542 FREEZING AND BURNING. 



cellular spaces. If the water were forthwith frozen inside the cells, between the 

 groups of molecules of the living cell-body and its wall by a few degrees of cold, 

 fundamental displacements and disorganizations of the groups of molecules would 

 be unavoidable. On the other hand, the ice-crystals on the exterior of the cells do 

 not produce such destruction. In the intercellular spaces they can form large 

 clusters, the spaces may be even enlarged by them, and the adjoining portions of 

 tissue may be compressed and split, without a disorganization of the molecular 

 structure of the living cells occurring simultaneously. 



It is shown by numerous other phenomena that the excretion of water described 

 does not connote the death of the living cells. It is also beyond doubt that the 

 excreted water can be again received back subsequently under favourable conditions; 

 and that by slow thawing of the ice formed in the intercellular spaces, the water 

 again returns to the places it previously occupied within the cell. If, on the other 

 hand, the cells are no longer able to take back the separated water, or if the cold 

 becomes so severe that finally the water retained by the protoplasm and indispens- 

 ably necessary to its existence, becomes changed into ice, then a disorganization of 

 the molecular structure is the natural consequence; or, in other words, the proto- 

 plasm of the cells in question has been killed by the loss of heat. Then we say 

 the plants are frozen dead. 



Thus the difference between mere freezing and freezing to death is made clear; 

 and at the same time the experience of gardeners is confirmed, that the former is 

 not necessarily attended by the latter. 



At what degree of cold freezing occurs, and at what freezing to death, depends 

 first of all on the specific constitution of the protoplasm of the various species, but 

 also, in each individual species, upon the stage of development arrived at by the 

 organs exposed to the cold. Just as the water in various salt solutions becomes 

 changed into ice at various temperatures, so the protoplasm of one species exhibits 

 a different behaviour to that of another. It has been mentioned above that the 

 hydrophyte Nitella syncarpa is frozen at a temperature of — 4° C. Other aquatic 

 plants bear a much greater degree of cold without their protoplasm being killed. 

 S'phcerella nivalis, which produces the red colour of snow, is exposed in the winter 

 for months to a temperature of — 20° C. in Arctic regions, and is not destroyed 

 thereby. This SphoBrella is also frequently exposed to very severe cold on the 

 snow-fields of the Alps during winter nights, and the same remark appUes to 

 various species of the genera Epithemia and Navicula and to other Diatomaceae 

 which are to be found together with Si^hcerella nivalis living on the glaciers. It 

 may be mentioned here in passing that there are also animals which live with these 

 unicellular plants in the ice regions, and are not killed although they remain frozen 

 for months. As soon as they thaw, these Rotifers bring their cilia into action 

 again; the black Poduras, known by the name of glacier-fleas, take their flying 

 leaps, and the spotted spiders again stride with their long legs over the sun-illu- 

 mined ice-fields; while, on the other hand, the insects driven by tlie wind to these 

 glaciers are in a very short time killed by the frost 



