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 ^he 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*0006866 mm. 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"ooo5889 mm. 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 speciabilis 

 on bright days it was from 8-100/0; for Typha laiifolia 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. 



(c) Formation 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 



1 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 Godlewski, Arb. d. Bot. Instituts in Wlirzburg, Heft 3, 1873. 



3 Sachs, Ueber die Auflosung und Wiederbildung des Amylums in den Chlorophyll-kornern bei 

 wechselnder Beleuchtung : Bot. Zeitg. 1864, p. 289. 



