501 



normal process of yellowing is found, beginning at the edge of the leaf, and 

 followed by the drying of the tissue, toward the centre of the leaf, or the 

 yellowing and drying do not follow parallel, but rather opposite paths, i. e., 

 the process of turning yellow begins at the petioles and the larger veins and 

 advances toward the periphery, so that the edge is colored last of all, while, 

 nevertheless, the first to dry subsequently. I observed the last course espe- 

 cially well in Acer platanoides, less constantly in Acer Pseudoplatanus. The 

 middle surface showed an uniform brilliant quince yellowy while the peri- 

 pheral zone was still green. With advanced lowering of the temperature, 

 many leaves showed a turning brown and dying of the outermost edge of the 

 still green part of the leaf peripher}^ while the yellow, middle field did not 

 yet show any dead places in the tissue. 



This case can also occur with Tilia, and in fact usually on one side, 

 since only half of the leaf shows the process. Nevertheless in the linden, 

 the coloration, advancing, from the edge toward the centre, is more frequent. 

 The investigations of numerous cases show that the irregularities of color- 

 ation are connected with the irregular dying of the vascular bundles. 



The normal autolysis in the autumn sets in when the whole vascular 

 bundle system of the roots has still retained its functioning and the dying 

 back only begins at the finest ends of the ner\'es at the edge of the leaf. 

 Then the leaf discolors and dries firr.t along the edge ; the discoloration ad- 

 vances gradually in the portions of the leaf between the smaller veins and 

 finally also between the larger ones toward the midrib and the petiole. If, 

 on the other hand, the functioning of the ducts is prematurely destroyed in 

 the branch or in the petioles, which can be perceived from the browning of 

 the vascular bundles, then the discoloration begins at the petiole, or the 

 larger veins, and extends irregularly toward the periphery. 



The course of the dying back, due to continued summer drought, re- 

 sembles the normal autumnal autolysis, inasmuch as the parts of the leaf 

 receiving the least amount of water are the first to discolor. Besides the 

 dr}4ng of the leaf edges, however, that of the middle region of the larger 

 intercostal fields becomes more noticeable here, because these lie fartherest 

 from the strongly developed conducting strands ; thus especially great de- 

 mands are made upon them because of excess of light and heat. 



The autumn coloring begins with a change of the chlorophyll often 

 accompanied by the appearance of a red coloring matter. At first a change 

 in the position of the chloroplasts is noticed, and a tendency to unite. I 

 found in the spruce that the individual chloroplasts form radiating processes 

 which unite with those of the adjacent ones. The red coloration is condi- 

 tioned by the presence of ferments and related bodies. Many evergreen 

 plants turn a dirty brownish green. According to Kraus^ this coloring is 

 produced as follows : fine grained protoplasmic masses, colored a bright 

 reddish green to copper red, occur in the palisade parenchyma in place of 



1 Kraus, tjber die winterliche Farbung immergTiiner Gewachse. Sitzungsber. 

 d. phys.-med. Soc. Erlangen; cit. in Oekonomische Forstchritte 1872, Nos. 1 u. 2. 



