Januaet 15, 1915] 



SCIENCE 



103 



tion of the abscission layer does not interfere 

 with the translocation of materials between 

 stem and leaf, although, as Tison demonstrated, 

 thyloses may form in the vessels, and callus 

 in the sieve tubes, but other observed changes 

 (lignification, cork formation and stoppage by 

 wound-gum) take place only after the leaf 

 has fallen. 



To test Tison's statements as to the forma- 

 tion of thyloses and callus. Swart employed 

 indigo-carmine, a stain that does not pene- 

 trate the protoplast, and is not poisonous (like 

 eosin) in concentrated solution. Branches 

 containing leaves that had begun to turn 

 yellow, as well as green leaves, were placed in 

 deep blue solutions in light, and the penetra- 

 tion of the stain followed by observing cross 

 sections. In all cases, within a few hours or 

 longer (varying according to the species or 

 the length of the branch), the stain had ad- 

 vanced into the vascular bundles of the petiole, 

 and even into the veins of the blades of the 

 yellowed leaves. The stain appeared earlier in 

 the green than in the yellow ones, although 

 the green ones were farther away, and this 

 may be explained by the stoppage of the sto- 

 mata above described. In how far the closing 

 of the stomata or the changes in the leaf base 

 are to be considered as the primary cause of 

 the diminished suction in the yellow leaves 

 was not determined. The fact remains that 

 any considerable stoppage of the path of liquids 

 is not the rule. The observation that com- 

 pletely yellowed leaves often remain for many 

 days in their exposed places on the tree with- 

 out becoming dry, leads us to the same conclu- 

 sion. The author also finds that the leptome 

 elements of the vascular bundles are not cut 

 off by the abscission layer, so that the trans- 

 portation of substances from the leaf is not 

 hindered in that manner. These facts meet 

 all objections to the transportation theory 

 based on the argument that the abscission 

 layer, being formed before yellowing begins, 

 prevents the passage of materials to and from 

 the leaf. 



We are accustomed in our thoughts always 

 to associate leaf fall with a degenerative 



change of color; but that leaves out of account 

 the periodic or the continual leaf-fall outside 

 of temperate regions, and even often in au- 

 tumnal leaf-fall, where we observe, especially 

 on certain trees, that many leaves fall when 

 still green, or only half colored. On more 

 careful investigation, however, these latter 

 eases are found to be due to some unfavorable 

 external conditions, such, for example, as 

 frost, which may cause a rupture of the leaf 

 by the formation, at night, in the abscission 

 layer, of ice crystals which thaw on the follow- 

 ing day. Other causes mentioned are storms, 

 sudden increase of turgor in the active zone, 

 disease of the roots or of the leaves them- 

 selves, and the leaving of collected plants for 

 too long a time in the collecting case. Thus 

 it is seen that plants shed their leaves, not 

 alone when they have ceased their function as 

 organs of nutrition, but also when the life of 

 the plant is threatened, especially as a result 

 of too great transpirational activity. 



Mention is made of such trees as the beech, 

 hornbeam and oak, many or most of whose 

 leaves remain on during the winter, on account 

 of their failure to form the abscission layer 

 until the following spring. 



The red pigment of the anthoeyan group, 

 occurring in solution in the cell-sap, and to 

 which the autumn landscape owes its special 

 charm, is also formed at other seasons. Its 

 appearance before the death of the leaf is re- 

 stricted to a relatively small number of spe- 

 cies, and then it does not replace the yellow 

 pigment, but only masks the latter. It often 

 occurs in mature leaves when there is no out- 

 ward indication of initial degeneration, such, 

 e. g., as the disintegration of chlorophyll. An 

 active assimilation, combined with a migration 

 of carbohydrates induced by nocturnal cold, 

 is a conditio sine qua nan for the accumula- 

 tion of sugar in the cell-sap, which, in turn, 

 is the antecedent condition for the formation 

 of the red pigment. Thus it is that we have 

 the most beautiful red colors in autumn when 

 cold nights alternate with warm days. 



There follows a brief discussion of Pick's 

 theory that the presence of anthoeyan in leaves 



