376 Ewarts — The Action of Cold and of 
Protococcus pluvialis , Oscillaria sancta , and vegetative Dia- 
toms and Desmids are killed by air-drying and also by 
being completely frozen. The zygospores of Zygnema can, 
however, withstand several years’ air-drying, and are there- 
fore hardly likely to be affected by cold b The filaments of 
Vaucheria and the normal vegetative mycelia of Penicillium , 
Mucor , and Physomyces are killed by drying and also by 
being completely frozen at — io° C. to — 1 2° C. In Mucor and 
Penicillium the mycelia may be made more resistant by 
growing them on concentrated nutrient solutions, while in the 
form of Mucor yeast, the resistant powers are very different 
from those of the normal vegetative Mucor mycelium 2 . 
With regard to marine Algae, many of these are, as the 
researches of Kjellmann upon the Arctic Flora have shown, 
very resistant to cold, and it is possible, and indeed probable, 
that as a class they are more resistant to cold than those 
fresh-water Algae are, which are purely aquatic. According 
to Pfeffer 3 , in sea-weeds the internal isosmotic force which 
the cell-sap exerts must be greater than in land-plants, and 
also than in fresh-water plants, for the former are frequently 
subjected to great external pressure and are also surrounded 
by a weak saline solution, and yet the phenomena of turgid ity 
which they present are in general similar to those exhibited 
by land or fresh-water plants. Hence, in seaweeds the cell- 
sap will be more highly isosmotic, or more highly concentrated 
than in fresh- water plants, and therefore a lower temperature 
will be required to completely freeze the former than the 
latter. Similarly, just as in Penicillium , where the adaptation 
to growth in concentrated solutions causes an increased 
1 See Schroder, Bot. Unters. Tubingen, Bd.i, Heft i, 1886 ; Ewart, Assim. Inhib., 
1895, 1. c., p. 375, &c. 
2 Chodat, 1 . c., supposes that spores of Mucor mucedo which had commenced to 
germinate might nevertheless resist exposure to -70° C., but the details of his 
experiments do not support this conclusion, and it is certain from my own observa- 
tions that the young vegetative mycelium of Mucor is killed by several hours’ 
exposure at a temperature of from -10 to -12 0 C., although if any acrospores 
or endospores have been formed, these will germinate and produce new mycelia 
when restored to a normal temperature. 
3 Pflanzenphysiologie, 2. Aufl., Bd. i, sec. 24. 
