509 



the formation of ice, as was evident in the number of plants which, without 

 injury, endure the formation of ice in their tissues. These plants are called 

 "resistent to ice" and they freeze only if the parts, which have been frozen 

 stifif, are cooled down below this specific minimum. 



This specific minimum is not fixed but rises with the amount of cell sap, 

 i. e., death from cold occurs at a higher temperature and, conversely, a loss 

 of water will cause an increase in resistance to all factors^ and therefore, 

 with frost, will cause death only at a lower temperature. 



Mez- adds to these the following observations : Any aqueous solution 

 of a substance must be cooled down below the freezing point of water 

 before ice can be crystallized out. In dilute solutions, as they exist under 

 normal circumstances in cell sap, the lowering of the freezing point is pro- 

 portionate to the molecular concentration (Raoult's law^). Dalton's law in 

 regard to the solution of osmotic substances which contain several sub- 

 stances in solution, holds good here. According to it, the amount that the 

 freezing point is lowered equals the sum of those amounts which each sub- 

 stance would produce of itself. 



Since now each cell in the same plant may have a content gradually 

 dififering from that of the other cells, the point of minimum cooling of the 

 cell sap will be a constantly changing one. Since the composition of the cell 

 sap within the latitude of the specific limits of all varieties of plants fluctu- 

 ates according to the nutrition, it is easy to understand that the various 

 individuals possess a different resistance. This also explains the different 

 behavior of dry and juicy parts of plants. The fact that in seeds, which 

 may be dried, death can result also from a removal of water is explained by 

 Miiller and Molisch by the assumption that it takes place because of the 

 sudden formation of ice in the supercooled plant, whereby the water is veryr 

 rapidly removed. Pfeffer* opposes this hypothesis and his book contains 

 a thorough treatment of the pertinent literature. Mez's studies, already 

 mentioned, support Pfeffer, for his investigations led to the following 

 results. The fall in temperature, indicating the end of crystallization, did 

 not lie, in any of the objects tested, below 6 degrees C. below zero. (The 

 experiments were made with petioles of Helleborus, Saxifraga and Strelitzia, 

 with leaves of Sempervivum and sprouts of Opuntia, Asparagus, Begonia, 

 Peperomia, etc.). 



"But the cell sap, capable of coagulation and not absorbed, stiffens be- 

 tween o and 6 degrees C. below zero. Accordingly, at 30 degrees C. below 

 zero, no greater dr}dng of the protoplasm, resulting from the removal of 

 water in the formation of ice, takes place than at 6 degrees C. below zero. 

 A plant which always survives the formation of ice in its tissues, does not 



1 Pfeffer, Pflanzenphysiologie, 2d ed., p. 315, note. 



2 Mez, Carl, Neue Untersuchung-en iiber das Erfrieren eisbestandiger Pflanzen. 

 Sond. Flora oder Allgem. Bot. Z. 1905, Vol. 94, Part I. 



3 Raoult's law: cit. Nerst, Theoretische Chemie, 4th ed. 1903, p. 152. 



■i See the chapter on "Die Ursachen des Erfrierens"' in "Pflanzenphysiologie," 

 II. Vol., 1904, p. 314. 



