ICE IN PLANT TISSUE. IJ 



Muller-Tluirgau could detect this increase in ice content by 

 simply sectioning^ various tissues under diiTerent temperatures. 

 The writer found it very apparent in the case of winter buds. 

 Sections taken at — 20° F. showed very little ice. It was found 

 necessary to work at a temi)erature of 0° F. in order to find the 

 ice content well developed. At this temperature, in the buds of 

 about nineteen out of twenty-seven species of trees and shrubs 

 the ice could be plainly seen and in the majority it was very con- 

 si^icuous. The remaining eight were sectioned again at — 15° 

 F. and all but two showed ice, but in small scattered crystals. 

 The buds of these eight species, butternut, l)eech, witch-hazel, 

 hickory, ash, oak, hazel and chestnut all contained little cell-sap 

 and small cells with rather thick walls. 



It has been found by Miiller-Thurgau, Molisch and others cited 

 earlier in this paper that in no case can the death of anv plant 

 be traced directly to absolute cold alone at temperatures below 

 the freezing point. At present there seems little if any evidence 

 that death is due to shock, over-stimulation, or any other action 

 of cold which might produce the so-called " cold rigor," although 

 there are several cases yet unexplained by the drying theorv.* 

 It may well be that in these and other cases additional secondary 

 changes are produced. An unconvincing attempt has recently been 

 made to show that in no case is death due to the amount of ice for- 

 mation and drying out.f If death were due to shock we should 

 have to assume a special sensitive point at a temperature a few 

 degrees below freezing, which is unlikelv, especially since the 

 frequent death at this temperature can be more easily explained 

 in another way. Most plants are killed by the first ice formation 

 within the tissue. If they survive this, a considerably lower tem- 

 perature is reciuired to kill them, or they may be capable of en- 

 durinof anv deirree of cold. It has been demonstrated bv means 

 of orchids, the sap of which turns blue when the cell is killed, 

 and Ageratu]n,% which gives oiT a peculiar odor under similar 

 circumstances, that, in the case of delicate tissues at least, death 



* See Pfeffer and Ewart. Ph\-siology of Plants, p. 244. 

 t Mez, C. Neue Untersuchungen uher das Erfrieren eisbestandiger 

 Pflanzen. Flora, 94 : 89. 1905. 



§ See Molisch. 



