Plants as Affected by Cold. II3 
18%. Potato Foliage is often Injured by Sun Heat in 
summer, as is shown by the browning of the leaves from the 
$ (j tip and edges toward the center, or on 
i f/the border of holes made by insects. 
y This affection, known as “ tip-burn,” is 
: v// ¥ due to the destruction of the protoplasm 
| : in the cells, and is often mistaken for 
(23the work of a fungus. Itis most serious 
(ad in dry seasons, No 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 EXCESSIVE 
CoLp 
18S. 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 
te Hi Me ce temperature, ice crystals form within 
Siete 
wn i by ae “the tissues and the succulent parts of 
a eee nail ,. 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 
