<i W iiifrr Whad \ (iiicdts kikI Wijifi r-Killini/ 



tlian 2° C. and was corrected for in the usual way. By this method 

 (hiphcates often varied as much as 0-03° C, and in later collections it 

 was abandoned in favour of the standard Beckmann method, by which 

 it was always possible to obtain checks agreeing within O-Ol^ C. 



The work of Dixon and Atkins (lO), e.xtended by Gortner, Lawrence 

 and Harris (lu), has shown the necessity of rendering the cell membranes 

 permeable by freezing the tissue previous to sap extraction in order that 

 a representative sample may be obtained. However, having regard bo 

 the observation of Harvey (i!») that freezing permanently lowered the 

 hvdrogeu-ion concentration of cabbage juice, and (-1) that on the other 

 hand a certain amount of dilution did not affect this value, the samples 

 of the first collection were not frozen before, expressing the sap, as it 

 was desired to study particularly the relationship of this constant to 

 liardiness. fiut the following coni^Jarisons of sap exjiressed from duplicate 

 samples with and witliout previous freezing indicated that for wheat at 

 least Harvev's observation does not hold true. 



Therefore in hiter collections preliminary freezing of the tissues was 

 carried out, and the expressed sap used for all constants studied. 



The technique of (iortner and Harris (if)) was followed in the main. 

 The rubber-stoppered bottles containing the samples were packed in 

 a slushy mixture of ])ulverised ice and salt in an earthenware jar, which 

 fitted snugly inside a well-insulated "tireless cooker." Jn this condition 

 the contents remained frozen solid until required for use. never in any 

 case for less than \'2 hours. For the freezing mixture, coinmou salt was 

 used at first, and later calcium chloridi". To thaw th(> samples, tlie bottles 

 were placed under running water, then rinsed with distilled water and 

 wiped dry before ojiening. The leaves were folded in pieces of strong 

 cotton previously boiled in three changes of distilli'd water and dried 

 free from dust, and the sap expressed either in a hydraulic press under 

 400 atmospheres pressure, or in a large hand screw press with a small 

 steel cup which permitted the application of heavy pressure. The parts 

 of the press with which the juice came in contact w6re kept coated with 

 a thin layer of paraffin wax. 



The depression of the freezing point was first determined with a 

 Beckmann apparatus. Then the conductivity was measured with a, 



