511 



The increase of the danger of freezing to death exists in all conditions 

 which hasten the appearance of a fatal supercooling. 



Thus, for example, the anatomical structure of the individual, depend- 

 ing upon the vigor of nutrition, can influence this. In very luxuriant 

 growth, the lumina of the cells and ducts are wider and the intercellular 

 spaces larger. However, the wider the duct, the more the lowering of the 

 freezing point is suppressed by capillarity. We find this fact emphasized 

 by Bruijning^ He found that the extract of Taxus leaves, in narrow 

 capillary tubes, has a freezing point of 8.8 degrees C. below zero, while the 

 same extract in open reagent glasses freezes at 1.3 degrees below zero. 



Besides the greater amount of water in the tissues, the constitution of 

 the air (amount of humidity contained) and its movement come under con- 

 sideration. In the later connection, attention should be called to the wide- 

 spread discovery that, in protected positions (in narrow valleys, fields sur- 

 rounded by woods, etc.) plants freeze which would remain uninjured in 

 regions accessible to the wind. 



In order to explain this circumstance, we will have to recall the fact 

 that air in motion increases evaporation and thus concentrates the cell sap. 

 With stronger evaporation the formation of ice will occur more quickly, 

 whereby supercooling will be avoided, and, at the same time, protection 

 of the remaining heat in the tissue will be brought about. 



In its prevention of supercooling by the superimposed ice, may be 

 found the advantage of the "open furrow" for winter grain ; it retains 

 snow much longer. 



Fog will also act as a protection. We find a recent example of this in 

 the observations made by Thomas"-, who, in Thuringia, found that the foli- 

 age of young beeches, on the heights covered with fogs was uninjured, 

 while in the valleys it was brown and wilted as a result of frost. In this 

 case, an evident boundary line could be found. In mountain forests, the 

 covering of clouds is a protection against frost which one should not 

 underestimate. 



We will now turn once again to the fact that in many cases a rapid 

 thawing of frozen plant parts can bring about death, while a slow warming 

 does not kill. The correctness of this assertion is often contested. If it is 

 given as an universal rule, it seems inconclusive; but if it is limited to cer- 

 tain cases, it certainly is of value. An older and very instructive example 

 is given by Karsten^. A large shipment of tree ferns (Balantium) had to 

 withstand 20 degrees below zero enroute. Some of the plants, when they 

 arrived, were put, in a still frozen condition, into a warm place and were 

 killed, while almost all of those first thawed in cold water and then taken 



1 Bruijning, F. F., Zur Kenntnis der Ursache des Frostschaden. Sond. 

 Wollny's Forschungen auf dem Gebiete d. Agrikulturphys. 1896; cit. Centralbl. f. 

 Agrikulturchemie 1898, p. 173. 



- Thomas, Fr., Scharfe Horizontalgrenze der Frostwirkung an Buchen. Thiir- 

 inger Monatsblatter 1904, 12. Jahrg., No. 1. 



3 tJber die Wirkung plotzlicher bedeutender Temperaturanderung- usw. Bot. 

 Z. 1861, No. 40. 



