502 



the disappearing chloroplasts. The further the cells of the leaf flesh are 

 separated from the brown upper surface, the more transitions are noticed 

 from these reddened cytoplasmic masses to the normal chloroplasts. 



All these changed tissues may, in many cases, be brought back to the 

 normal color, if cut branches are brought into a warm place. In this, how- 

 ever, the intensity of the light is not increased, and this may explain why 

 only a lowering of the temperature should, in general, be considered as the 

 cause of the autumn coloring. A further proof lies in the fact that in the 

 autumn natural ripening only the ripened places, i. e., the places most cooled 

 down by heat radiation, change their color, while the parts inside the top of 

 the tree and co\^ered by the outer leaves, show no change. 



In regard to the change in the coloring matter of the chlorophyll, it has 

 been proved by Frank^ and Wiesner- that the chlorophyll passes over into a 

 substance which Pringsheim called "Hypochlorin"'-\ This is an oily body, 

 usually dark colored, which is produced from chloroplasts by the action of 

 anorganic and organic acids and finally crystallizes into needles or whip-like 

 brown crystals. Tschirch^ has proved that this hypochlorin is identical with 

 the "Chlorophyllan" of Hoppe-Seyler and that it should be considered as the 

 first product of the oxidation of the chlorophyll (and in fact of only one 

 part of the raw chlorophyll, viz., the cyanophyli of G. Kraus). This product 

 is formed of itself if a chlorophyll solution is left standing for some time'. 



Tschirch found that the formation of chlorophyllan or hypochlorin, 

 increasing according to the amount of acid, could be proved (tytrimetri- 

 cally, by means of normal alkali) in the parts of the plants. Besides water 

 plants, there may be only a few plants, the cell sap of which does not have 

 a marked acid reaction. In genera which contain little acid, the formation 

 of the chlorophyllan will be small and the extract made from this will have 

 to stand some time, while in strongly acid plants (Aesculus, Rumex) the 

 oxidation proceeds so fast of itself that no purely green extract can be made, 

 since it at once exhibits the peculiarities of the modified chlorophyll and, 

 even when chilled, deposits chlorophyllan. 



It is worth mentioning, .for our consideration, that according to Tschirch 

 even carbon dioxid is able to change the chlorophyll into chlorophyllan. 

 Also the tannic substances with which the red coloring matter is certainly 

 related, will have to be reckoned among those bodies with an acid reaction 

 which attack the chloroplasts. It is thus a question whence it comes that 

 this discoloring influence of the acid cell sap makes itself felt in the chloro- 

 plasts only in the autumn. This can be explained either because in the course 

 of the summer so little free acid is available in proportion to the rest of the 



1 Silzungsber. d. Bot. Ver. d. Prov. IJrandenburg XXIII, v. 24. Feb. 1882. 



a Bemerk. iiber d. Natur d. Hypochlorins. Bot. Centralbl. 1882, Vol. X, p. 260. 



3 Untersuchungen iiber Lichtwirkung. Pringsheims Jahrbiicher 1880, Vol. XII. 



4 Sitzungsber. d. Bot. Ver. d. Prov. Brandenburg XXIII, v. 28. April 1882. 



5 Concentrated hydrochloric acid breaks down the chlorophyllan into a body 

 dissolving in hydrochloric acid with a blue color, the "Phyllocyanin" of the authors, 

 and a brown body insoluble in hydrochloric acid, but soluble in ether, the "Xanthin" 

 of C. Kraus. (Tschirch, Untersuchungen iiber das Chlorophyll III. Ber. d. deutschen 

 Bot. Ges. Vol. I, Parts 3 and 4; cit. Centralbl. 1883, Vol. XIV, No. 25, p. 356. 



