MACDOUGAL: MECHANISM AND CONDITIONS OF GROWTH 7 



concerned. No other single agency or external force acting 

 upon an adequate food-supply may exert so wide a range of 

 efTects, upon the processes necessary in the adduction and forma- 

 tion of building material necessary for growth. 



In the incorporation of this material, however, some energetics 

 are involved in which light also may play a part. Turgidity and 

 its resultant pressure-effects, hydratation capacity and its re- 

 sultant swelling in the cell colloids, while modified by temperature, 

 are also highly respondent to the acidity and alkalinity of the 

 solutions which penetrate them, in consequence of which light, 

 as will be shown later, may play a more important role than in 

 other phases of growth. This form of energy, especially in the 

 shorter wave lengths, may neutralize or coagulate protoplastic 

 colloids, especially in the minute plants and it is probable that the 

 eiihciency of this agency in sterilization is because of this action. 

 Its partial action would decrease hydratation capacity and thus 

 tend to lessen growth. 



The author carried out some extensive experiments upon the 

 course of growth and development of plants in darkness, previous 

 to 1904, and in connection with this study of the features of 

 enlargement and differentiation of shoots uninfluenced by growth 

 obtained some insight into the positive effects of light on growth. 

 The results of this work carried on at The New York Botanical 

 Garden, 1 899-1 904, showed that the total amount of growth 

 accomplished by individual plants of a hundred species selected 

 for the experiments might be more or less in any given dimension 

 in darkness than in light, that the rate of growth might be af- 

 fected in a similar manner and that the effect of light on growth 

 was not invariable. This rendered unsafe at once the trite con- 

 clusion that "light retards growth," current from the time of Sachs 

 to the present day and still repeated in text-books and lectures. 



My own results^ on the matter were presented only in a gener- 

 alized way, and their validity was not considered by authors of 

 text-books and reviewers, including Barnes, Jost, and others. 



Blaauw,- a Dutch investigator, has recently studied the question 



1 MacDougal, D, T. Influence of light and darkness on growth and development. 

 Mem. N. Y. Bot. Garden 2: 307, 308. 1903. Also, Light and the rate of growth in 

 plants. Science II. 41: 467-469. 1915. 



2 Blaauw, A. H, Licht und Wachstum. Zeitsch. Bot. 6: 641-703. 1914. Also, 

 7: 465-532- 1915- 



