RESEARCH METHODS IX STUD! OF FOREST ENVIRONMENT. 105 
the species, the following correlation between the two measures is 
given tentatively : 
Sap 
density. 
10 
Osmotic 
pressure. 
Atmosphere. 
4.5 
10.0 
15 15.7 
20 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. When 10 to 15 grams of material has been secured, the flask 
is 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 be avoided by making 
all weighing with the stoppers removed. 
2. The flasks are now placed in the drying oven for a period of not 
less than 12 hours. It Avill 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. 
3. Each flask now has added to it distilled water to the extent of 
five times the weight of the green plant material and is then again 
placed in the oven for an hour, the temperature attained in this time 
