54 



xVeilher llie Imperial pholomeler nor the Wiesner insolalor gives 

 perfeclly exact eslimalions. But if Ihe experimenls are underlaken 

 only during Ihe lighter hours of Ihe day (8.30 a. m. — 3. 30 p. m ), 

 when the speciral composition of Ihe light of heaven is fairly con- 

 stanl, and if one does not "svork under a cloudy sky, the values 

 may be considered approximately parallel with the intensity of the 

 assiinilaloiy light. In the forest the deviations are greater. Since 

 earlier measurements have been undertaken with similar methods, 

 my values may be compared with those. An exact idea of the 

 assimilatorily active light in the forest can be arrived at only 

 photometrically and on the basis of the secondary assimilalion- 

 curve, which however has not yet been worked out. 



The material was taken directly from the place of growth. 

 However, in regard to shade-plants more parlicularly, it is wise 

 to place the severed shool or leaf for some time under a glass 

 bell, in air saturated with moisture, so that it may recover. The 

 degree of openness of the stomata, which can be measured by 

 infiltration with xylol, seems to be of much greater importance 

 for assimilation in the case of sun-plants than in the case of shade- 

 plants. My experience goes to indicate that a number of shade- 

 plants {Oxalis, Stellaria nenioriim), even with apjiarently closed sto- 

 mala, are able to absorb carbon dioxide, especially when this has 

 more than normal tension. Several unsuccessful series of experi- 

 ments with Oxalis however are in my opinion to be explained by 

 the faci that the cuticular CO 2 dilTusion was not sufficient for 

 more intensive assimilation, so that the above mentioned previous 

 treatment of the material should not be neglected. 



I always tried to lake leaves of about the same age, which is 

 of importance since the chlorophyll percentage and the assimilation 

 power vary with age (Ibving, Willstätter and Stoll 191<S), and 

 further the stomata in older leaves may also be conslantly closed. 

 The light-intensily on the habitat also exercises an influence upon 

 the quantily of chlorophyll and the qualily of the leaf (Lubimenko 

 1908), and the material should therefore be taken from similar 

 habitals. The varying assimilation of the sun- and the shade-Ieaf 

 in the same species or individual has been investigated by Hessel- 

 MAN (1904, p. 397), Bovskn-Jensen (1918), and StÅlfelt (1920). — 

 The experiments were carried out between 8 a. m. and 4 p. m., 

 with 3 or 4 apparaluses working simultaneously. 



