1884.] Influence of Heat and Light upon Vegetation. 585 
for the wild birds against the modiste and the phimassier 
no less than against the Whitechapel bird-catcher and the 
Cockney sportsman. 
IV. THE INFLUENCE OF HEAT AND LIGHT 
UPON VEGETATION. 
R. HELLRIEGEL has been engaged for some years 
in studying the action of heat and light upon the 
development of plants. His results, which are very 
important both from a practical and a theoretical point of 
view, were made the subject of an independent work,* and 
have also appeared in an abridged form in certain German 
periodicals. 
He lays down the fundamental position that the stock of 
nutritious matter which is placed at the disposal of a plant 
is capable of being utilised and assimilated only when a 
given quantity of heat and light is simultaneously offered. 
The life of the plant lies within two well-defined limits of 
temperature. These limits differ not alone for every vege- 
table species, but for each physiological function. 
Near the lower limit of temperature all the vital functions 
of a plant are effected with extreme sluggishness, but as the 
heat augments the energy of vegetation increases, and 
reaches its maximum at a certain degree, which may be 
called the optimum temperature. If this limit is overstepped 
the vital actions of the plant become feebler again, — pro- 
bably by reason of abnormal processes and decompositions 
in the interior of the cells, — and cease entirely at a third 
upper limit. 
The optimum temperature for the different physiological 
functions ranges between 68° and 104° F. (20° to 40° C. ; 
temperatures above 122° F. (50° C.) have a destructive action 
upon the more highly-organised plants. 
Light behaves in a manner very similar to heat. In 
complete darkness no green (chlorophyilaceous) plant can 
* Beitrage zu den Naturwissenschaftlichen Grundlagen des Ackerbaues. 
Braunshweig. 
VOL. VI. (THIRD SERIES). 2 Q 
