527 



i8 hours, while those kept at 8.5 degrees C. turned green only after 5 days. 

 The plants left out of doors, with a maximum temperature of about 4 

 degrees C. became green only after 20 days when the temperature of the air 

 had risen. These observations favor my theory, that the red coloring is 

 conditioned by the preponderance of a process of oxidation, connected with 

 the action of light, over the process of assimilation. A\'ith equal amounts 

 of light, a rise in temperature so increases assimilation that the process of 

 turning green preponderates. 



To avoid a fixation of the morbid yellowish ai)pearance of leaf tips, 

 l)leache(l by frost, it is advisable to remove the winter coxering gradually, 

 or,, for the first few days, to spread a light layer of brush over the plants. 



Defoliation Due to Frost. 



The sudden falling of the foliage during and after the appearance of 

 the first autumnal frost is only one form of the autumnal defoliation which 

 should be designated death from senility (in contrast to the cases already 

 described of abnormal defoliation after excessive heat, drought, lack of 

 light, excess of moisture and other causes, producing a sudden loss of func- 

 tion of the organ). The leaf has simply lived out its life. A normal death of 

 this kind has the least disadvantageous results for the trunk which remains 

 alive. From the senile leaf apparatus man\- plastic as well as important 

 mineral substances gradually wander back into the trunk and are used again 

 in the following period of growth. The retention of abundant amounts of 

 organic structural substances and the leaching of easily soluble nutritive 

 substances by rain are ven' disadvantageous in leaves which die in a juven- 

 ile stage, since these are thus lost to the trunk. But both processes have 

 but little significance when the leaves die of old age. In this case, as has 

 recently been emphasized repeatedly by B. Schultze\ the assimilation of 

 carbon dioxid may well be proved, up to the last moment, even if naturally 

 with weakened power. Through the preponderance of the processes of 

 decay over those of construction the leaf's supply of easily soluble proteins 

 is especially impoverished. A\'ith the increasing thickening and calcification 

 of the membranes, the conducting of new nutritive substances becomes 

 constantly more difficult, so that the demonstrable reduction- of nitrogen, 

 phosphoric acid and potassium thus becomes explicable, even if no consid- 

 erable process of retrogression is accepted. 



After all that has been said in earlier sections on the influence of 

 position, soil constitution and weather, it is not necessary to emphasize here 

 the fact that the life period of the leaves can be proved to be very difl^erent 

 for the same species and that in this frost also acts on leaves which \ary 



1 Schultze, B., Stvidien iiber die Stoffwandlungen der Blatter von Acer Negundo 

 L., 76 Versammlung d. Ges. Deutsch. Naturf.; cit. Centralbl. f. Agrikulturchemie 

 1906, p. 35. 



- Fruwirth, C. and Zielstoff, W., Die herbstliche Riiclvwanderung von Stoffen 

 bei der Hopfenpflan^e. Landw. Versuch.sstat. 1901; cit. Bot. Jahresb. 1901. Part 

 2, p. 161. 



