540 FREEZING AND BURNING. 



the fluid cell-sap, and consequently the walls of the cells are torn and burst like 

 a glass bottle in which water has frozen. A tissue whose cells are rent can, 

 however, no longer perform its functions. Moreover, although the ice melts by 

 and by, the damage to the ruptured cell-walls is irreparable. Besides, the cell-sap 

 streams from the cell-chambers of a thawing plant, and the leaves and stem which 

 have thawed after freezing are seen to be not only blackened, soft, and pulpy, but 

 are also covered over with a watery film which is never absorbed again into the 

 interior. 



Renewed investigation has shown that this idea of the freezing of plants needs 

 revision. First, in that no rupturing and bursting of the cell-walls occurs by the 

 pressure of the ice formed in the interior of the cells. In the tissues of plant- 

 organs surrounded by air the formation of ice does not usually commence in the 

 interior of the cells, but in the intercellular spaces; and the ice-crystals are first 

 formed in the interior of the cells only in those aquatic plants in which intercellular 

 spaces are absent. 



If Nitella syncarpa, belonging to the Characeae, which is met with in the clear 

 water of lakes and pools in Central Europe, is exposed to a temperature of 0° C, 

 its vital activity is not disturbed. Even the streaming of the protoplasm in the 

 cells is still very active, and even if by further cooling of the surrounding water 

 to —2° needles of ice are formed, the streaming of the protoplasm may still be 

 recognized. The cells are indeed somewhat compressed by the ice-needles, but the 

 protoplasm is never killed, even at —3°. It first begins to shrivel up between —3° 

 and —4°, gives up a portion of its water, shrinks away from its cell-wall, forms a 

 folded, contracted sac in the middle of the cell, whilst the water excreted stifiens 

 into ice between this sac and the cell-wall. If this Nitella be again exposed to 

 a higher temperature the ice melts, the protoplasm expands, and lies close to the 

 cell- wall; but it is incapable of again producing the streaming movement; it has 

 ceased to live; its molecular constitution has evidently become so fundamentally 

 altered by the separation of water from it that a reconstruction is no longer 

 possible. 



In the stems and leaves of plants surrounded by air, the ice always first arises, 

 as remarked above, in the intercellular spaces. But since usually air, and not 

 water, is contained in the intercellular spaces, the water stifiening into ice in them 

 must have been first excreted from the neighbouring cells shortly before the 

 freezing. And this is what actually happens. The structure of the ice-crystals 

 plainly shows that the water has come from within through the cell-wall to the 

 exterior, and that not once, but over and over again; for on the outer walls of 

 the cells which face towards the intercellular spaces the ice is seen in the form 

 of small discs placed above one another and combined into pillars, and these discs 

 can have been only gradually formed one after the other. This observation, 

 however, raises the questions. What portions of the cell give up the water? and 

 why does the water freeze in the intercellular spaces and not in those places 

 which it occupied before the frost? In answer to these questions it must first 



