678 BIOLOGICAL EFFECTS OF RADIATION 



and 1881 Siemens (67) reported results obtained in the culture of a num- 

 ber of spegies with the electric-arc lamp used to replace daylight and to 

 supplement the latter so as to afford continuous illumination. Disclaim- 

 ing any intention of generalizing, Siemens reached the conclusion that 

 under suitable conditions electric light may effectively replace or supple- 

 ment daylight and that plants apparently do not require a daily rest 

 period. The continuous stimulus of light was found to be favorable for 

 healthy development at a greatly accelerated rate through all stages of 

 the annual life cycle of the plant from the early leaf to the ripened fruit. 

 Peas grown under continuous light produced viable seed. 



Kjellmann (35), during a sojourn on the north coast of Siberia in 

 1878 and 1879, as a member of the Vega Expedition headed by Norden- 

 skiold, conducted experiments with the Arctic species, Catabrosa algida 

 and Cochlearia fenesirata, which were exposed to the continuous northern 

 daylight and to a 12-hr. day. As compared with the shortened daylight 

 period, the continuous illumination induced a more rapid rate of growth, 

 and earlier and more profuse flowering. These results were taken to 

 indicate that plants may continue the assimilation processes throughout 

 the continuous daily light period, and as a result their development is 

 materially hastened. The effects on flowering, however, were of a quanti- 

 tative rather than a qualitative character and were not particularly 

 striking. In the period 1891 to 1893 Bailey (8) conducted investiga- 

 tions with electric light from arc lamps used for a portion or all of the 

 night as a supplement to daylight, primarily with a view to forcing vege- 

 table crops. The additional light hastened the growth of lettuce and 

 induced early flowering and formation of viable seeds in spinach. It 

 was concluded that periods of darkness are not necessary to the growth 

 and development of plants. Rane (56), working along similar lines with 

 the incandescent carbon-filament lamp, obtained much the same results 

 with lettuce and spinach and observed earlier blooming in certain flower- 

 ing plants. 



Bonnier (9) carried out experiments with a large number of species 

 in 1894 and 1895 in which the arc lamp was utilized to produce a daily 

 light period of 12 hr. as well as continuous illumination. Control plants 

 were exposed to natural conditions of light during the winter months at 

 Paris. Comparison of the experimental and control material was based 

 almost entirely on anatomical observations. In general, the continuous 

 illumination was found to cause thickening of the leaf and leaf stalk, 

 reduction in size of leaf, poor development of mechanical tissue and to 

 jiroduce a superabundance of chlorophyll. The marked reduction in 

 differentiation of tis.sues was considered strongly suggestive of the 

 etiolated condition resulting from prolonged exposure to darkness. 

 Using the incandescent gas light, Corbett (13) demonstrated that night 

 illumination as a supplement to daylight stimulated markedly top growth 



