On Ni(jht-Frost<i. 17 



coverlet which hinders further loss of heat. Thereby freezing of the cell- 

 water of vegetables is prevented for a short time. If tlie loss of heat still 

 continues, the cell-water freezes and causes the death of the plants. The 

 manner in which this occurs lias not been stated with certainty. Earlier it was 

 admitted tliat the destruction took place by the bursting- of the walls of the 

 vessels caused by the augmentation of volume which took place at the freezing. 

 Låter researches have shown that this opinion is wrong and that the destruction 

 is caused by the diminishing of water which the protoplasma is undergoing at 

 freezing. The principal tliing is that the ice must appear in the interspace 

 of the cells, before death can occm-. 



Not only the low température, but also the time of its duration, will be 

 décisive for the destiny of the plant. Then, if we assume, that not the sap 

 between the cells but the surrounding dew is frozen, the final destiny of the 

 plant still dépends on the way in which the thawing takes place. Sliould it 

 happen to be very rapid, for instance should the rays of the sun, immediatel}' 

 after its rise, reach the plant, the ice will not only melt but also evaporate, 

 consuming a great quantity (686 Cal. for every kilogram) of heat. The greatest 

 part of this heat naturally comes fi-om the sun, but one part is still derived 

 trom the little store of the plant, and it is probable that the last determining 

 cause of the damage done by the night-frost often dépends on this circumstance. 



It would be wrong to believe that whenever rime round the plants is 

 produced, the suni'ise being clear, damage by frost will instantly occur. 

 As mentioned above, the rye was not damaged, a short time before its bloom- 

 ing, though the température was 2'^ near the ground and feil still more in 

 1 ^ hours, the sunrise being clear. Hence follows that vegetables possess a 

 certain power of résistance against frost and that they are not in general 

 destroyed, if they are covered with ice at a température of '2°, near the 

 ground, by clear sunrise, the time of the duration of the low temperatui-e not 

 surpassing 1 ^ hours. 



ON THE QUANTITY OF HEAT TO BE RESTOßED TO A FIELD FOR 

 PßEVENTING DAMAGE BY FßOST. 



In order to give an idea of the quantifies of heat that ai'e to be commu- 

 nicated to the plants of a field during a frosty night, to prevent damage, 



3 



