86 BOTANY. 
and prevent starch-making. -At the expiration of a fortnight ex- 
amine and compare the tubers of the two plants. 
(f) Germinate a handful of Indian corn in moist clean sand, and, 
as the plants grow, taste the kernels from time to time. The sweet 
taste shows that the starch has changed into sugar for the nourish- 
ment of the growing plants. 
(g) Cut off a stem of geranium and apply the moist surface to a 
bit of blue litmus-paper. The latter will turn red on account of the 
presence of an acid in the water of the cells. 
171. Temperature.—It may readily be seen that plants 
are active only between certain temperatures. There is 
for every plant a certain highest temperature (maximum) 
beyond which there is no activity. Likewise there is also 
a lowest temperature (minimum) below which activity 
ceases. Between these there is a best temperature (opti- 
mum) at which the plant is most active. We have thus 
three temperatures which should be studied for each plant, 
viz., lowest, best, highest, or, as they are commonly referred 
to in botanical works, the minimum, optimum, and mazi- 
mum. 
172. The lowest temperature for plants ranges from near 
the freezing point of water to 10 or 15 degrees Cent. above 
it (32° to 50° or 60° Fahr.). It is not the same for differ- 
ent plants, some being active at much lower temperatures 
than others. Moreover, in each plant, the lowest tempera- 
ture varies for the different parts; thus roots are active at 
much lower temperatures than leaves. As a rule, also, 
metastasis can take place at lower temperatures than as- 
similation. 
173. The highest temperature for plant-activity ranges 
from about 35° to 50° Cent. (95° to 122° Fahr.), varying 
somewhat for different plants, and varying also for differ- 
ent parts or different functions of the same plant. 
174. The best temperature varies still more than the 
