OSMOTIC PRESSURES AND ELECTRICAL CONDUCTIVITIES 153 



between 2 p.m. and 4 p.m. during dry weather in the third 

 week of May. The research was carried out with the object 

 of ascertaining whether there was any correlation between 

 the freezing-point of the sap and the resistance of the 

 plants to a severe frost which occurred at the end of the 

 previous April. In view of the many factors which in- 

 fluence osmotic pressure in leaves, sufficing as they do to 

 cause alterations of nearly 100 per cent, within a few 

 hours in extreme instances, it is not surprising that no 

 very close parallel could be found between the two sets 

 of records. 



The small number of observations recorded by Marie 

 and Gatin (1912) also fail completely to make good the 

 validity of any such general relation between osmotic 

 pressure and resistance to cold. The not very marked 

 differences recorded by them between plants from the 

 mountains and the same species from the plains may be 

 readily accounted for as due to inequalities of illumination, 

 temperature, or age. In addition Dixon and Atkins (1910) 

 have shown that whilst storage of the pressed and filtered 

 sap alters its freezing-point but little within twenty-four 

 hours, yet the keeping of leaves in a closed vessel in the 

 dark usually leads to an increase in osmotic pressure, 

 probably owing to the mobilization of starch resulting in 

 the production of maltose. To such storage the plants 

 used by Marie and Gatin were subjected during trans- 

 mission from the mountains to Paris. 



The physiology of resistance to cold has, however, been 

 worked out by Maximo w in a series of papers, which he 

 has recently (1914) summarized. 



