118 



Principles of Plant Culture. 



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

 mer, as is shoTin 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. ISTo 

 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- 

 CESSi\'E 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- 

 r f tion of temperature, ice crystals form 

 ifj' within the tissues and the succulent 

 parts of the plant assume a glassy 



Fig. 59. Trunk of ap- "^ rr^, j- i ■ j- 



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 



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