112 



PRINCIPLES OF PLANT CULTURE 



the same as when a tree is exposed to the heat of a 

 fire. Sun-scald is most common in young trees, es- 

 pecially in those recently transplanted or overpruned. 

 It appears to be due, in some cases, 

 to the superheating of the cambium 

 in summer — in others to a re- 

 turn of severe freezing weather 

 after a period sufficiently warm 

 to excite the cambium cells to 

 activity. A preventive is to shade 

 the trunk and larger branches by 

 inclosing them with straw or simi- 

 lar material, or with a lath screen 

 (Fig. 59). 



186. Tip-burn of potatoes. — 

 Potato foliage is often injured by 

 sun heat in summer, as is shown 

 by the browning of the leaves from 

 the tip and edges toward the center, 

 or on the border of holes made by 

 insects. This affection, known as 

 tip-burn, is due to the destruction 

 of protoplasm in the cells and is 

 often mistaken for fungus work. 

 It is most serious in dry seasons. 

 No specific remedy for it is known, 

 but it may be in part avoided by 

 selecting varieties of potatoes least 

 subject to it, and in part by such 

 attention to soil and cultural prac- 

 tices as will tend to conserve the moisture during dry 

 weather. 



Fig. 59. — Trunk of 

 apple tree inclosed in 

 lath screen. 



