26 A TEXTBOOK OP BOTANY [Ch. Ill, 3 



separated, in part at least, from tlieir existent unions in 

 those substances. But ))oth carbon dioxide and water are 

 very stable compounds, and therefore their dissociation or 

 separation into their constituent atoms rerjuires the applica- 

 tion of much power, the basis of which is energy. This 

 energy is loiown to be supplied b}' the sunlight, of which 

 the role in photosynthesis is thus exjjlained. Now the 

 energy in the hght cannot of itself effect this dissociation 

 (else obviously no carbon dioxide or water vapor could re- 

 main in the atmosphere), and accordingly there is also neces- 

 sary some agency by which the energj' in the light can be 

 applied to the actual work of dissociating or splitting the 

 molecules of carbon dioxide and water into their constituent 

 atoms. That agency appears to be the chlorophyll, though 

 it is not yet certain in precisely what way it accomplishes the 

 result. Thus the sun supplies the energy for photosynthesis, 

 and the chlorophyll applies it as power to the actual work. 

 This is why both are essential. 



The stud}' of chlorophyll by aid of the spectroscope shows 

 that practically only certain red and the blue rays are ab- 

 sorbed by chlorophyll from the many contained in the 

 white sunlight ; but these are Icnown to be the raj's effec- 

 tive in photosynthesis. Since those rays are absorbed, they 

 do not come to our ej-es from the leaves ; ]]ut the unabsorbed 

 rays, those useless in photosynthesis, reach our eyes in a 

 mixture which collectively gives the sensation of green. 

 Thus the greenness of vegetation is due to the light rejected 

 by the chlorophyll after removal of the rays useful in photo- 

 synthesis. 



The photosjT.ithetic formation of grape sugar is often 

 compared with a process of manufacture carried on by man. 

 The leaf is the factory constructed for the work : the epider- 

 mis forms the external walls, giving shelter from weather, 

 while the chlorenchyma cells are the working rooms, and 

 the veins, with storaata and air spaces, the passages 

 for access and removal of materials; the simlight is the 



