RESEARCH METHODS IjST STUDY OF FOREST ENVIRONMENT. 105 



the species, the following correlation between the two measures is 

 given tentatively : 



Sap 



density. 



Osmotic 

 pressure. 



Per cent. 



5 



10 



15 



20 



Atmosphere. 



4.5 



10.0 



15.7 



22.4 



For rough approximations, the osmotic pressure in atmospheres 

 may be considered equal to the sap-density percentage for plants of 

 this class. It is probable that further data will bring out specific 

 differences worthy of consideration. It is, therefore, believed that 

 where freezing-point determinations are impracticable because of 

 lack of apparatus, or of freezing mixtures; or when, as frequently 

 happens with the foliage of conifers, the material is so dry that even 

 with grinding it lacks free moisture, so that a distinct end point can 

 not be secured, the sap-density method may be of very great assist- 

 ance. 



After considerable experimentation with a number of methods giv- 

 ing essentially the same results, the following simple practice has 

 been developed, which is designed primarily to eliminate the need for 

 evaporating the large volume of water used in extracting the 

 solutes: it also greatly reduces the opportunity for loss of material 

 during the operation. 



1. The plant material, usually consisting of the more exposed and 

 consequently the drier portion of the needles, is secured by carrying 

 into the field the desired number of wide-mouthed liter flasks, a fun- 

 nel, and a pair of shears. The plant, or branch of a tree, is held over 

 the funnel, and the leaves are snipped off in sections not over one- 

 half inch long, the outer one-half to two-thirds of all needles being 

 taken. A\'l j «'i i 10 to 15 grams of material has been secured, the flask 

 stoppered. As soon as a collection has been completed, the flasks 

 are taken in and weighed with their contents, the flask weights hav- 

 ing previously been recorded. Confusion may lie avoided by making 

 all weighing with the stoppers removed. 



•i. The flasks are now placed in (lie drying oven for a period of not 

 less than \-i hours. It will usually be found convenient to have the 

 specimens ready for extraction early in the morning. At this stage 

 the weight of flask and dry contents is secured, and by the difference 

 between this and the earlier weight, the original water content, is 

 obtained directly. 



.".. Bach flask now has added to it distilled water to the extent, of 

 five time-, the weight of the green plan! materia] and is then again 

 placed in the oven lor an hour, the temperature attained in this time. 



