Plants as Affected by Cold. 121 



branches by inclosing them with straw or similar ma- 

 terial, or with a lath screen (Fig. 59). 



186. 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 cen- 

 ter, or on the border of holes made 

 by insects. This affection, known 

 as tip-burn, is due to the destruc- 

 tion of protoplasm in the cells and 

 is often mistaken for fungus work. 

 It is most serious in dry seasons. No 

 remedy for it is known, but it may 

 be in part avoided by selecting va- 

 rieties least subject to it. 



B — The Plant as Affected by 

 Excessive Cold. 



187. The Immediate Efifect of 

 Cooling the Plant is to check the 

 activity of its vital processes. When 

 a certain degree of cold is reached, 

 the protoplasm loses its power to 

 imbibe water (62) ; hence the plant 

 tissues become less turgid, and the 

 foliage droops somewhat (102). 

 With, a sufficient reduction of tem- 

 perature, ice crystals form within fig. 59. .Trunk of 



*^ ' " apple tree inclosed m 



the tissues and the succulent parts latt screen. 

 of the plant assume a glassy appearance. The foliage 

 of many plants, as celery, parsnip, etc., assumes an ab- 

 normal position when frozen. 



