Plants as Affected by Cold. 113 
18%. Potato Foliage is often Injured by Sun Heat in 
summer, as is shown by the browning of the leaves from the 
(7 tip and edges toward the center, or on 
: f/the border of holes made by insects. 
This affection, known as “ tip-burn,” is 
due to the destruction of the protoplasm 
in the cells, and is often mistaken for 
(3 the work of a fungus. Itis most serious 
in dry seasons. No remedy for it is 
known, but it may be in part avoided by 
selecting varieties least subject to it. 
B— Tue Puant aS AFFECTED BY EXCESSIVE 
CoLp 
188. The Immediate Effect of 
Cooling the Plant is to check the ac- 
tivity of its vital processes. When a 
certain degree of cold is reached, the 
protoplasm loses its power to imbibe 
water (63); hence the plant tissues be- 
come less turgid, and the foliage droops 
’ somewhat. With a sufficient reduction 
wh We an temperature, ice crystals form within 
ythe tissues and the succulent parts of 
Fic. 58. Appletree in. the plant assume a glassy appearance. 
cased in lath sereen. The foliage of many plants, as celery, 
parsnip etc., assumes an abnormal position when frozen. 
189. The More Water Plant Tissue Contains, the 
Sooner it Freezes. 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 
