106 BULLETIN 1059, U. S. DEPARTMENT OF AGRICULTURE. 



being practically that of boiling water, which will greatly facilitate 

 the diffusion of the solutes. While this warming is being accom- 

 plished, a filter may be prepared, corresponding to each flask. The 

 filter papers are dried and weighed before using, and their weights 

 credited to the samples with which they will be used. At the end 

 of the hour the liquid contents of each flask are poured into their 

 appropriate filters. 



4. Water is again added to each flask, in the same amount as be- 

 fore, and the process repeated. After the third extraction, with 

 possibly a little cold rinsing of the pulp, filter, etc., the filter, with 

 whatever solids it has accumulated, is placed in the flask, and this 

 is returned to the oven for its final drying. After this, making allow- 

 ance for the filter-paper weight that has been added, the loss of 

 solutes is readily computed. It should also be realized that this loss 

 will include a small proportion of water which was hygroscopically 

 held by the solutes in the previous drying. Such loss, however, will 

 possibly compensate for solutes not removed. While the three ex- 

 tractions, theoretically, should remove more than 99.9 per cent of 

 the solids, it is probable that they fall appreciably short of this. 



Since it is believed that investigations along this line will de- 

 velop increasing importance in forest ecology, it seems advisable to 

 make available a table of osmotic pressures for freezing-point de- 

 pressions to 5.999° C, as worked out by J. A. Harris and published 

 in the American Journal of Botany, 2 : 418-419, 1915. This is an 

 extension of the work begun by Harris and Gortner in 1914. The 

 table will be found in the appendix. 



Of almost equal ecological importance with the increase in osmotic 

 pressure and absorbing capacity which accompanies greater concen- 

 tration of the cell sap, is, perhaps, the very great decrease in the prob- 

 able rate of evaporation from the leaves. It is especially desired to 

 call attention to this, since the earlier announcements of the findings 

 of physical chemistry have led many biologists to believe that 

 a considerable change in the osmotic pressure of the plant 

 solution could have little effect on evaporation. Thus Livingston 

 (130), in 1911, argued that the greatest concentration of the cell sap 

 would only create a depression of 10 per cent in the vapor pressure 

 over the solution, and consequently could have no important effect 

 on the evaporation rate. 



As early as 1915, Bates (105) had observed in the artificial drying 

 of pine cones, for which a calorimetric kiln was used, a very great 

 increase in the amount of heat consumed as the drying advanced. In 

 certain instances this was nearly three times as great, per unit of 

 water evaporated, in the final stages as when beginning with very 

 green cones. When, therefore, he found, in 1917, a great decrease 

 in the transpiration rate of those species of conifers which showed 



