14 Wliitir Wlnnt Vftrirflcs hikI \Vi iiI< r-f\illing 



latter author pictures plant protoplasm as a pentosan-proteiu colloid, 

 and considers that the character and amount of the pentosans largely 

 determine the hydration reactions of the protoplast. Winter conditions 

 where the soil freezes solid are really xerophytic conditions, since the 

 plant's usual water supply is cut ofT. Having this in mind, it may be 

 reasoned from the evidence of MacDougal and Rpoehr that under winter 

 conditions pentosans would accumulate in the cell, contributing largely 

 to the formation of a protoplasmic gel of high imbibitioiml powers. 

 Investigation of this point will be reserved for a later paper. 



The relative importance of sugars and electrolytes in osmotic pressure 

 is considered in Table IV. Unfortunately there is no known method 

 whereby the relative proportions of osmotic pressure contributed by 

 electrolytes and non-electrolytes can be calculated accuratelv from con- 

 ductivity and freezing point data. These proportions have sometimes 

 been estimated by taking tlie value for electrolytes as equal to that of 

 a solution of potassium chloride having the same conductivity. But it is 

 manifestly unjustifiable, as Miss Haynes(22) has pointed out, to assume 

 that the cell sap electrolytes are dissociated to the same extent as potas- 

 sium chloride, and produce ions of the same mobility. In the present 

 instance the sugar percentages were known and these were assumed to 

 represent the non-electrolyte materials. The theoretical osmotic pressure 

 exerted by the sugars is calculated from the quantities of reducing sugars 

 and sucrose determined by analysis to be present. The difTerence between 

 this value P^ and the total osmotic pressure P (i.e. P— P,) may be 

 attributed chiefly, though probably not entirely, to electrolytes. In the 

 coliec'tion of November 12, the sugars present were sufficient to account 

 for an average of about .38 per cent, of the osmotic pressure: in that of 

 December 9, for about 34 per cent. 



The probable effect of non-electrolytes in depressing the conductivity 

 cannot be determined by ordinary viscosity measurements, as these do 

 not distinguish between substances in molecular solution and colloids, 

 and the latter substances, as is well known, do not interfere appreciably 

 with conductivity^. However, the correction suggested by Miss Haynes 

 for the depression of the conductivity by the sugar content of the sap 

 has been applied to the observed values. The corrected specific con- 

 d)i(tivity A'„ is derived from the observed \-;iIue K . hx the formula 



lOOA" 

 '""^lOO-oa;' 

 where n = 2 (constant for sugar) and x — per cent, sugar in sap. 



' Fur i'xam])l(', spp Af.rson (2H), Table V. 



