IS 



ACTION OF LIGHT ON VEGETATION. 669 



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 

 exhaling oxygen agrees still more exactly with the curve of brightness than 

 represented in Fig. 446, which was drawn from only a few data obtained 

 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 the following 

 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 efi'ect. 

 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 0-0005889 mm. 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. 652) 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 ^ 



{c) Formation of Starch in the Chlorophyll'^ . The yellow chlorophyll-grains 

 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 they 

 arc contained. It is only after they have assumed their green colour and under 

 the continued action of more intense light, in other w^ords under conditions favour- 

 able to assimilation, that the formation commences of the starch which is enclosed 

 within the chlorophyll-grains (see p. 46). When cells whose chlorophyll has already 

 produced starch on exposure to light are again placed in the dark, the starch is 

 absorbed and disappears completely from the chlorophyll-grains, and does so the 

 quicker the higher the temperature. If light is again allowed access starch is 

 again formed in the same chlorophyll-grains ; and the formation of starch is there- 

 fore a function of chlorophyll exposed to light, its absorption a function of chloro- 

 phyll not exposed to light. If complete or partial darkness is continued for a 

 length of time, the chlorophyll is usually itself destroyed; it first loses its form, is 

 then absorbed, and finally disappears from the cells together with the colourless 

 protoplasm ; in the case of leaves of rapidly growing Angiosperms this takes place 

 after a few days when the temperature is high. Cactus-stems with slow growth 

 and the shoots of Selaginella on the contrary remain green for months in the dark. 



The absorption and re-formation of starch in the chlorophyll— a process which 

 1 was the first to demonstrate in the leaves of Phanerogams— can be seen more 

 readily in Algoe of simple structure like Spirogyra, which may therefore serve for 

 purposes of investigation. I had already shown that the formation of starch in 



' 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. 



'' Sachs, Ueber die Auflosung und Wiederbildung des Amylums in den Chiorophyll-kurncrn bej 

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



