V. The Inlluence of Light upon the Growlh of Leaves. 
115 
niore uniform. Sachs explains these phenomena by ascribing the decrease 
in the rapidity of growth which immediately succeeds the attainment of 
the maximum to the aclion of the increasing light, and by regarding the 
increase which follows the occurrence of the minimum as being due to 
the gradual diminution of this retarding action. It must be noliced, that 
the maximum does not fall wilhin the period of darkness, and that the 
minimum does not occur at the time, when the light is most intense. For 
the maximum is atlained some time after davvn, the minimum toward 
sunset. The explanation given of this is, that the action of light upon 
the growlh ol plants is a gradual one; its effect in producing retarda- 
tion is slowly manifesled, and as slowly disappears. Ilence the minimum 
of growth occurs in the afternoon after that daylight has acled upon the 
growing cells for several hours, and as the retarding influence gradually 
diminishes so does the rapidity of growth increase, until it reaches its 
maximum shortly after dawn, when the action of light begins again to 
make itself feit. 
A daily periodicity has been observed by Praxtl *) to occur in the 
growth of the leaves of Dicotyledonous plants. A comparison of his 
curve I wilh curves 5 and 6 of Sachs shew s that they are very similar, 
and at ouce suggests, that the growth of these leaves is influenced by the 
action of light in the same way as that of internodes. Prantl’s experi- 
menls. in which he varied the time of exposure to light, the results of 
which are given in his curves 2, 3 and 4, prove conclusively, that this 
Suggestion is correct. 
The same periodicity has been found by Stkehl 1 2 ) to occur in the 
When exposed to the light, it also grows 4 mm in an hour, the temperature being 
22«. — ln ocder to find the true value of the retarding effect exercised by light 
upon its growth, it is evidently necessary to estimate the effects due to the rise of 
temperature. 
Here, x = 4, 
then 
l = 20, and m : 
x 4 
t—m 20—10 
•due to eacb degree of temperature. 
ln the second case, x — 4, t = 
then X 
■ any number between 0 and 20; say m = 10; 
= 0-4, that is the uniform acceleration of growth 
22 and »i = 10 , 
4 ^ 
12 
= — = 0.3. 
t—m 22—10 
Here the uniform acceleration for each degree of temperature is 0.1 less than 
in the preceding case and this represents the value of the retarding action of light. 
This formula is empirical in so far that the increase of rapidity of growth has 
not been proved to be accurately proportional to the rise of temperature, and further, 
in that a value for m has to be assumed. Its value, as a means of eliminating varia- 
tions due to changes of temperature, cannot be doubted, as a comparison of the obser¬ 
ved and calculated curves on Sachs’ Plate VII will shew. 
1) Arb. des bot. Inst, in Würzburg. Heft III. 1873. 
2) Unters, üb. Längenwachsthum der Wurzeln. Diss. Leipzig 1874. 
8 * 
