198 PLANT PHYSIOLOGY 



oats, with a light intensity of 300 candle-meters; while the com- 

 pensation point of the very shade-tolerant liverwort Marchantia^ 

 growing well in the deepest shade, lies at a light intensity of 

 100 candle-meters. Correspondingly, the respiratory intensity 

 of mustard per 50 sq. cm. equaled 0.87 mg. CO2 per hour; of 

 oats, 0.37 mg.; and of Marchantia, 0.06 mg. The assimilation 

 intensity of these three plants varies but little. The exact 

 numerical expression of the amount of radiant energy required 

 by the plant for its normal development can be obtained only 

 when it is possible to grow normal plants entirely in artificial 

 fight under controlled conditions. Several such attempts have 

 been made. By using the light of strong electric lamps of several 

 thousand candle-power, it is possible to grow plants and to make 

 them bloom and mature fruits (Fig. 60) in a room entirely 

 deprived of daylight, for instance, in a cellar. 



Some data may be given regarding the strength of illumination 

 in candle-meters sufficient to induce flowering and fruiting of 

 plants: peas, 1,100; beans, 2,400; barley and wheat, 1,800 to 

 2,200; radish, 4,000; tobacco, 2,200 to 2,800; corn, 1,400 to 8,000; 

 buckwheat, 850 to 1,100; etc. (Harvey, 1921). By way of 

 comparison, it should be mentioned that direct sunfight at noon 

 gives about 30,000 to 40,000 candle-meters (9,250 foot-candles 

 at 42 deg. N. Lat. on June 21 at noon). It is interesting to note 

 that plants grown under conditions of artificial lighting develop 

 much better when continuously illuminated during day and 

 night than when subjected to an 8- or 12-hr. period of darkness. 

 Hence, the ''night's rest" is not necessary for plants. The 

 possibility of work without rest for several months seems an 

 amazing fact. But it must be kept in mind that in photosynthe- 

 sis there is a continual input of energy and that rest as in animals 

 is necessary only for repairs and for relaxation of the nervous 

 system. 



In the growing of plants in artificial light, the greatest difficulty 

 lies in providing them with light, not only of a sufficient intensity 

 but also of an adequate quality, that is, of such wave lengths as 

 permit the normal course of the photosynthetic process. The 

 light of the strong incandescent electric lamps usually applied 

 for illumination, as compared with daylight, is poor in rays of 

 the blue-violet half of the spectrum, from 4000 to 5800 A., and 

 too rich in those of the yellow-red half, especially in infrared rays, 



