118 



Principles of Plant Culture. 



187. Potato Foliage is often Injured by Sun Heat in sum- 

 mer, 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 the work of fungus. It 

 is most serious in dry seasons. ]^o 

 remedy for it is known, but it may be 

 in part avoided by selecting varieties 

 least subject to it. 



B THE PLANT AS AFFECTED BY EX- 

 CESSIVE COLD 



188. The Immediate Effect of Cooling 



the Plant is to check the activity of 

 its vital processes. When a certain 

 degree of cold is reached, the proto- 

 plasm loses its power to imbibe water 

 (63); hence the plant tissues become 

 less turgid, and the foliage droops 

 somewhat. With a sufficient reduc- 

 tion of temperature, ice crystals form 

 within the tissues and the succulent 

 parts of the plant assume a glassy 



FIG. 59. Trunk of ap- A _ _. _ 



pie tree inclosed in lath appearance. The foliage of many 

 screen, plants, as celery, parsnip etc. , assumes 



an abnormal position when frozen. 



189. The More Water Plant Tissues Contain, the Sooner 

 they Freeze. Since the water of plants is not pure, but 

 is a solution of various substances, it does not freeze at 

 the freezing point of pure water (32 F. ), but at a lower 



