124 PHYSIOLOGY OF NUTRITION. 



being frozen with the water, and subsequently thawed out, they 

 were found to be dead, and no longer exhibited assimilatory 

 activity. 



I have found it very instructive to expose leaves of Begonia 

 manicata, either in the open or indoors, to a temperature of 5 

 C. or 10 C., placing them, under a bell-glass, with their stalks 

 dipping into water, but with their blades exposed to the air. In 

 freezing, the leaves become discoloured, and the discoloration 

 does not disappear on thawing. Exposure to the low temperature 

 causes disorganisation of the protoplasm, so that the acid cell-sap 

 is able to act on the chlorophyll bodies, and destroys their pig- 

 ment. If we examine microscopically surface sections of leaves 

 of Begonia manicata which have been killed by freezing, it is in 

 fact seen that the chlorophyll bodies are not, as normally, green, 

 but yellow in colour. Experiments with these leaves are instruc- 

 tive, since the change of colour appearing on the reduction cf 

 temperature indicates directly that the mere freezing brings about 

 the death of their cells. 1 



It is further an important fact determined by various physiolo- 

 gists, that the same structures which suffer if frozen when rich in 

 water, are not injured by exposure to cold when poor in water. 2 

 We may readily prove that this is the case by investigating air- 

 dry seeds and soaked seeds respectively of Phaseolus, Pisum, 

 Triticum, etc. For example, I have taken on the one hand air- 

 dry wheat grains, and on the other wheat grains which had been 

 lying in water for seven hours, and exposed both for fifteen hours 

 to a temperature of 10 C. in small glasses. I found that the 

 former, when placed on moist sand under normal conditions for 

 germination, are still capable of germinating, while the latter do 

 not germinate p,nd perish. 3 



All investigations clearly bring out this fact, that different 

 plant structures, and the same structures in different conditions, 

 are by no means equally sensitive to the influence of low tempera- 

 tures. 



1 See Detmer, Botan. Zeitung, 1886, No. 30. 



2 See Detmer, Vergleicliende Pliysiologie d. Keimunysprocesses d. Samen, 1880, 

 p. 392. 



3 Further literature: Sachs, Versuchsstationen, 1860, Berichte d. sacks. 

 Gesellschaft d. Wiss., 1860, Bd. 12, p. 27, and Flora, 1862 ; Goppert, Wfirmecnt- 

 wichelung in der Pflanze, 1830. 



