Oct. 14. 1918 Hardening Process in Plants loi 



The midribs and petioles of fresh cabbage leaves were cut out. This 

 separation gave a more or less quantitative division of the tissue, although 

 a large amount of vascular tissue was still left in the leaf. Juice was ob- 

 tained from these portions by grinding them in a meat chopper and then 

 expressing. Samples of a large quantity of juice were then taken for 

 the determination of the freezing point and the acidity. The remainder 

 was frozen, and the concentrated juice expressed from the ice. This 

 concentrated juice was then sampled for freezing-point determinations 

 and for acidity. The graphs in figure 2 show on the y-axis the increase 

 in acidity on concentrating by the amount shown by the depression of 

 the freezing point given on the x-axis. Graphs 3 and 4 represent the 

 changes of acidity of juices expressed from cabbage leaves with the mid- 

 rib removed. Graphs i and 2 give the acidity changes for juices ex- 

 pressed from the midrib and petiole alone. The original acidity of the 

 juice from the midrib is a little less than that for the juice from the rest 

 of the leaf, although the freezing point is practically the same. On 

 concentrating, however, the acidity of the midrib juice does not increase 

 nearly so rapidly as that of the juice from nonvascular tissue. 



The concentrated juices were rediluted by adding the ice from which 

 they had been expressed and then allowing the mixture to thaw. There 

 is, of course, some loss in expressing, but this is comparatively small, as 

 shown by the freezing point values. Petiole juice having a freezing 

 point of —0.88° C. and an acidity of i.46Xio~^H+ was concentrated 

 to a freezing point of —1.94° and an acidity of 1.92 X io~*H"*', All of 

 the ice removed was saved and then remixed with the concentrated 

 juice, giving the rediluted juice a freezing point of —0.85° C. and an acid- 

 ity of o.96Xio"^H+. In a similar manner the values for juice from 

 the rest of the leaf were originally: Freezing point —0.90°, acidit)'- 

 2.5Xio"^H"''; after concentration, freezing point —1.96° C, acidity 

 5.45Xio"^H+; and on rediluting, freezing point —0.85° C, acidity 

 2.33X lo'^H''". The concentration was the same for both samples as 

 measured by the freezing point; still there was a much less change in 

 the acidity of the juice from the petiole than from the nonvascular tis- 

 sue. On rediluting, the acidity of the petiole juice is less than its original 

 value, while that for the rest of the leaf is practically the same. The 

 same amounts of acid were added to the same volume of juices expressed 

 from the midrib and from the rest of the leaf, and then the vials were 

 placed on ice. The petiole juice precipitated much more quickly than the 

 other sample. 



BUFFER EFFECT OF JUICE FROM MIDRIB AND REST OF LEAF 



The juice from the petiole and midrib of cabbage has a lesser buffer 

 effect than that from the rest of the leaf. This is shown on titration 

 with Njio sulphuric acid and Njio sodium hydroxid (fig. 3). The number 



