110 ON THE POSSIBILITY OF PREVENTING DAMAGES BY FROST. 



that the destruction is caused by the diminution of water which 

 the protoplasm undergoes at freezing, as mentioned above. 



There has also been a controversy with regard to the time 

 when frost proves fatal. While Goppert concludes that death 

 occurs during the continuance of the frost, Sachs is of the 

 opinion that the tissues die only after they have thawed, and 

 that a fatal issue depends very much on the manner and rate of 

 thawing. The two views may to a certain extent be reconciled, 

 for it is possible that during winter death occurs during the con- 

 tinuance of the frost, whereas in the case of a summer night- 

 frost it appears at the moment of thawing. 



All plants are not equally effected by a low temperature. 

 Among our common European vegetables the potato is one of 

 the most sensitive. P^ar less susceptible are the cereals, as oats, 

 barley, rye, wheat; more sensitive pea, &c., at least during the 

 first stages of growth. 



Certain plants are seriously injured by low temperatures 

 which are considerably above the freezing point of water, but 

 these are exceptional cases. In some of our familiar spring 

 plants of Europe the leaves may be frozen and thawed without 

 apparent mischief, but in general the thawing must take place 

 slowly ; if it proceeds rapidly the plant may be irreparably 

 injured. There are however also well known cases in which 

 plants may be thawed quickly without serious injury. Sachs has 

 showii that the leaves of the cabbage, turnip, and certain beans, 

 frozen at a temperature of from — 5" C. to — 7" C. and placed 

 in water at 0° C. are immediately covered with a crust of ice, 

 upon the slow disappearance of which they resume their former 

 turgidity. If such frozen leaves are placed in water of -\- 75" C. 

 they at once become flaccid. 



The behaviour of certain plants during exposure to low 

 temperatures affords some of the best illustrations of the adap- 

 tability of vegetation to its surroundings ; and the question as 

 to increasing the tolerance of a given species or variety to the 

 adverse influence of cold by careful selection of seeds for a series 

 of years has been successfully answered by cultivators in some 

 northern countries of Europe. 



Apart from specific peculiarities we also find individual 

 differences, and it is this fact which makes it possible for us to 

 acclimatize plants. As the ability to resist frost varies amongst 



