40 Peach-Growing 



"bottoms" none occurs and perhaps tender vegetation is 

 green for days or even weeks in the autumn after all but 

 the hardiest plants in the low places are dead from 

 frosts. 



The stratification of air according to its temperature is 

 visualized in a remarkable manner in Plate IV. It shows an 

 orange tree injured by a low temperature in December, as 

 it appeared the following February. It is well known that 

 when green foliage is entirely killed by a sudden and very 

 severe freeze or frost, it will dry up on the tree and may 

 cling to it for a long time; whereas a temperature that is 

 barely low enough to kill the foliage will result in the leaves 

 dropping at once. These conditions are indicated in Plate IV. 

 The leaves on the lower branches were so severely frozen 

 that they dried up and remained on the tree. The stratum 

 of air having this severe temperature apparently extended 

 from the ground to a level just above the height of a man's 

 head. In the middle section of the tree, the temperature 

 was low enough only to cause the foliage to drop, while at 

 the level of the upper third of the top no injury is apparent. 

 Thus, within the height of the tree there were strata of air 

 having three different temperatures, each resulting in a 

 different expression in terms of tree life. In reality these 

 results were due directly to differences in elevation. 



The same expression is indicated in Plate III. The orchard 

 is situated on the slope of a mountain ridge. The illustra- 

 tion was made early in the morning when there was no wind, 

 but a current of cold air settling down the slope to a lower 

 level is carrying with it the smoke from a burning stump. 



Attention may now be called to the fact that topography 

 refers to the general conformation of the surface. In ref- 

 erence to orchard sites, it has to do with the physical fea- 



