Sunlight upon Aquatic Plants „ 373 
almost certainly be corroborated by the necessary experi- 
mental research. All plants growing entirely submerged in 
fresh water must be always in a condition of maximal 
turgidity, and contain the full amount of water possible. As 
is well known, it is those parts which are richest in water 
which are first affected by cold, and are most readily injured 
by being frozen. It does not, however, necessarily follow that 
a tissue rich in water will be killed by being frozen, as is 
shown by the high resistant powers exhibited by the leaves 
of Sempervivum , Sedum , &c., and by many Mosses as 
well k 
With regard to amphibious plants, i.e. plants which (1) grow 
partly in and partly out of the water, or (2) float upon its 
surface, or (3) may lead either a submerged or a subaerial 
existence, these seem to have acquired more marked resistant 
powers than are possessed by plants growing always com- 
pletely submerged in fresh water. This is only natural and 
to be expected, considering the variable conditions under 
which such plants live and the exposure to which they may 
be subjected ; but even then comparatively few, excepting 
Schizophytes, such as the Cyanophyceae and Bacteria, seem 
able to withstand complete freezing when in the actively 
vegetating condition. That the plants of the first class die 
down at the approach of winter is familiar to all, though in 
cases where the water remains comparatively warm throughout 
the winter, the submerged leaves may remain living and 
green, and even form starch. Of the second class Lenina 
affords a good example, as also do the floating leaves of 
Water-lilies, &c. That the leaves of Water-lilies are killed by 
being completely frozen is a matter of common observation. 
Lemna appears able to withstand partial freezing, especially in 
the younger parts, but all vegetative parts are killed by being 
completely frozen. 
The third class includes all forms which can grow in- 
differently in water, or upon mud, damp soil, or other 
See Assim. Inhib., Journ. Linn. Soc. Bot., Vol. xxxi, 1896, p. 389, 
