72 MASSACHUSETTS HORTICULTURAL SOCIETY. 



the structure of the organs is at first much modified or differen- 

 tiated ; but, if the light is intense, and continued during several 

 months, without check or diminution, the new organs formed by 

 the plants which adapt themselves to this light, present remarkable 

 modifications of structure in their various tissues, and are less 

 differentiated, while at the same time being rich in chlorophyll. 



"2. The direct electric light is injurious to the normal develop- 

 ment of tissue because of the ultra-violet rays, even at a distance 

 of three meters and more from the lamp." 



There remains one point to which I must call your attention : 

 When do the lighted plants grow? It is known that growth is 

 ordinarily more rapid at night. But when the sun supplies light 

 by day and the arc lamp supplies it by night, at what time will the 

 plant choose to perform this function ? It is probable that most 

 plants will grow more or less continuously, but more rapidly under 

 the electric light. In fact, we have found that a petunia plant 

 which was subjected to normal daylight, then to electric light until 

 eleven o'clock at night, and to normal darkness from that hour 

 until morning, grew most rapidly when the electric light was burn- 

 ing. Careful tests with lettuce gave the following conclusions : 1. 

 That the electric light did not determine the periodicity of 'growth. 

 2. That increase of growth under the light occurred only during 

 the first days of the experiment. 3. That growth in both the 

 light and dark houses took place in daylight as well as in darkness. 



I may now say, in conclusion, that the electric arc light exerts a 

 powerful influence upon plants in greenhouses, and that all species, 

 and sometimes even garden varieties, are affected differently. 

 This varying influence is the effort of the plant to adapt itself to 

 the new conditions in which it is placed, and the modification 

 undoubtedly bears some relation to the elasticity or fixedness of 

 the organization of the plant, and, therefore, to the conditions 

 under which the species has been evolved. And it must, there- 

 fore, follow that if plants were to be subjected to the electric light 

 through several or many generations, more or less adaptive varie- 

 ties would arise, in the same manner in which plants liave adapted 

 themselves by profound variation to the requirements of greenhouse 

 cultivation. The electric light affects plants by hastening growth 

 or maturity ; but this effect may be so powerful as to destroy the 

 plant. The injury is avoided, however, by screening the light 

 through a glass or by removing the plant to such distance that the 



