may produce a slight but by no means negligible quantity 
of heat, and a correspondingly higher temperature. Ordinary 
turgid plant-cells do not begin to freeze until the temperature 
falls from two to four degrees below zero. As ice-crystals 
form and water is extracted, the freezing-point for the more 
concentrated watery solution remaining is correspondingly 
lowered. Moreover, the freezing-point for capillary imbibed 
water undergoes an even more marked depression, for, as 
Dixon and Joly 1 have shown, the imbibed water in the walls 
of wood vessels does not freeze until a temperature of — io°C. 
or —ii° C. is reached. Hence it is only when the temperature 
is at least as low as this that the plant is completely frozen, 
though it commences to freeze at two or three degrees below 
the freezing-point for distilled water. 
The statement, therefore, that no purely aquatic fresh-water 
Alga, or, indeed, that no purely aquatic fresh-water plant, can 
withstand complete freezing when in the actively vegetating 
condition is one which, though sufficient evidence is not yet 
at hand to establish the generality of its application, will 
1 Dixon and Joly, Ann. of Bot., 1895, Bd. ix, p. 416. 
