JANFAKT 15, 1915] 



SCIENCE 



101 



general, in which are completed those proc- 

 esses which are associated with the degenera- 

 tion of the leaves, varies in length (according 

 to the species of plant) from a few days to sev- 

 eral months. In unusually dry, or in wet cold 

 years (e. g., 1911 and 1912 in Germany), this 

 period began several weeks earlier than nor- 

 mally, and was of briefer duration. Under 

 these conditions the loss of substance which 

 leaves suffer through autumnal translocation 

 is notably less than under normal conditions, 

 especially when an early night frost brings the 

 life of the leaf to an abrupt end. 



That the vascular bundles exert an influ- 

 ence upon the process of change of color is evi- 

 denced by the fact that the change always be- 

 gins in those parts of the parenchyma that are 

 farthest from the veins, and spreads thence by 

 •degrees, first to the smaller veins, then to the 

 main veins, reaching the petiole last of all. 

 As the yellow color spreads the stomata (as 

 Stahl has shown) quite generally become 

 ■closed. 



Swart then asks what has become of the 

 chlorophyll, where the yellow color has ap- 

 peared, and notes that this question can not 

 he considered to advantage, except in the light 

 of the newer conceptions concerning the na- 

 ture of chlorophyll. Then follows a review of 

 the chlorophyll studies of Sorby, Tswett, Will- 

 statter. Kohl, Czapek and others. According 

 to Kohl (1902), there belong to the yellow pig- 

 ments of chlorophyll, carotin and two xantho- 

 phylls, a and ^. Tswett (1903) distinguished 

 in addition to carotin, three xanthophylls, a, 

 Oi, /?. From subsequent researches of Will- 

 statter we have become better acquainted with 

 two of these yellow accompaniments of chloro- 

 phyll. One is an unsatisfied carbohydrate, of 

 the composition C^H^,, identical with the caro- 

 tin of carrots. The second, the xanthophyll of 

 older authors, is an oxide of carotin, with the 

 formula C^„Hj„0,. Both pigments are crystal- 

 lizable. According to Willstatter, both these 

 yellow pigments play no role in photosynthesis, 

 but are concerned in respiration, and with this 

 view Swart agrees. 



Eetuming now to the question as to the be- 

 havior of these green and yellow pigments. 



during the yellowing of the leaves. Swart re- 

 jects the hypothesis that the green ones disap- 

 pear from the leaves, while the yellow ones, as 

 such, remain behind. As to whether the chlor- 

 ophyll is merely translocated as such, or 

 whether it first undergoes a decomposition, 

 there is no a priori reason for considering the 

 first alternative as correct, while at the same 

 time no instance is knovra where the colorless 

 rhizomes of our perennial plants become green 

 in winter. Observations of the bark of girdled 

 branches above the girdling, as compared with 

 the bark below the girdling, also give negative 

 results, and so on this ground also, we may 

 reject the hypothesis of a translocation of 

 chlorophyll as such. 



If, however, the chlorophyll suffers a disin- 

 tegration, then either the disintegration prod- 

 ucts are removed from the leaf, or else they 

 remain where formed. Stahl (1909) showed 

 that the former is the case, and demonstrated 

 that the separation of a leaf from the stem, or 

 portion out of the leaf blade, caused a marked 

 retardation of the yellowing in the leaf or iso- 

 lated portion. The same result was obtained 

 on trees in the open, in the case of leaves at 

 the commencement of color change, by sepa- 

 rating a part of the blade from direct connec- 

 tion with the vascular bundles, by a cut or 

 simply by pinching. Swart has repeated these 

 experiments with the same results as those ob- 

 tained by Stahl. For their firmer establish- 

 ment, there is needed only the application of 

 the law of mass action. Swart, therefore, con- 

 cludes, with Stahl, that the chlorophyll passes 

 from the leaves into the stem in the form of 

 its disintegration products. 



The question of the fate of the yellow color- 

 ing matters is next considered. Kohl held that 

 the yellow pigment that remains behind in 

 yellowed leaves consists of carotin and also of 

 the xanthophylls, a and /?, found by him in 

 green leaves, but that the yellow pigment of 

 the normal green chloroplasts remains unal- 

 tered during the autumnal color changes. In 

 direct opposition to this Tswett, by means of 

 his adsorption method, found that, in most 

 eases, the larger part of the yellow pigments in 

 yellowed leaves represents a new formation, 



