RESEARCH METHODS IN STUDY OF FOREST ENVIRONMENT. 83 



be given for the formaldehyde to evaporate entirely. This will 

 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. arid 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 slightty 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 stern at the ground line. The phenomenon is 

 almost identical when the surface of the soil becomes dry iii advance 

 of tin- deeper soil. The seedling is undoubtedly vulnerable to water 

 loss and critical injury in the lower part of the stem. Under such 

 conditions i' IS noted that the younger seedlings usually succumb 

 first, and those which survive one exposure are killed by a repetition 

 which i - -t ill more severe. 



It i- evident, therefore, that age of seedlings will have an import- 

 ant influence on the results, though this will not he so important if 



