212 



PLANT PHYSIOLOGY 



amount of light is required by them to carry on the essen- 

 tial process of food-manufacture. A larger amount acts as 

 a stimulus to the formation of reproductive organs (pp. 

 264-8 ) . Otherwise the relations of these plants to light are 

 similar to those of land plants. Since the water and the 

 materials at the bottom of a natural body of water absorb 



a 10 



^1 



FIGUEE 16. DIAGRAM SHOWING THE GROWTH OF 



DIATOMS AND THE INTENSITY OF LIGHT 



AT VARIOUS DEPTHS. 



Water in Lake Cochltuate, Hassacbusetts, Nov. 29, 

 1895. Examined Deo. 9, 1895. Temperature 40-44°. 

 Color 0.33 (pt. standard) . Diatom schiefly AsUrion- 

 ella and Melosira. Intensity of light at different depths 

 calculated on assumption that layer of aq. 1 Jt. In depth 

 absorbs 25 per cent, of light failing upon it. 

 (After Whipple). 



light, plants living in such a situation receive less light than 

 the plants living on the bank near by. Plants living on or 

 over a dark or black bottom receive light only or mainly 

 from almost vertically above. The quality as well as the 

 quantity of light reaching water plants differs from that 

 falling upon plants in the air. Water absorbs the different 

 kinds of rays unequally, the luminous rays least, the actinic 

 rays more. Whipple" cultivated diatoms in bottles at dif- 



* Whipple, G. C. Some experiments upon the growth of diatoms. Tech- 

 nology Quarterly, vol. IX., 1896. Also Microscopy of Drinking Water, 

 1901. 



