66i 



how these are made use of. The more vigorously a plant grows, the more 

 organic substances it produces per gram ash. Therefore, a lesser assimil- 

 atory activity must be concluded each time, if the analysis proves a high 

 ash content in proportion to the dry substances. In the present case, the 

 scanty amount of light is the factor reducing production. 



Sensitiveness* to shade is, at any rate, connected with a definite limit of 

 value for each plant variety, but, as in all factors of growth, these values 

 can shift individually to a certain degree, so that, in the same species, there 

 may be races very sensitive to shade in which, in Nordhausen's^ opinion, 

 certain reduction phenomena become herditary. 



Each leaf in the plant has its special sensitiveness to shade, according 

 to the light conditions under which it was produced, and its position on the 

 axis. The shade produced by leaves higher up is the most important. The 

 amount of assimilation and respiration, as weU as of transpiration, is deter- 

 mined by this. In Griffon's experiments^, for example, it was found that 

 a leaf, as thick as that of Prunus Laurocerasus is not able, in direct sunlight, 

 to completely prevent the carbon dioxid decomposition in the leaf of 

 IJgustrum ovalifolium. Under two such leaves, however, the development 

 of carbon dioxid took place. Under such conditions, therefore, assimilation 

 was so reduced that respiration exceeded it. 



It naturally depends also upon what color the shaded plant parts are, 

 i. e., which light colors can still pass through them. 



According to Teodoresco" the leaf tissues develop most poorly ingreen 

 light; they are found to be better in red light; the best development, how- 

 ever, is found in blue light, and, therefore, the greatest enlargement. The 

 chlorophyll grains are also smaller in green Hght, less numerous and not so 

 regularly distributed as in red and blue light. 



The product of the activity of the chloroplasts, corresponding to their 

 development, is proved especially favorable in the most strongly refrangible 

 rays. Palladin* exposed etiolated cotyledons of Vicia in sugar solutions to 

 white and colored light and found that the assimilation of the sugar, as well 

 as the formation of active proteids, took place most vigorously in the more 

 strongly refrangible light rays and, therefore, respiration was more intensive. 



If the leaf, because of a scanty light supply, cannot work any longer, 

 it falls off, just as under the action of all other factors which suppress its 

 assimilatory activity ^ This explains the regular "summer leaf fall," which, 

 naturally, is different from leaf fall due to heat. Wiesner" explains the 



1 Nordhausen, M., tJber Sonnen unci Schattenblatter. Ber d Deut'sch Bot 

 Ges. Vol. XXI, 1903, p. 30. ... 



2 Griffon, Ed., L'assimilation chlorophyllienne dans la lumiere solaire qui a 

 traverse des feuilles. Compt. rend. CXXIX, Paris 1899, p. 1276. 



3 Teodoresco, E., Influence des diffgrentes radiations, etc.; cit. Bot. Jahresber 

 27. Jahrg". 1901. Part II, p. 133. 



4 Palladin, W., Influence de la lumiere, etc.; cit. Bot. Jahresber. Jahra- 1899 

 II, p. 134. 



5 Vochting-, H., fiber die Abhangigkeit des Laubfalls von seiner Assimilations - 

 tatigkeit. Bot. Zeit. 1891, Nos. 8 and 9. 



6 Wiesner, Jul., tJber Laubfall infolge Sinkens des absoluten Lichtgenusses 

 (Sommerlaubfall). Ber. d. Deutsch. Bot. Ges. Jahrg. XII, Part 1, 1904, p. 64. 



