352 PLANT PHYSIOLOGY 



The cause of the death of these plants must be sought not in the 

 rupture of the cell walls but in the changes of the protoplasmic 

 membrane, primarily its coagulation. This coagulation is the 

 result of the formation of ice in the intercellular spaces and the 

 withdrawal of water from the cells. The cell sap becomes 

 increasingly concentrated, while the protoplasm is gradually 

 deprived of water. Besides, the protoplasm is exposed to the 

 pressure of the growing ice crystals. As a result, an irreversible 

 coagulation of the colloidal substances of the protoplasm takes 

 place. After thawing, it is dead and has lost its permeability. 



Hence, frost killing must be regarded not as the direct influence 

 of cold on the protoplasm but as an indirect effect, the desiccation 

 of the protoplasm due to the freezing out of water. One of the 

 most direct proofs of this statement is the fact that in the con- 

 dition of undercooling, without ice formation, the plant is able 

 to endure such low temperatures as would kill it if freezing had 

 occurred. 



The increase in concentration of the cell sap occurring during 

 freezing likewise exerts a harmful effect upon the protoplasm. 

 The action of the anions and cations upon the protoplasm and 

 especially of the hydrogen ion contained in the cell sap is inten- 

 sified with freezing. The total acidity of the cell sap increases, 

 and this may also contribute to a coagulation of protein sub- 

 stances of the plasma. 



81. The Reasons for Different Frost Resistance of Plants. 

 Hardening to Low Temperatures. Methods of Determination 

 of Frost Resistance. — Not all plants succumb with equal ease 

 to the formation of ice. Some of them, the most sensitive, are 

 killed in the very beginning of this process. Others, such as 

 the cabbage or the onion, are able to stand moderate freezing. 

 Those in the third group may freeze to a hard condition and thaw 

 out afterward without losing their vitality. Many plants of the 

 polar regions have this capacity. Some winter cereals are able 

 to stand frosts of — 15 to — 20°C. even in snowless winters. Still 

 more cold resistant are the dormant buds of deciduous trees and 

 the needles of conifers. 



The reason for this hardiness was sought first in the fact that 

 because of the small amount of water in these dormant organs, 

 especially the buds and needles, no ice could form. This suppo- 

 sition, however, has proved erroneous. Observations have 



