728 BIOLOGICAL EFFECTS OF RADIATION 



with etiolation from 1686 to 1900. The reader is referred to these works 

 for reviews of the earher hterature. 



EFFECT OF DARKNESS ON GROWTH, FORM, AND STRUCTURE 



In studying the phenomenon of etiolation, it must be borne in mind 

 that the fully etiolated plant is one which has developed in complete 

 darkness. Friedel (36) demonstrated that there was a pronounced 

 difference between plants developed in absolute darkness and those 

 developed under bell jars covered with black paper. Lentils in complete 

 darkness had smaller leaves, fewer internodes, and less angular stems 

 than those grown behind black screens. He found absolutely no chloro- 

 phyll development in the onion in complete darkness, while other, less 

 careful workers, had previously reported that the onion could produce 

 chlorophyll in darkness. Likewise, exposure to light only a few minutes 

 each day for the purpose of examination will greatly modify the form of 

 the plant (76, 113). 



The typical etiolated plant is white or yellowish in color, has long 

 internodes, undeveloped leaves, thin watery sap, and, in general, rela- 

 tively undifferentiated tissues. 



MacDougal (65), during a period of seven years, cultivated 97 

 different species in darkness. Particular care was taken to exclude light 

 from his cultures. Although the different species presented several 

 unique reactions to darkness, certain features were common to all. The 

 outstanding features which he observed in practically all higher plants 

 were tall, weak, attenuated stems, and thin only partially unfolded leaves; 

 in fact, in many plants the only sign of foliar development was a distinct 

 plumular hook. The microscopic structure of stems and leaves showed 

 that the different tissues present in normal plants were generally present 

 also in etiolated plants, but the cells were usually thin-walled, loosely 

 organized, and less distinctly differentiated. Darkness appeared to 

 arrest development so that many tissues which form later in the life of 

 the plant were not present in etiolated forms. Similar structural 

 differences have been reported by practically all workers on etiolated 

 plants (114, 76, 78, 105, 89). 



Some of the special features observed by MacDougal are worthy 

 of note. Plants developed from root storage organs, in general, were 

 able to live longer in the etiolated state, and produced greater growth than 

 seedlings. In fact, some of them were capable of going through a com- 

 plete vegetative cycle in the dark and developed new storage organs at 

 the end of the season. Arisaema triphyllum which develops new corms 

 each season was capable of four successive seasons of activity in darkness. 

 The corms and other storage organs produced in the dark were smaller 

 and more watery but similar in structure to such organs developed in 

 normal plants ; however, the shoots produced from such etiolated storage 



