no PLANT PHYSIOLOGY 



he should consult the critique of optima by Blackman in 

 Annals of Botany, 19, 1905, 281. 



The exactness of the photosynthetic equation demonstrated 

 by the preceding experiment calls to attention the fact that, 

 as in all such equations, when certain quantities are known the 

 others can readily be determined. The student should now, 

 using the data given in Part III, calculate in grams or other 

 desired units, how many grams of the photosynthate can be 

 formed from a given number of grams of carbon dioxide or of water, 

 how many grams of oxygen are released in the process, how many 

 grams of carbon dioxide or of water are needed to form any given 

 number of grams of photosynthate or to release so many grams 

 of oxygen, and how much atmosphere must be emptied of its car- 

 bon dioxide to form any given quantity of photosynthate. The 

 student should make these various calculations, using definite 

 quantities, and should practice the calculations until he can 

 make them with ease, certainty, and even pleasure. 



There is yet another fact, of the utmost importance, shown 

 by the photosynthetic equation, namely, that either the carbon 

 dioxide or the water, or perhaps both, are dissociated in the 

 process. Since these are extremely stable substances, they 

 require a large amount of energy to dissociate them, and the 

 question arises as to the source of this energy. Here one at once 

 recalls the constant association of the process with light, as well 

 as the stoppage of a part of the light energy by the chlorophyl, 

 shown by the black bands of its absorption spectrum. These 

 facts suggest that the particular energy which does the photo- 

 synthetic work consists of those principal rays of light which 

 are absorbed by the chlorophyl. So important a matter requires 

 experimental study, which formulates an important problem as 

 follows : 



Are the rays of light absorbed by chlorophyl capable alone of 

 doing photosynthetic work? 



This may be determined by supplying to a green leaf rays spectro- 

 scopically equivalent to those absorbed by the chlorophyl, either with- 

 holding all others, or else, and better, supplying them as a control, 

 and noting whether photosynthesis takes place. 



