10 



hardiness of the different varieties of apples and the rate at which 

 water will be evaporated from their twigs. 



Another form of injury, at least an injury that has been attribut- 

 ed to the effect of low temperature, results in the formation of dead 

 areas on the bark of the tree trunk, especially near the top of the 

 ground or in the crotches formed by the branches. Such injury has 

 been studied by Goeppert 1 , Sorauer 2 , Grossenbacher 3 and others. 

 Grossenbacher finds this injury greater on the side of the tree next 

 to the prevailing wind, indicating that the great evaporation from 

 the bark during a long period when it is frozen, and especially the 

 tearing due to the bending of the tree when the bark is under high 

 tension, may have something to do with this form of injury. This 

 may not be a different form of injury from direct freezing to death 

 which will be discussed later. 



Sachs observed that the foliage of certain plants wilted following 

 exposure to a temperature above the freezing point. He and also 

 Muller-Thurgau 4 conclude that this wilting was not due directly 

 to the effect of cold, but indirectly to the inability of the roots to 

 take up moisture at so low a temperature to replace the evaporation 

 from the leaves. According to Molisch 6 , plants continuously exposed 

 to a temperature too low for normal metabolism, but above the 

 freezing point, will eventually die. Under these conditions, death 

 ensues more slowly than where plants are killed by a sudden freeze. 

 The plants gradually turn yellow and die, or dark colored dead 

 spots are formed on the foliage. 



Miiller-Thurgau 6 limits the term freezing to death ("erfrieren") 

 to the most common phenomena to which we have reference in speak- 

 ing of killing from cold. It is death of the tissue following, directly, 

 the lowering of the temperature below the freezing point, with the 

 accompanying formation of ice crystals. When plant tissue is thus 

 frozen to death in the case of growing plants, the foliage in prac- 

 tically all cases has a wilted or limp appearance. Pronounced color 

 changes take place. Thus in most cases the green color due to the 

 chlorophyll is lost, and the tissue takes on a brownish watery color. 

 Plant cells containing coloring matter give up this coloring matter 

 to the adjoining cells of the liquid in the intercellular spaces. Other 



'Ueber die Warmeentwickelung in dem Pflanzen, etc. Book, 1830. (Bibi. No. 44). 

 "Landw. Jahrb. Vol. 35, pp. 465-525. 1906. (Bibl. No. 105). 



»N. Y. Geneva, Agr. Exp. Sta. Tech. Bui. 23, (Bibl. No. 50) ; N. Y. Geneva, Agr. Exp. 

 Sta. Tech. Bui. 12, (Bibl. No. 51). 



<Landw. Jahrb. Vol. 15, p. 453, 1886. (Bibl. No. 78). 

 'Untersuchung fiber das Erfrieren, etc. 1897, Book. (Bibl. No. 75). 

 6 Landw. Jahrb. Vol. 15, p. 453, 1886. (Bibl. No. 78). 



