Oct. 14. 1918 Hardening Process in Plants 85 



molecular" forms of the proteins to simple forms, which are not so easily 

 precipitated by the salting-out process. 



In the plants which have been tested in this study all of these factors 

 seem to play a part. In addition to these, there appears another im- 

 portant factor, which for the most part has been disregarded, the change 

 in the actual acidity or hydrogen-ion concentration of the plant juice 

 on freezing. It seems that this factor supplies the deficiencies of the 

 other factors in explaining frost injury. 



FIRST INDICATIONS OF FROST INJURY AND THEIR DEVELOPMENT IN 

 CABBAGE AND TOMATO 



The first indication of frost injury in the herbaceous plants which have 

 been observed is in the appearance of injected areas. Where the ex- 

 posure to low temperature has been of short duration, these areas appear 

 as isolated dots over the surface of the leaf, as is well shown by cabbage 

 and tomato (PI. 7) ; and occasionally along the stem also, observ'ed on 

 sunflower (Helianihus annuus). The injected spots when observed by 

 transmitted light are more transparent than the remainder of the leaf. 

 The transparency is due to the displacement of air, which is ordinarily 

 present within the intercellular spaces of the spongy parenchyma, by 

 water which has been withdrawn from the cells during the process of 

 freezing. It was shown by Sachs {42, 43) that in the process of freezing, 

 water passes out from the protoplast and freezes in the intercellular 

 spaces. On thawing, this water is then left in the intercellular spaces 

 until such time as it is evaporated or reabsorbed by the cells which have 

 been plasmolyzed by freezing. Under certain conditions it requires con- 

 siderable time for the protoplast to return to its former position against 

 the cell wall and to regain its turgidity. 



The tomato leaf does not survive ice formation in the tissue; conse- 

 quently the injected spots appear as brown areas after a few hours. 

 Sections of these spots made immediately after freezing show a collapsed 

 condition of the palisade cells (PI. 8, A). The collapse of the palisade 

 causes depressions in the leaf surface, and these areas dry up after a 

 few days. Death of the injured cells produces the spotted appearance 

 shown in the leaf at the left of Plate 7, B. The celb around these areas, 

 although exposed to the same temperature, show no injury because there 

 was no ice formation. The effect of short exposures to low temperature 

 appears to be nil, the injury being an accompaniment of ice formation. 

 Voigtlander {47) has previously shown that there must be ice formation 

 in the tissues to produce frost injury. 



MECHANISxM OF FREEZING 



The undercooling of the cell solution is a factor of great importance 

 in the resistance of cabbage to freezing. Those plants which have the 

 most bloom on the leaf surface are most resistant to the formation of 



