[October, 
58$ Influence of Heat and Light 
respiratory process — till lately very frequently overlooked in 
plant-life — requires the smallest share of heat, and goes on 
even in the total absence of light. The assimilation of 
carbon makes higher demands both for heat and light, whilst 
the formation of certain chemical compounds requires a 
still more liberal supply of both. 
These and other single physiological functions obtain a 
varying importance for the general life of the plant at its 
different vital epochs. Hence, as a necessary inference, the 
entire plant requires at its different stages different quanti- 
ties of heat and light. As a general rule, as a plant 
approaches and reaches maturity, the more heat and light it 
requires. If the maximum intensity of heat and light does 
not coincide with the last period of the plant’s life, but with 
the epoch of the most aCtive production of leaves, the total 
growth is checked ; the production, and consequently the 
development of the seed, is retarded. 
If in this last period the supply of heat and light is insuf- 
ficient, the reserve matter which has been accumulated in 
the leaves does not transmigrate completely to the seeds, 
but remains to a greater or less extent in the leaves, in the 
stalks, and in the seed-capsules. Hence the leaves do not 
wither, but remain green and juicy. In bad cases new 
shoots are pushed out, and the crop never ripens. 
As the intensity of light and heat increases regularly from 
winter to summer, and reaches its maximum from July to 
the middle of August, the vegetative times of plants should 
be so arranged that the epoch of the formation of seed may 
fall about the end of uly. The seed-times for the different 
crops are therefore dictated neither by custom nor by eco- 
nomic circumstances, but by natural laws, — the relations of 
the plant to the earth’s annual movements. 
It is therefore evident that the productiveness of a country 
depends not so much on its mean temperature as on the 
question whether its supply of heat is received at the right 
time. It has been demonstrated by other observers, pro- 
ceeding by the statistical method, that, other things being 
equal, the wheat-harvest in England is simply a function of 
the mean temperature of the months of July and August, 
unusual heat after harvest or in winter being, as far as corn- 
crops are concerned, simply wasted. 
Dr. Hellriegel illustrates his conclusions as to the coinci- 
dences of the epoch of maximum heat with that of the 
formation of seeds by the following experiments : — He 
sowed barley, under otherwise identical conditions, on April 
2ist, May 28th, June 28th, August 2nd, and September 1st. 
