748 GENERAL CONDITIONS OF PLANT-LIFE. 



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

 the continued action of more intense light, in other words under conditions favour- 

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

 within the chlorophyll-granules (see p. 46). When cells whose chlorophyll has pro- 

 duced starch after exposure to light are placed in the dark, the starch is absorbed 

 and disappears completely from the chlorophyll-granules, and does so the quicker 

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

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

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

 of chlorophyll-granules not exposed to light. If complete or partial darkness is 

 continued for a length of time, the chlorophyll-granule 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 Angio- 

 sperms 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-granules — a pro- 

 cess which I was the first to demonstrate in the leaves of Phanerogams— can be seen 

 more readily in Algae of simple structure like Spirogjyra, which may therefore serve 

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

 chlorophyll-granules depends on conditions which favour assimilation, and that the 

 principal feature of this process, the evolution of oxygen, proceeds vigorously in light 

 transmitted through potassium bichromate, and consists therefore of red, orange, 

 yellow, and to a certain extent green rays ; while the more strongly refrangible half 

 of the spectrum, consisting of green, blue, violet, and ultra-violet rays, obtained by 

 passing the light through ammoniacal copper oxide, has only a very slight effect. 

 The conclusion at once followed from this, that the formation of starch must take 

 place in the set of rays first named to the same extent that it does in full sunlight, 

 but only to a very small extent in the latter set. This was confirmed by Famintzin's 

 experiments ^ in which he found that in Spirogyra the formation of starch in the 

 chlorophyll-granules took place only in the mixed yellow light (that had passed 

 through potassium bichromate), and not in the mixed blue light (that had passed 

 through ammoniacal copper oxide) in which the starch already formed even disap- 

 pears. Since however a small exhalation of oxygen takes place even in the mixed 

 blue light, it must be supposed that a small production of starch occurs in it. Kraus's 

 experiments^ with Spirogyra, Funaria, and Elodea, confirm this. He also found that 

 in plants of Spirogyra which had lost their starch from exposure to dark, the forma- 

 tion of this substance in the chlorophyll-granules recommenced in five minutes in 

 direct sunlight, in two hours in diffused daylight. In Funaria the formation of 

 starch recommenced in the same manner within two hours in direct sunlight, within 

 six hours in diffused daylight; and similar results were obtained with leaves of 

 Elodea, Lepidium, and Betula^. 



^ Famintzin, Action of Light on Spirogyra; Melanges biologiques, Petersburg 1865, Dec; and 

 1867, p. 277. 



^ Kraus, Jahrb. fiir wissensch. Bot. vol. VII. p. 51 r. 



' [From the observations of Weber (Ueb. specifische Assimilationsenergie, Arb. d. bot. Inst, in 



