26 HORTICULTURAL SOCIETY OF NEW YORK. 



The most important factor of all, however, is the question 

 of temperature, especially that of the minimum winter tempera- 

 ture. Two classes of plants can be distinguished here, viz., 

 those that are killed by a more or less long-continued period of 

 cool nights which do not reach the freezing temperature, and 

 those which endure such periods without injury but which are 

 killed or seriously injured by a comparatively slight and short 

 drop in temperature below the freezing point. It is of certain 

 factors affecting the acclimatization of this latter group that I 

 wish to speak. 



It has been long recognized that the slope of the land has a 

 great bearing upon the liability to injury of fruit trees from late 

 spring frosts. So, for example, where danger of frost injury 

 to peaches is to be feared, the orchards are planted on hill slopes 

 or near steep inclines which will drain away the cold air. Dur- 

 ing the winter this air drainage is of no avail since the whole 

 mass of air is cold. In the spring and fall, however, when the 

 general mass of air is warm the air next to the ground is cooled 

 by radiation, forming a cold layer of limited depth. 



In southern Florida, e. g., in the vicinity of Miami, the tem- 

 perature of the mass of air never falls low enough to cause death 

 of plants by freezing. By radiation, however, the layers of air 

 next to the ground sometimes become colder than freezing tem- 

 perature. It is under such conditions that air drainage becomes 

 a factor in acclimatization. If the general air mass should 

 occasionally become cooled below freezing temperature, those 

 plants mentioned above which are subject to injury by such 

 temperature would be killed out, regardless of location, but since, 

 at Miami, the cold air is only of limited depth the location deter- 

 mines whether the plant is killed or not. 



The principles stated a*bove were clearly demonstrated at 

 Miami last winter (1906). During the daytime of December 

 23d, 24th and 25th, there was a heavy, cool northwest wind, but 

 the sun shone brightly and the temperature remained about 50* F. 

 At sunset each of these days the wind ceased almost absolutely, 

 and the temperature began to fall at once. On the morning 

 of the 24th, at the Subtropical Laboratory building about seven 

 feet from the ground, the temperature was 32 F, ; on the 25th, 



