EESEAECH METHODS IN STUDY OF FOREST ENVIRONMENT. 83 
be given for the formaldehyde to evaporate entirely. This Avill 
doubtless occur before the soil is perfectly dry. 
When germination is fairly complete, the seedlings well established 
so as to reach all parts of the soil, and the tendency to succumb to 
damping off, if any, outgrown, the surfaces of the pans are sealed 
over by pouring on the top of the soil, previously leveled, about 50 
grams of a melted mixture of paraffin and petrolatum (veterinary 
vaseline is one of the least expensive forms) in the proportion of 
2 : 1. This congeals at 40° C. and may be applied at 50° C. without 
any injury to the stems of the seedlings. Not infrequently, if the 
wilting process requires many days, the seal will draw away from 
the edges of the pan, but this is easily rectified by the use of a blunt, 
smooth stick. At any rate, it is not essential absolutely to prevent 
direct evaporation from the soil, though a more even distribution of 
moisture may be expected if such loss is kept at a minimum. 
The weight of paraffin added is determined by weighing the beaker 
from which it is poured before and after each application. This 
makes a further addition to the tare. The soil should be fairly moist 
when the paraffin is applied, so that the latter will not penetrate. 
With coniferous seedlings, provided a good stand has been 
secured, the withdrawal of moisture and the sealing of the pans 
may usually be undertaken at four to six weeks after sowing ; though 
in the case of spruce and perhaps other species which root rather 
slowly a slightly longer perior may be desirable. As has been 
pointed out, the difficulty of detecting wilting increases as the seed- 
lings become older and more completely lignified. It is also un- 
mistakably true that the older the seedling the more difficult it is 
to kill. This is probably due in part to greater resistance to drying 
out and in part to deeper or more extensive rooting, which would be 
an advantage if the moisture at, say, the bottom of the pan, were 
not being drawn on as freely as that near the surface. However, 
observations on the wilting of seedlings under direct insolation point 
unmistakably to resistance increasing with age. When the surface 
of the soil becomes extremely warm, even if there is an abundance 
of moisture within reach of the roots, wilting is likely to be evidenced 
by collapse of the stem at the ground line. The phenomenon is 
almost identical when the surface of the soil becomes dry in advance 
of the deeper soil. The seedling is undoubtedly vulnerable to water 
loss and critical injury in the lower part of the stem. Under such 
conditions it is noted that the younger seedlings usually succumb 
first, and those which survive one exposure are killed by a repetition 
which is still more severe. 
It is evident, therefore, that age of seedlings will have an import- 
ant influence on the results, though this will not be so important if 
