ACTION OF LIGHT ON VEGETATION. 
747 
If allowance is made for the small error mentioned above incident to the 
method of counting the number of bubbles, we find that the curve of capacity for 
exhaHng oxygen agrees still more exactly with the curve of brightness than is 
represented in Fig. 475, which was drawn from only a few data obtained with 
difficulty. 
Since a comparison of the curve of brightness with that of the evolution of 
oxygen, otherwise convenient, has turned the attention of observers in a wrong 
path, and has led to many erroneous theories, it will be convenient to state the 
only relation between the two with which we have to do here, in precise 
terms : — The evolution of oxygen caused by chlorophyll is a function of the 
length of the waves of light ; only those wave-lengths which are not greater than 
0'ooo6866mm. and not less than 0-0003968 mm. being able to produce this effect. 
Starting from the two extremes, the capacity of light for causing evolution of 
oxygen rises till it reaches its maximum at a wave-length of o'ooosBSQmm. Or, 
starting with the medium wave-lengths of the coloured region of the spectrum 
measured in hundred-thousandths of millimetres, the evolution of oxygen is effected 
by waves of light of a minimum length of 39 ; it increases with the increase of wave- 
length until the latter reaches about 59 ; it then diminishes if the wave-length con- 
tinues to increase until it entirely ceases when the wave-length is 68. It will be at 
once seen that we have here a similar phenomenon to that of the relation of vegetation 
to temperature ; for we found (see p. 729) that this function also rises with the rise of 
temperature, attains a maximum at a definite temperature, and again decreases as 
the temperature rises still higher ^ 
Godlewski^ obtained the following results by a long series of eudiometric ex- 
periments as to the influence of the percentage of carbonic acid in the air upon the 
extent of the decomposition of this gas and upon the corresponding evolution of 
oxygen. An increase of the amount of carbonic acid present in the air, up to a 
certain limit (optimum), increases the evolution of oxygen, an increase beyond this 
limit diminishes it. The limit is different for different plants ; for Glyceria speciahilis 
on bright days it was from 8-100/0; for Typha latifolia from 5-7% ; for the Oleander 
probably rather lower. The increase of the evolution of oxygen consequent upon 
an increased amount of carbonic acid being in the air is much greater than the 
diminution produced when the optimum is exceeded by an equal amount. The 
greater the intensity of light, the more is the evolution of oxygen promoted by an 
increase of the carbonic acid up to the optimum, and the less is it diminished by 
excess. It follows that the influence of light upon the evolution of oxygen is the 
greater the more carbonic acid is contained in the air. 
(r) Fonnaiion of Starch in Chlorophyll-granules'^ . The yellow chlorophyll (etiolin)- 
granules formed in the dark are small ; after turning green on exposure to light they 
become considerably larger, corresponding to the increase in size of the cells in which 
^ The same law of dependence is also evidently applicable to the sensitiveness of the eye to 
brightness ; and this is the cause of the curve of the brightness of light running nearly parallel to 
that of the evolution of oxygen, 
2 Godlew^ski, Arb. d. Bot. Instituts in Würzburg, Heft 3, 1873. 
^ vSachs, Ueber die Auflösung und Wiederbildung des Amylums in den Chlorophyll-körnern bei 
wechselnder Beleuchtung: Bot. Zeitg. 1864, p. 289. 
