F. F. Blackman. 
356 
resists freezing till it has been over-cooled to —10 n C so the capillary 
.mycelium of Phycomyces may, in air, be cooled to -c-17°C before ice 
is formed in it. 
It is exceptional for ice to form inside cells, and on cooling cells 
mounted in water, the outside water freezes first and then water is 
gradually withdrawn from within the cell to form more ice on the 
outside of the cell-wall. A filament of Spirogyra when frozen in 
this way is so shrunken, that under the microscope it looks exactly 
like a shrivelled dried-up thread. When tissues of higher plants 
are frozen, films of pure ice form on the walls abutting on inter¬ 
cellular spaces, and these films grow steadily to quite large lumps 
of ice causing disruption of the tissues. 
Muller-Thurgau and Molisch held that the fatal effect of this 
freezing should be traced to the resulting dessication of the proto¬ 
plasm whereby its structure was irrecoverably disorganised. As 
long as the ice-formation is staved off in sappy plants the cold does 
not kill. For sudden death on cooling ice-formation is essential, 
whether it acts as Molisch suggests straight upon the protoplasm, 
or only indirectly. The reverse is, of course, not at all true, as 
many plants recover readily after being frozen solid. 
Dominant by the authority of Sachs the erroneous view had 
long been held that it is only on the thawing of the cell that the 
fatal disorganisation sets in, and that if thawing proceeds very 
slowly recovery may take place which would not be attained with 
quick thawing. Molisch has ingeniously and finally demonstrated the 
complete incorrectness of these views. He cooled the red marine 
Alga Nitophyllwn to —5°C when it froze and on keeping it frozen 
the brilliant orange red fluorescence of the phycoerythrin in the cell 
soon became very obvious. In the living cell the pigment exhibits 
no fluorescence and this appearance is a sure sign of death. By 
similar treatment of the leaves of Ageratum mexicanum the odour 
of coumarin was soon developed, without any thawing having 
taken place, and this odour, which results from post-mortem ferment 
action within the cell, is also evidence of the death of the cell. 
Definite experiments made with a number of plants showed 
that only in one exceptional case, Agave, did the rate of thawing 
make any difference to the question of death or recovery. 
In 1905, Mez developed a different point of view. He thinks 
that death by “ cold-dessication ” is only exceptional, perhaps true 
only for water-plants and holds that protoplasm is directly susceptible 
