THE INFLUENCE OF LIGHT AND DARKNESS 

 UPON GROWTH AND DEVELOPMENT. 



HISTORICAL. 



The more apparent features of the behavior of plants in darkness 

 must have been a matter well known to cultivators from the earliest 

 times, but a conception of the influence of light upon growth and 

 form seems to be a distinctly modern idea. John Ray appears to be 

 the first botanist to make mention of the more apparent features of 

 etiolation, and of the relation of color to illumination.^ 



The quantitative effects of illumination of various degrees of 

 intensity was known to Hales in a way, and he saw that diffuse light 

 did not permit or induce normal development as he says : " Beans and 

 many other plants, which stand where they are much shaded, being 

 thereby kept continually moist, do grow to unusual heights, and are 

 drawn up as they call it, by the overshadowing trees, their parts being 

 kept long, soft and ductile." Hales also made the observation that 

 plants become heavier at night. ^ So far as ma}^ be learned from the 

 records consulted, Bonnet may be designated as the pioneer in actual 

 experimental investigation of the subject. He carried out a series of 

 tests upon beans, peas, and branches of the vine, from which it was 

 seen that elongated internodes and small yellow leaves were produced 

 in darkness. Etiolated plants became green after exposure to illumin- 

 ation for 24 hours. Green leaves placed in darkness did not blanch, 

 but fell from the stems. The wood of etiolated stems did not "harden " 

 and cuttings from etiolated stems could not propagate the plant. '^ 



After Bonnet, Mees may be named as having carried out the next 

 important experimental observations, which covered a large number 

 of the phases of the question still under discussion. Mees saw that 

 some seeds germinate in darkness as well as in light, that red color 



1 Raj, J. Historia Plantarum, i : 15. 1686. Also in imprint of 1693. 



2 Hales, S. Statical Essays, i : 334. 1727. Also p. 336. Ed. of 1769. 

 •'Bonnet, Ch. Usage des feuilles, p. 254. 1754- 



