RELATIVE RESISTANCE OF TREE SEEDLINGS TO EXCESSIVE HEAT 7 



bulb partly covered by the soil, the other suspended in the air with 

 the bulb iii the same plane as the " crowns " of the seedlings. Ex- 

 posures were for five minutes or more, but in all cases so short 

 as to cause little drying of the soil. In consequence, the thermometer 

 exposed in the air and nearer to the heating coil than the soil ther- 

 mometer always recorded the higher maximum temperature. Since 

 the injury was almost wholly to the leaves of the seedlings, it is this 

 temperature of the air thermometer, receiving the same direct radia- 

 tion as the seedlings, which Avill be referred to. The temperatures 

 were observed throughout the period of test, and the maxima, which 

 ordinarily were obtained near the end of each test, were recorded. In 

 the tables all temperatures noted are the maxima, in degrees Fahren- 

 heit. 



After each subtest the pot was watered and kept shaded for a. 

 half day or so to note any immediate results, after which it was trans- 

 ferred to the seed-testing table in the greenhouse, where it attained a 

 daily maximum temperature of approximately 77° F. at a depth of 1 

 inch in the soil, or about 85° F. at a depth of one-fourth inch. Daily 

 observations were made to note wilting. 



The third test, made 64 days after sowing, employed four pots 

 which had been used in the second test and had lost no seedlings in 

 the exposures of that test. The seedlings were subjected to the direct 

 radiation for periods of 6, 8, 10, and 12 minutes, at such distance 

 from the coil as to produce in all cases maximum temperatures of 

 about 150° F. 



For the fourth and final test, 90 to 92 days after sowing, the same 

 procedure was employed as for the second test; except that, instead 

 of placing the seedlings in rows, the four species were scattered pro- 

 miscuously, using as before six seedlings of each species in each pot. 

 The pots were exposed several days before testing to make certain 

 that the seedlings were well established, and the final tests used the 

 electric coil in conjunction with solar heat. The period of time 

 varied from 12 to 128 minutes, and the thermometer recorded air tem- 

 peratures from 110.84° to 161.06° F. 



After the completion of the fourth test on September 30, the pots 

 were placed under observation until November 17, since the seedlings 

 by this time had become hardened to the extent that many of them 

 survived for several days. The immediate results in each test were 

 noted for each species; such as, two killed, four badly cooked, one 

 slightly injured. The judgment formed at this time as to the extent 

 of injury, however, had no bearing on the final results. The seedlings 

 were not removed from the pots until obviously dead, and none were 

 removed until the second day. At the expiration of the periods of 

 observation in each of the three tests, the seedlings were classified as 

 dead; injured, will eventually die; injured, will recover; and unin- 

 jured. 



RESULTS IN MOIST AIR 30 DAYS AFTER SOWING. 



In test 1, the seedlings were subjected to hot-air temperatures in a 

 saturated atmosphere, ranging from 101° F. for two minutes, which 

 caused no loss, to 150.5° F. for four minutes, which caused the death 

 of all seedlings. The results are shown in Table 1. where the subtests 

 are arranged in the order of increasing temperatures. 



