FREEZING AND BURNING. 541 



be remarked that the water absorbed by the plants only partly enters into 

 chemical combination with the materials of the cell-body and cell-wall; that 

 another part, which we have called the water of imbibition, is not chemically 

 combined. The cell-wall and cell-body are saturated with this latter, and the 

 cell-sap in the vacuoles of the protoplasm also contains a large quantity of such 

 water. In the cell-sap it appears as the solvent of the acids, salts, and other 

 materials there present. The water by which the protoplasm and cell-walls are 

 saturated, and which we must imagine filling the interstices, like capillary spaces, 

 between the groups of molecules, is indeed held fast by the molecules of the 

 protoplasm and cell-wall, and the water in the cell-sap by the molecules of acids 

 and sidts, but yet certainly not so energetically as the cheiliically-combined water 

 in the albuminous substances of the protoplasm. 



What happens now in a body which holds fast the water in its smallest 

 intei-stices, like paste, for example, or in which the water appears as a solvent 

 as in an alum solution, when warmth is withdrawn, and when it is cooled down 

 to the freezing point of water? It is very remarkable that the water does not 

 immediately stiffen into ice as long as it is retained in the capillary spaces, or 

 as a solvent, and manj'^ salt-solutions can be cooled down to 5° C, some even to 

 10°, below zero without freezing. When at length under the influence of still 

 lower temperatures a stiflTening occurs, a separation has always taken place 

 previously; the water has run together from the finest interstices of the paste 

 into its larger spaces; it is first changed into ice in these cavities, and the water 

 of the salt solution has separated from the molecules of salt, and is then first 

 changed into ice-crystals. 



The same thing occui's, however, with the water saturating the cell-wall and 

 protoplasm, and serving as solvent of certain materials contained in the cells. The 

 formation of ice occurs in a very few species only on cooling the plant-tissues down 

 to —1°; in most instances the temperature must sink to —2° or — 3° in order 

 that ice may be formed in the cooled tis.sue. And indeed the water here has sepa- 

 rated from the molecules by which it was hitherto held fast before it congealed, 

 and it does not freeze in the interior of the cells, but outside them in the inter- 

 cellular spaces. In order that the water should get from the interior of the cell 

 into the adjoining intercellular spaces, a pressing and squeezing is necessary, and 

 this pressure can only proceed from the living pi'otoplasm in the cell-chambers; 

 consequently the process of freezing can be most correctly represented in this way, 

 viz. that the protoplasm becomes stimulated and roused by the lowering of the 

 temperature to transport a portion of the water from the interior to the exterior of 

 the cell, by means of contraction and pressure. What happens there is accordingly 

 not unlike the excretion of watery sap into the intercellular spaces in the stimulated 

 pulvini on the leaf-stalk of Mimosa; but the advantage obtained by the excretion 

 of water in the two cases is very different. In the cooled leaves the benefit of 

 course is to be sought for in the fact that the living portion of the cells is protected 

 from destruction as long as possible by the formation of ice-crystals in the inter- 



