86 Journal of Agricultural Research voi. xv. no. 2 



ice within the tissue. Cabbages which are well covered with wax show 

 no indications of freezing after several hours' exposure to a temperature 

 5 degrees below that at which ordinary plants show ice formation. 



When one considers the physical mechanism of freezing in the leaf, 

 the cause of the resistance is found to be this coating of wax. In the 

 natural state moisture is usually present on the surface of leaves, even 

 though in minute quantities. This may be deposited by condensation 

 from the surrounding atmosphere. In cooling down to only slightly 

 below zero this water freezes, for the plant is seldom motionless enough 

 to allow much undercooling, as shown by Voigtlander (47). In plants 

 which have but little wax greater amounts of moisture stick to the leaves, 

 while those covered with wax are not wet. This can be observed if 

 such leaves are immersed in water. Those with a thick coating of wax 

 have a bright silvery sheen, owing to the lack of wetting of the leaf sur- 

 face. If the wax is rubbed ofiF, the surface then becomes wet and loses 

 its ability to form a mirror surface. 



Water which freezes on the leaf surface serves to inoculate the under- 

 cooled solution within the leaf; in fact, the injected spots observed 

 are caused by this inoculation. When once begun, the freezing process 

 is transmitted rapidly through the undercooled leaf tissue, and the 

 frozen spot enlarges until the whole leaf may be frozen. That water 

 on the surface may cause the inoculation is easily shown by placing a 

 drop of waler on the leaf and exposing it to — 3° C, when the area beneath 

 the drop will be found to freeze first and show injection. Inoculation 

 probably takes place through stomata or through cracks in the wax and 

 will take place less frequently if the wax is thick. 



Bigelow and Rykenboer (2) have recently shown that a very great 

 undercooling can occur in capillar}' tubes. Stomata and cracks in the 

 waxy covering of the leaf are of small enough dimensions to allow con- 

 siderable undercooling without ice formation taking place through them 

 to inoculate the tissue beneath. It would appear consequently that 

 where inoculation occurs the openings are largest. 



Freezing in spots is of common occurrence in plants. It has been 

 observed to occur on cabbages when they are exposed to frost in the 

 open. The injected areas have been produced on various greenhouse 

 plants, including begonia, salvia, geranitun, coleus, bryophyllum, lettuce, 

 sunflower, hydrangea, and Aucuha japonica. 



Unhardened cabbage leaves survive ice formation within the tissue 

 when the injected area does not cover too great a part of the surface. 

 Only slight plasmolysis can be observed in sections of the injected spots 

 in this case. On fixing with acid alcohol (Camoy's solution) cabbage 

 leaves which have been frozen in spots, large masses of spherocrystals 

 were found, quite sharply limited to the injected areas and often not to 

 be found in the rest of the tissue. These crystals (PI. 8, B) appear to be 



