128 PHYSIOLOGY OF NUTRITION. 



which emerge the stems of the thermometers to be used for 

 tracing the variation in temperature of the objects, and of the air 

 surrounding them. Finally each tray is provided with a wooden 

 cover, which just comes up to the upper edge of the wooden box. 

 This freezing chamber is placed in a situation where the tem- 

 perature is as constant as possible, e.g. in a room with a north 

 aspect. With a little attention it is then possible in winter to 

 keep the temperature in the apparatus very constant below zero 

 for days. In conducting the investigations the objects must be 

 placed well in the middle of the case. The upper ends of the 

 thermometers are fixed by suitable holders standing on one of the 

 halves of the cover. The thermometers are graduated to tenths 

 of a degree, and must frequently be compared with a standard 

 instrument. Naturally they mast be so constructed that the parts 

 of the scale indicating temperatures from C. to say 8C. may 

 rise outside the case. The readings are to be made at intervals 

 of a minute, and clearly tabulated. 



In order to grasp the fact that plant structures exposed to 

 temperatures below experience supercooling, and that their 

 freezing point is not at 0C,, but at a lower temperature, we must 

 recall the behaviour in freezing of salt solutions and of water re- 

 tained by solid bodies by adhesion. The cells of plants, indeed, 

 contain not pure water, but an aqueous solution of various sub- 

 stances, and the organised plant structures hold water. Pure 

 water freezes ordinarily at about C., but a solution (e.g. of 

 common salt) always freezes at a lower temperature, and the 

 amount of supercooling experienced before the formation of ice 

 begins, varies with the strength of the solution. Then the tempera- 

 ture of the solutions suddenly rises, till the freezing point proper, 

 which always lies below 0C., is reached. We can easily prove 

 these relations by suitable experiments. To investigate the be- 

 haviour, in freezing, of water retained by the force of adhesion, I 

 have employed the method described by Miiller-Thurgau. Round 

 the bulb of a mercury thermometer was wrapped blotting paper 

 which had been soaked with water and then externally allowed to 

 dry. When placed under a bell-glass surrounded by a freezing 

 mixture, the temperature of the blotting-paper gradually fell to 

 3 C. (the maximum of supercooling), and then suddenly rose to 

 a point somewhat below C. 



Tlie water of solutions and the water retained by solid bodies 

 by the force of adhesion and water occurs in plants under both 



