i7 
Leaf Surface of Cucumis sativus. 
Differentiating this with respect to time gives 
dA rA 
■ dt ~~ t 
We see that the rate of increase in area still depends 
r 
existing, but the constant r has been replaced by - • 
1 / 
(* 3 ) 
on the area already 
Hence in this case the 
rate of interest falls off continuously with time. In other words, we have 
a time factor at work, which continually tends to decrease the rate of 
expansion of the leaf surface of the plant. It is suggested that this detri- 
mental factor is bound up in some way with the low intensity of light and 
the shortness of the day during the winter months, since the detrimental 
factor does not come into action in March and June, and can hardly be 
Fig. 14. Equation of curve of closest fit : log A = 1-756 log/ — 0.354. 
a temperature effect as the average temperature in the greenhouse in March 
was only 1-5° C. higher than in December. 
The increase in total leaf area of a plant depends upon two factors : 
(1) the rate of production of successive leaves, (2) the rate of expansion of 
individual leaves. It is clear that the rate of production of successive leaves 
is a function of the activity of the vegetative point in which rudiments of 
the leaves are laid down, and one would expect that this would be regulated 
mainly by the temperature relations, though the rate of supply of available 
material for growth may act as a limiting factor. Thus plants which have 
large reserves of material will, when grown in the dark, produce rudimentary 
leaves until the reserves are exhausted ; these leaves, however, usually fail 
to expand. On the other hand, evidence has been produced above to show 
that the area of the leaf surface is closely related to the intensity of the 
incident radiation. 
Temperature, however, has another important effect on the plant, in 
