FREEZING AND BURNING. 541 



)e remarked that the water absorbed by the plants only partly enters into 

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

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

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

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

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

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

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

 3etween 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 

 md salts, but yet certainly not so energetically as the chemically-combined water 

 n the albuminous substances of the protoplasm. 



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

 nterstices, like paste, for example, or in which the water appears as a solvent 

 18 in an alum solution, when warmth is withdrawn, and when it is cooled down 

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

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

 IS a solvent, and many 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 

 ower temperatures a stiffening occurs, a separation has always taken place 

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

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

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

 3hanged into ice-crystals. 



The same thing occurs, 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 tissue. And indeed the water here has sepa- 

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

 ind 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 protoplasm 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 

 Qot 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- 



