632 SUMMARY OF CURKENT RESEARCHES RELATING TO 



freezing upon their tissues. Sections made of pieces at — 5° to — 10° C 

 exhibit ice in the tissues ; it forms two layers of icy rods, and is found 

 between, not within, the cells, which are somewhat compressed and 

 separated by the intervening crystals, but resume their original 

 position more or less when the ice melts. 



Experiments show that it is only pure water, not an organic 

 fluid, which thus freezes in the tissue. A lower temperature than 

 the freezing-point of the substances contained in the tissues is 

 required, in order to produce the ice ; this is in accord with the well- 

 known phenomena exhibited by solutions of certain salts which 

 submit to temperatures below their freezing-point without solidifica- 

 tion, if not disturbed. 



Capillarity also appears to exercise an influence on the freezing- 

 point ; fluid which is unfrozen at — 7° or — 10° becomes solidified 

 when brought into contact with a crystal of ice, and the temperature 

 then rises almost to 0° C. ; this is what appears to take place in the 

 minute intercellular passages. 



The solution of various substances in the intercellular fluid has 

 also some hand in the lowering of the freezing-point, but is not the 

 only cause. Proceeding on the hypothesis that the cell-wall consists 

 of minute particles (" micelli ") closely apposed, but with spaces 

 between them, Miiller-Thurgau explains the fact that the cell-wall 

 does not freeze even when the ice forms on its exterior, by the 

 fact that capillarity has greater power in these excessively narrow 

 spaces between the micelli than in the intercellular spaces, and 

 considers that, further, a thin lamina of unfrozen water separates the 

 cell-wall from the ice. 



The contained gases of the fluids are given out in the process of 

 freezing and are visible as bubbles in the ice; the dissolved sub- 

 stances accumulate between the ice and the cell-wall. 



The effect of bringing a potato at a temperature of 15"^ C. into an 

 atmosj)here which is at from — 4° to — 4 • 5° C. is first to gradually 

 lower the temperature to about — 3° C ; it then rises rapidly 

 to — • 8° C, and after an hour begins to fall again ; when the potato 

 is brought back into the warm chamber the temperature rises with 

 great and increasing slowness : the sudden rise of temperature in the 

 cold chamber is due to the commencement of ice formation as above 

 explained. The actual freezing-point of the tissues is — 5°. The 

 freezing-point of plants which contain little water (e. g. the leaves of 

 conifers), is lower than that of those which contain much (e. g. 

 Sempervivum leaves). 



Development of Heat during- the Germination of Plants.* — The 

 attempt has been repeatedly made to demonstrate the development of 

 heat in plants in such a way as to apply the thermometer to it ; but 

 although the existence of this phenomenon has been determined, 

 i. e. by comparative experiments, it has been hitherto out of the 

 question to measure its amount ; the results afforded by the ther- 

 mometer depend so entirely upon a number of external conditions, 



* Bull. Soc. Bot. Fiauce, xxvii. (1880) p. 141. 



