[October, 
586 Influence of Heat and Light 
prolong its normal existence. The germinative process 
alone is effected better in darkness than in the light. If a 
plant which has passed this stage is completely secluded 
from light the formation of chlorophyll is prevented, and 
any which has been previously formed is destroyed. The 
plant becomes pale, the stem and leaves assume a quite 
anomalous form, and the amount of total dry matter 
dccrcssGSi 
The most important phase of the nutrition of plants, the 
decomposition of carbon dioxide (carbonic acid) and the 
assimilation of the carbon in the chlorophyllaceous cells, is 
purely a function of light, — in other words, it increases 01 
decreases with the increase or decrease of light. 
In a faint light the process of assimilation is feeble and 
tardy; it increases with the intensity of light, and reaches 
an optimum point, which, however, does not coincide with 
the maximum of light. Heat and light, together with the 
rainfall, form that complex fadtor in fertility which is under- 
stood by the terms “ weather” and “ climate.” This fadtor 
determines the quantity of crops more frequently, and to a 
greater degree, than the natural fadtor known as fertility of 
the soil, and the artificial fadtor of manures. 
It is generally supposed that the lowest limit at which 
the majority of our field-plants are capable of germinating 
is about 39 0 to 41 0 F. (4 0 to 5 0 C.). Observation, however, 
proves that this limit lies lower, approaching 32" F. (o° C.). 
To decide this point Dr. Hellriegel, in the winter 
1871-72, sowed the seeds of eighteen plants which' he wished 
to study in a number of flower-pots. No. 1 was kept at the 
constant temperature of 8*7° C., No. 2 at 5 0 , No. 3 at 3 0 , 
No. 4 at 2°, and No. 5 at o° C. The soil and the proportion 
of water were alike in every case. In this manner the pots 
were allowed to stand for 55 to 60 days. The temperature 
of the soil was determined at different times of each day, 
and the number of seedlings was noted. 
At the end of the experiments the young plants and the 
seeds which had not germinated were washed out of the 
soil, and more closely examined. The seeds experimented 
on were winter rye, winter wheat, barley, oats, maize, rape, 
flax, tares, peas, horse-beans, clover, carrots, cress, and 
cucumbers. Winter rye was found to require the lowest 
temperature for germination. It sprouted at 32° F. (o° C.) 
quite normally, and developed both its seed-leaf and root- 
germ, though slowly. Winter wheat came next ; it germi- 
nated at the same temperature, though more slowly than 
rye. Barley and oats required more heat ; they put forth 
