357 
Vegetation and Frost. 
to cold and that there is for each cell a fatal “minimum” temperature. 
Further, for plants which can stand the formation of ice in their 
tissues he thinks it is an advantage that in a cold environment (he 
worked with —14 - 5 n C experimentally) the ice should form as quickly 
as possible. The protective action of the ice lies in this, that 
conduction away of the plant-heat is not half as quick in solid ice 
as it is in a liquid or watery tissue. Consequently the inner parts are 
much longer in cooling to their fatal minimum. 
The formation of ice from sap in tissues leads to the liberation 
of the latent heat of the water and crystallising out of sugar from 
the sap would also generate heat. If these radiations were 
adequately isolated by non-conductors from the outside cold, the 
delay in cooling down to the minimum might be very considerable. 
Mez holds that the storing-up of sugars in the periderm of trees 
and in evergreen leaves (cf. Lidforss) is a preparation of substances 
which will give out a little protective warmth when concentrated to 
crystallisation by ice-formation. It is clear that these principles 
must be at work in the plant, but it is difficult to determine the 
magnitude of their biological significance. 
Mez shows clearly that when a sappy piece of tissue (he used 
largely the nodes of stems of Impatiens) is cooled down and its 
internal temperature registered by an imbedded thermo-junction it 
goes through the same phases as a solution of its constituent 
substances would. The falling temperature on cooling passes 
through a phase of separation of pure ice into a phase where the 
solutes have become so concentrated that they and the ice crystallise 
out at an equal rate. During this phase the temperature remains 
constant and only when it is over and the whole mass is frozen 
solid does the temperature once more fall. 
According to Mez this freezing temperature was never observed 
to be below —6"C, so that in time “ cold-dessication ” should be 
complete at this temperature and the protoplasm should not suffer 
any more by further cooling, on the hypothesis that all the injury is 
simply due to dessication. This clearly goes against the view of 
Muller-Thurgau and Molisch, and we shall see in the work of 
Gorke that there are other ways in which withdrawal of water may 
injure the protoplasm besides direct dessication. 
In 1906, Gorke made experiments on changes in composition 
produced by freezing, either whole plants or their expressed sap, 
which brought to light a quite new factor. It is well known that 
