Chap, x The Problems of Growth 449 



the plant were observed by Janczewski in 1885 ; he found 

 certain aerial roots of orchids tend to become radially 

 organized in darkness, but to be flattened or bilateral 

 in light. Vochting showed, in 1894, that in the phyllo- 

 clades of Opuntia and Phyllocactus, a radial organization 

 is induced by darkness. 



Anatomical changes also occur ; Kraus showed in 1870 

 that in darkness the thickness of cell walls becomes less 

 pronounced, and in 1883 Stahl found that the palisade layer 

 of leaves may remain undeveloped in the absence of light. 

 He pointed out that not only is the development of this 

 layer correlated with the intensity of light, but that in the 

 absence of the latter no internal differentiation of the 

 mesophyll occurs. Haberlandt confirmed Stahl's observa- 

 tion on the palisade layer of leaves in 1896. In 1878 

 Kraus, Koch, and Rauwenhoff severally showed that the 

 epidermal and parenchymatous cells of elongated etiolated 

 internodes are much longer than those of normal ones. 

 Kraus attributed this to the thinness of the cell walls of 

 such etiolated cells and to the consequent yielding to the 

 longitudinal tensions. Rauwenhoff observed that the fibres 

 of the elongated internodes do not become excessively long 

 if the longitudinal tensions remain small during the whole 

 period of growth. 



The effect of darkness upon the growth and development 

 of fungi was examined by several authors. Brefeld, in 1889, 

 found both processes were greatly impeded, the sporangia 

 often being either totally suppressed, or only partially 

 developed. Lendner found in 1897 that the sporophore 

 of many Mucorini becomes elongated in darkness. The 

 development of several Algae and Bryophyta has been 

 found to suffer from the same cause. Some more ex- 

 tended investigations into their behaviour were made by 

 Klebs in 1896. 



The way in which the absence of light operates in caus- 



GREEN F f 



