Oct. 14, 1918 Hardening Process in Plants 1 03 



allowed, lesser concentrations of acid precipitate the proteins. In fact, 

 if phosphoric acid is added until the hydrogen-ion concentration is 

 increased to the point to which it is increased by freezing, a precipitation 

 of the protein occurs in about the time necessary to kill plants by freezing. 

 However, changes in state much less than precipitation of the proteins 

 may be sufficient to cause changes in permeability or even the death of 

 protoplast. 



DIFFERENCES BETWEEN THE MIDRIB AND THE REST OF THE LEAF 



The foregoing data indicate that there is a difference between juices 

 expressed from the midrib and from the rest of the leaf. The proteins in 

 the midrib juice seem to be more easily precipitated by increase of acidity, 

 and the juice has a lesser buffer effect. Simultaneous with the precipita- 

 tion of proteins there is a decrease in acidity of the rediluted petiole 

 juice to a value less than the original. The change of acidity is comparable 

 to the conditions causing a change from red to blue in coleus leaves when 

 they are thawed out. In juice from the rest of the cabbage leaf this 

 does not occur to so great a degree. The irreversible precipitation of 

 proteins is held to agree with the greater injury or stimulation of the cells 

 of the midrib and those at the hydathodes than to those in the spaces 

 between the veins, as shown by tumor formation both on freezing and on 

 subjecting to ammonia fumes. It remains to be seen whether the cells 

 of the midrib in plants generally may not have greater powers of regen- 

 eration than other cells ; yet it is indicated by the more frequent regenera- 

 tion from such cells in preference to other areas. It is suggested also that 

 the point of regeneration may be determined by conditions similar to 

 the above on stimulation in a similar manner by acidity increased through 

 desiccation. 



PRECIPITATION OF THE PROTEINS OF HARDENED AND NONHARD- 

 ENED CABBAGE ON FREEZING 



The precipitation of proteins on freezing was found by Schaffnit (44) 

 to be greater in the case of juices expressed from greenhouse plants than 

 from plants taken from the open in winter. Chandler (5) doubts that 

 protein precipitation can be the cause of frost injury. However, his 

 results show that piotein precipitation occurs on freezing and in the 

 case of hardened succulent plants to a lesser degree than in nonhardened 

 plants. The differences in the precipitation of proteins given in his last 

 report (5, p. 186) are small, but he suggests that they are within the 

 range of experimental error. He does not state the relative hardiness 

 of the plants used, and it is entirely possible that they were not thor- 

 oughly hardened so as to be frozen without injury. 



The writer has determined the relative protein precipitation in hard- 

 ened and nonhardened cabbages. Plants from the same lot were grown 



