212 Plant Physiology 



rapid diffusion of CO 2 from the soil, due largely to the activ- 

 ity of microorganisms of the soil. In consequence there 

 may be a stratum near the soil so much richer in C0 2 as 

 to be distinctly advantageous for low-lying or rosette- 

 forming plants. 



115. Light the source of energy. - - It has been indicated 

 that an important feature of the work of chlorophyll 

 is the absorption of light, or the taking over of energy. 

 If a beam of sunlight is dispersed by a suitable prism, it is 

 found to consist of groups of rays of different wave length, 

 refrangibility, and " color " ; the beam is thus separated 

 into the well-known spectrum, the visible portion of which 

 presents a series of colors as follows : red, orange, yellow, 

 green, blue, indigo, and violet. 



If in the path of light there is interposed a weak solution 

 of chlorophyll in a suitable glass vessel, certain definite 

 absorption bands appear in this spectrum. There are, 

 in fact, seven of these, four in the region of red to green 

 and three beyond the blue, generally rather indistinctly 

 demarcated, and in strong solutions cutting out the visible 

 rays beyond the blue (Fig. 56). The four bands in the 

 red end of the spectrum are those of the blue-green solu- 

 tion, cyanophyll ; and the most important absorption 

 band is in the red, corresponding to wave lengths of 650 w 

 and thereabout. 



A considerable amount of experimental work has been 

 done to determine the rate of photosynthesis under the 

 different monochromatic lights. Colored glass screens 

 and double-walled vessels containing colored liquids have 

 been much employed. Since such materials seldom afford 

 pure monochromatic lights, they give only a crude idea of 



