102 



SCIENCE 



[N. S. Vol. XLI. No. 1046 



whicli lie provisionally designated as autumn- 

 xanthophyll. The question as to whether it is 

 simple or complex, Tswett did not solve. If 

 Tswett's idea is correct, then it is highly prob- 

 able that we have to do here with a derivative 

 of the normal yellow pigment of the leaf — pos- 

 sibly with a further oxidation product of caro- 

 tin. Swart refers to the contrast (first noted 

 by Stahl) in the behavior of the yellow and 

 green pigments, and the biological significance 

 of yellowing and etiolation, showing how the 

 yellow pigments containing the elements 0, H, 

 and O, remain in the leaf, while the green pig- 

 ments, which contain also N and Mg (two ele- 

 ments whose retention by the plant is more im- 

 portant than that of the other three), are 

 transplanted back into the plant before the 

 death of the leaves. 



Here is a lack of agreement with the results 

 of the chemical analyses, which show that the 

 magnesium is not removed from yellowing 

 leaves, and a satisfactory explanation of this 

 contradiction in favor of the results above 

 recorded is not at hand. One may consider 

 that the magnesium contained in chlorophyll 

 represents only a very small part of the entire 

 amount of that element which is present in 

 leaves, so that, considering the variations 

 which the results of the analyses always show, 

 the translocation of this small part may be 

 often not noticeable in the end result. Unless 

 we abandon the idea of a translocation of the 

 chlorophyll in the form of its disintegration 

 products, we must establish the fact that the 

 magnesium very probably takes no part in this 

 movement. 



Swart then gives attention to the changes in 

 the cell contents of the yellowing leaves dis- 

 closed by a microscopical examination. Four 

 different processes (to which Sachs first called 

 attention) are studied: (1) The destruction of 

 the outer form of the chlorophyll grains, (2) 

 the disintegration of the green pigment, (3) the 

 translocation of the chlorophyll substance, (4) 

 the translocation of the starch. These proc- 

 esses, which may occur in any order or simul- 

 taneously, are studied and described. Con- 

 trary to Mer, Swart believes that the oil, 

 formed by the disintegration of the chloro- 



phyll, remains in the leaf at the time of leaf 

 fall. Wide variations are noted in the behay- 

 ior of the starch. In some leaves which have 

 just begun to turn yellow, very little starch 

 was found, while in others, quite yellow, the 

 yellow portions were found to be full of starch". 

 Swart confirms the observations of Mer as to 

 the sequence of steps in the disappearance of 

 the starch, namely, first from the parenchyma 

 cells of the leaf blade, then from the veins of 

 the blade, and finally from the vascular bundles 

 .of the petiole. Small traces of starch remain 

 with the intact chloroplasts in the guard cells 

 of the stomata. It is not strange that traces 

 of starch remain in completely yellowed leaves, 

 when we recall that the solution of starch is 

 greatly inhibited by the very low temperatures 

 which often occur suddenly in autumn, and 

 that there is also a high content of insoluble 

 carbohydrate, as shown by chemical analysis. 



As to the behavior of the plasma itself. 

 Swart failed to confirm the observation of 

 Kienitz-GerlofE that only disorganized rem- 

 nants of plasma are to be found in yellowed 

 leaves. The mesophyll cells give the impres- 

 sion of being very poor in plasma, in compari- 

 son to the parenchyma of the vascular bundles, 

 but the plasma utricle and the nucleus remain 

 intact. The cells of the leaf, even after leaf- 

 fall, are by no means dead. That a transloca- 

 tion of nitrogenous substance occurs (as the 

 chemical analyses conclusively demonstrated), 

 was also determined by the microscopical study. 

 This may be associated with a destruction of 

 the plasma and a transfer of the disintegration 

 products, but Swart could find no evidence of 

 a translocation of the plasma itself, as held 

 by Kienitz-GerlofE. 



In 1860, von Mohl showed the untenableness 

 of Schacht's view that the death and fall of the 

 leaf was caused by the formation of the ab- 

 scission layer of periderm at the leaf-base, for 

 in many plants this layer does not form, and 

 in the cases where it is present the vascular 

 bundles are not interfered with by its forma- 

 tion. Observations to the same effect by Tison 

 were confirmed by Swart, who considers that 

 there is no longer any doubt that the forma- 



