ECOLOGICAL 65 



temperature does not quite reach the freezing-point. The leaves 

 become discoloured and die. It seems that the chemical routine or 

 metabolism may be fatally disturbed although there is no actual 

 freezing. Much depends on the length of the exposure. 



It is possible to surround the stage and lower parts of a microscope 

 with a freezing mixture, so that the process of congealing can be 

 intimately studied, and this has been done in great detail by Prof. 

 Molisch. Living matter is a colloid, that is to say, a fluid in which 

 there are suspended or dispersed innumerable ultra-microscopic 

 particles and unmixing droplets, presenting an immense superficial 

 area if all their minute surfaces could be added together. In a very 

 fluid colloid or "sol" the suspended particles or droplets are in a 

 state of quivering "Brownian movement", for they are being bom- 

 barded by the freely moving molecules of the medium in which they 

 are suspended. But this readily passes into a "gel" state where the 

 particles become "set" in a network or the like which imprisons little 

 vesicles of the fluid medium. We have already referred to the 

 fundamental fact that living matter or protoplasm is in a colloidal 

 state, and many a living cell can readily pass from "sol" to "gel" 

 and back again. 



But when such a colloid as the milky juice of the indiarubber- 

 tree is frozen, the droplets of rubber rush together to form a network 

 and ice-crystals form in the meshes. As the crystals go on growing 

 they necessarily fix more and more of the water, and this is the sort 

 of thing that happens when a living cell freezes. Molisch has shown 

 (i) that the ice may be formed inside the cell-substance, as happened 

 when he froze an Amoeba or the stamen-hairs of Tradescantia; or 

 (2) that it may form a wall around the outer surface of the cell, as 

 in the case of the common green threads called Spirogyra; or (3) 

 that there may be a combination of these two processes. But in all 

 cases there is a withdrawal of water to form ice and a consequent 

 shrivelling of the protoplasm. In the majority of vegetable tissues 

 the ice is formed outside the cells, and this is less likely to be fatal 

 than when it is formed in the very heart of life. Whether the forma- . 

 tion of ice is fatal or not depends on various factors, finally perhaps 

 on the specific structure of the living matter of the plant. In many 

 cases, such as daisy and conifer, the tissues may be frozen hard 

 without losing their power of recovery; in other cases, such as potato 

 tubers, tobacco plants, and the leaves of the vine, the internal 

 formation of ice is always fatal. 



As we have already hinted, there are many ways in which plants 

 may circumvent the intensity of the frost. There are external 

 protections, such as hairs and bark; there is a sacrifice of vulnerable 

 parts, such as leaves ; there is the assumption of a very dry state with 

 water-content at a minimum, as in seeds. Moreover, cell-sap may 

 not freeze as easily as pure water, and the minuteness of the capillary 



VOL. I F 



