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



exposure to acid or alkaline vapors, to freezing, or to drying by excessive 

 transpiration from exposed areas. The factor active here seems to the 

 writer to be a change in the reaction of the plant juice, accompanied also 

 by an increase in the concentration of the salts of the cell sap. That a 

 change of acidity might occur from exposure to ammonia or acetic- 

 acid vapor is not difficult to see. The change in reaction of the plant 

 juice on concentration by transpiration is shown by the following experi- 

 rfient: 



Fresh cabbage leaves were taken and divided into two lots. The first 

 half was kept in a cbol, moist atmosphere; the other half was allowed to 

 wilt in a dry room until turgidity had been lost. Then both samples 

 were ground in a meat chopper, and the juice was expressed. The 

 acidjty of the leaf juice was increased by the wilting from 2.05 X lo"" to 

 5.37 X io~^ H"*". Evidence of the change of reaction on freezing will be 

 given later. 



By placing plants which have been injected in spots into tap water 

 for from 5 to 12 hours the formation of tumors can be stopped. This 

 inhibition was accomplished by inverting a potted cabbage over a beaker 

 of tap water so that all or part of the leaves were immersed. The coating 

 of wax on the leaves prevents their wetting by the water and holds a 

 layer of air about the leaf. This treatment effectually stops transpira- 

 tion, although it may also produce abnormal conditions, such as a poor 

 oxygen supply. When treated in this manner, the injected areas fail to 

 develop into tumors, and this hindrance is confined to those spots which 

 are immersed. It is interesting in this regard to note the old observation 

 of Sachs (42, 43) that slow thawing, as he regarded it, decreased frost 

 injury. His method was to place the frozen tissue into water and allow 

 it to thaw out slowly. Miiller (57, 39) showed that this process in fact 

 caused a very rapid thawing, owing to the rapid thermal exchange. 

 Hence, the lessened injury must rest on some basis other than slow thaw- 

 ing of the tissue. Greenhouse men have generally observed that frost 

 injury is much less if the frosted plants are at once sprinkled thoroughly. 



PLASMOLYTIC BEHAVIOR OF TUMOR CELLS 



The cells of young cabbage tumors are plasmolyzed at lower concen- 

 trations and in shorter time than the mesophyll cells in adjoining normal 

 tissue. Sections were made from fresh tumors, thin enough to be 

 observed by the low power of the microscope and still thick enough to 

 leave several uninjured cells. It was found to give greatest regularity 

 if the tumor cells were compared with the normal mesophyll cells from 

 the same section for their plasmolytic behavior. Observations on the 

 plasmolytic limits of such tissues are very difficult and are mainly com- 

 parative rather than to be taken as a measure of actual osmotic pressure 

 or permeability. 



