C54 Journal of Agricultural Research voi. xviii, no. h 



axis and branches and to increase the superficial area of the foliage of 

 many species. Also, the thickness of the leaf lamina may be reduced, 

 and there may be marked departures from the normal in internal 

 structure, the tendency being toward a less compact structure. So far 

 as is known, no important general relationships between differences in 

 light intensity and reproductive processes have been experimentally 

 demonstrated. 



The comparative effects produced by different regions of the spectrum, 

 including the ultra-violet, have been extensively investigated but with 

 more or less conflicting results. The most extensive investigations on 

 the subject, perhaps, have been made by Flammarion (<?). It was found 

 that there is abnormal elongation of the principal axis in several species 

 under the influence of the red rays, while growth is markedly reduced 

 under the green and especially under the blue rays. In some plants, 

 however, such as corn, peas, and beans, growth is greatest in white light. 

 Some plants blossomed considerably earlier in red light than in white. 

 White light produced the greatest weight of dry matter. Leaves of 

 Coleus developed decided differences in color patterns under differently 

 colored lights. In subsequent work Flammarion has extended his studies 

 to a large number of species. 



The duration of the daily exposure to light needs to be considered in 

 three separate phases — (i) continuous illumination throughout the 

 24-hour period, (2) continuous darkness throughout, leading to the phe- 

 nomena of etiolation, and (3) illumination for any fractional portion of 

 the 24-hour day. Under natural conditions continuous sunlight through- 

 out the 24-hour period occurs, of course, only in very high latitudes. 

 Schiibeler {23) observed the behavior of several species transported from 

 lower latitudes and grown in northern Scandinavia under continuous sun- 

 light lasting for a period of two months. In the species under observation 

 the vegetative period was shortened and the seeds produced were larger 

 than the normal. It is stated, also, that there was an increased formation 

 of aromatic and flavoring constituents. Another method of securing 

 continuous illumination consists in the use of artificial light for illumina- 

 tion or in the supplementing of normal daylight with artificial light, 

 though, of course, the quality and the intensity from the two sources will 

 not ordinarily be the same. Using electric light alone, of an intensity 

 one-third that of sunlight. Bonnier {6) observed a marked increase in 

 chlorophyll formation which extended inwardly to unusual depths. 

 He found also incomplete differentiation of the tissues, recalling, in this 

 respect, the effects of continued darkness. In some instances the color 

 of blossoms was deepened. 



Etiolation, resulting from exposure to continuous darkness, has been 

 the subject of much study. In this connection special mention should 

 be made of the work of MacDougal {18) covering a very large number of 

 species. This author also presents a comprehensive survey of previous 



