246 THE POWER OF RESISTANCE TO EXTREMES 



whether in such cases death follows the least formation of ice, although 



o 



death occurs when ice is formed at the primary freezing-point, in which case 

 the total quantity is insufficient to cause a seriously injurious withdrawal of 

 water. If the foregoing is actually true, then it needs to be explained why 

 the potato and beet-root are not killed by temperatures below their freezing- 

 point so long as no ice is actually formed. Possibly more than one factor 

 may enter into play, and the actual cooling may even render the plant 

 especially sensitive to gelation and its attendant consequences. Among 

 plants which can withstand severe cold, involving the conversion of most of 

 their water into ice, some are included which are readily killed by drying. 



The formation of ice-crystals in the protoplasm may possibly exercise 

 a special influence, and in fact causes the death of Amoebae and of the 

 plasmodia of Myxomycetes. Other protoplasts may, however, be more 

 resistant. The fact that starch-paste after freezing forms a coarsely porous 

 mass from which water can be pressed as from a sponge l is probably the 

 result of a change of molecular or micellar structure. Since dried starch- 

 paste does not take up the original percentage of water, whereas resistant 

 protoplasts assume their original condition when moistened, it follows that 

 the observations on starch-paste cannot be directly applied to all living 

 protoplasts. 



Death by cold is in many cases, though not in all, due to the withdrawal 

 of water by freezing, but this says nothing as to the protoplasmic 

 peculiarities which determine whether a particular plant shall survive or 

 not. The same applies when death occurs above the freezing-point, and 

 indeed the functional disturbances leading to death may have various modes 

 of origin and action. 



The visible changes and deformations produced in the protoplasm 

 have been proved by Klemm 2 to be mainly the result of sudden change, 

 and hence are not shown when cells are gradually warmed or cooled. 

 This reaction is, however, of interest since sudden changes of temperature 

 exercise for the most part no permanent injurious effect. 



Death produced by the depressant effect of low temperatures usually 

 requires long exposure, but may also take place rapidly, while an increased 

 degree of cold may hasten death, but does not necessarily always do so. 

 The sudden formation of ice in a sub-cooled plant may induce such 

 disturbances as to cause rapid death even when this is not directly due to 

 the withdrawal of water. 



We do not know the nature of the peculiarities which determine the 

 different powers of resistance of individual plants, and of the same plant 



1 Vogel, Gilbert's Annalen, 1820, Bd. LXIV, p. 167. Coagulated egg-albumin and solid 

 gelatine show similar changes on freezing. Cf. Sachs, Versuchsstat., 1860, Bd. n, p. 192; 

 Miiller, Landw. Jahrb., 1880, Bd. IX, p. 140; Molisch, 1. c., p. 7. 



a Klemm, Jahrb. f. wiss. Bot., 1895, Bd. xxvni, p. 641. 



