308 ASSIMILATION. 



818. The conversion of the energy' of the motion of the ethe- 

 real medium (in radiant light) into chemical separation of oxj-- 

 gen from the carbon of carbonic acid, and the production of 

 this treasured energy- under other forms, is the chief office of the 

 plant. 



819. Attention has alreadj' been called (see page 306) to the 

 well-known fact that a beam of suuhght is composed of rays or 

 lines of undulations differing both in respect to their amplitude 

 and \'elocity. Hence it is to be expected that in their action on 

 the plant these raj's, which are iu fact vehicles of kinetic energy, 

 must have diverse effects. 



820. Classiflcation of the rays of the spectrum. When a beam 

 of sunlight is transmitted through a triangular prism, it is broken 

 up into its constituent rays, which, falling upon a screen, form 

 what is known as a spectrum. The colors of the spectrum 

 grade from red at one end, through orange, yellow, green, 

 blue, and indigo, to \iolet. The violet rajs are bent further 

 from their course by the prism than any of the others above 

 spoken of, and hence are termed the most refrangible ; experi- 

 ment has also shown that these highly refrangible rays are most 

 efficient in producing the chemical changes long known to be 

 attributable to light : for this reason they have been denomi- 

 nated chemical (or sometimes actinic) rays. The red rays are 

 bent far less from their course than anj^ of the others above men- 

 tioned, and hence they are termed the least refrangible. It is 

 at the red end of the ^-isible spectrum that the greatest amount 

 of heat is found. The rays which constitute .yellow and orange 

 light are of medium refrangibility ; they arc the most distinctly 

 luminous. It is proper, therefore, for convenience, to distin- 

 guish rays of the solar spectrum as chemical, luminous, and 

 heat rays, according to the dominant eflfect which thej' produce. 

 But it should be stated that each of these three groups may 

 share some of the work specially belonging to the others ; and 

 further, that bej^ond the visible spectrum are rays which are 

 efficient in accomplishing certain kinds of work. These latter 

 are known respectivel}' as the ultra-violet and the ultra-red 

 rays. 



Before examining the action of these different rays of light 

 upon the assimilative activity of chlorophyll granules, inquiry 

 must be made as to 



absorption, wliicli is essentially a process of moleciilai' adhrsion, is accompanied, 

 as is capillary attraction, by electrical disturbances. In no case is energy lost . 

 one form disappears only to reappear in some other, 



