OsMOTIC CONCENTRATION OF LEAF SAP 281 
It is perhaps evident from the preceding paragraphs that the 
four lines of investigation now most needful for a more complete 
understanding of the relationship between osmotic concentration 
and level of leaf insertion are: (a) more extensive measurements 
of the resistance opposed to the transpiration stream by the con- 
ducting tracts; (b) detailed investigations of the properties of the 
fluids of the transpiration stream at different levels in the trunk 
throughout the year; (c) measurements of the two chief atmos- 
pheric environmental factors, evaporating power of air and insola- 
tion at different levels; and (d) determination of the extent of the 
rdle played by the imbibition pressure of leaf colloids. 
The question of the resistance of the wood system has been 
touched on above. Further measurements for a great variety of 
species are needed. The importance for plant physiology in 
general of the investigation of the composition of the transpiration 
stream at different levels has been shown by Dixon and Atkins 
who have provided a method* and have published several series 
of measurements on trees examined during the winter and vernal 
season.f It seems rather improbable that such differences as 
they were able to demonstrate during the period of mobilization 
of reserve materials should be of great influence at the time at 
which our determinations were carried out. Nevertheless the 
subject deserves far more detailed study than Dixon and Atkins 
have been able to give it in their splendid pioneer work. 
Differences in the evaporating power of the air and in insolation 
would (theoretically) influence osmotic pressure by increasing the 
concentration of solutes in the leaves by evaporation and through 
differences in the rate of photosynthesis in varying illumination. 
* Dixon, H. H., & Atkins, W. R. G. Osmotic pressures in plants. IV. On the 
constituents and the concentration of the sap in the conducting tracts, and on the 
circulation of carbohydrates in plants. Sci. Proc. Roy. Dublin Soc. N. S. 14: 374- 
392. tI915. Also in Notes, Bot. Sch. Trin. Coll. Dublin 2: 275-293. 1916. 
Dixon, H. H., & Atkins, W. R. G. Osmotic pressures in plants. VI. On the 
composition of the sap of the Conanctag tracts of trees at different levels and at 
different seasons of the year. Sci. Proc. Roy. Heee Soc. N. S. 15: 51-62. 1916. 
Also in Notes, Bot. Sch. Trin, ok. ey 2: 335-346. 1916. 
t Note that ‘eis and Atkins find that the Sus of carbohydrates 
increases from lower to higher levels of the trunk. Possibly this may have some 
relation to our own findings with regard to the increase of osmotic concentration 
but the decrease of conductivity, or at least of the ratio of conductivity to freezing 
point lowering, in leaves inserted at higher levels. See discussion below 
