PHYSICAL AND METEOROLOGICAL RESEARCHES. 



105 



A photograph taken October 22 shows at a glance the influence of 

 different solar rays (flg. 2). The difference observed might have several 

 causes, viz: (1) The difference in the solar rays admitted; (2) difference 

 in temperature which, compared with that in the white house, was lowest 

 in the blue and highest in the red house; (3) difference in the amount 

 of light, the intensity of which was greatest in the white hothouse 

 and least in the blue; and (4) to difference in humidity of the soil and 

 the air, which was lowest in the white hothouse and highest in the 

 bine. 



It would be interesting to know whether the results obtained are due 

 to these differences. If the differences depend 

 upon temperature, they would differ accord- 

 ing to the seasons. In the spring the temper- 

 ature of the hothouses during daytime remains 

 below the optimum for the growth of plants. 

 The amount of growth, therefore, would follow 

 in this order : White, red, green, and blue; and 

 during the summer, when the temperature 

 often surpasses the optimum for plant growth, 

 the maximum of growth would occur in the 

 inverse order, namely: Blue, green, red, and 

 white. The blue hothouse, the temperature of 



Fig. 2. — Sensitive plants growu in different colored light. 



which is nearest the optimum, should favor the greatest growth. On the 

 contrary, the results obtained in the different houses were absolutely 

 identical in spring and summer, notwithstanding the difference in tem- 

 perature. The greatest development was always produced in the red 

 hothouse, and the minimum in the blue, where there was almost no 

 growth. The results indicate that the inequality in growth is not due 

 to differences in temperature. The sensitive plant in the white hot- 

 house owes its feeble growth to the combined action of excessive light 

 and temperature. 



In 1896 experiments were begun to compare the development of 



