The Prevalence of Green Color in Plants 31 



Thus is our equation triumphantly vindicated, and we shall 

 know it henceforth as the photosynthetic equation. Its importance 

 and meaning may thus be expressed as another of our botanical 

 verities, — that the 'photosynthetic sugar made in green leaves in 

 light is constructed from water drawn from the soil, and carbon di- 

 oxide derived from the atmosphere, with an incidental release of pure 

 oxygen, according to the jjhotosynthetic equation 6 COo + 6 H/) = 



It may interest the reader now to know what quantities of 

 these gases are necessary in the making of the sugar. For one 

 gram thereof there are required 750 cubic centimeters (about 

 f of a quart) of pure carbon dioxide, which is all that is con- 

 tained in 2 cubic meters of atmosphere, and there is released the 

 same quantity of pure oxygen. This, therefore, is the amount 

 of those gases absorbed and released by a square meter (or yard) 

 of green leaf each hour on a bright sununer day. This release 

 of oxygen, by the way, explains the remainder of the fact earlier 

 mentioned, that plants purify the air which animals vitiate, for 

 the plants not only remove the poisonous carbon dioxide from the 

 air, but replace it by pure oxygen. And it may interest the reader 

 to know how this balance of purification and vitiation works out 

 between green leaves and men. Calculations have shown, in 

 brief, that about 25 square meters (or yards) of green leaf are re- 

 quired to balance the respiration of a man on an ordinary sum- 

 mer day. But as the release of oxygen stops at night, it takes 

 about 60 square meters of leaf working for a day to balance the 

 man's respiration for 24 hours, and about 150 square meters work- 

 ing through the summer to balance his respiration for a year. 



In composing the foregoing paragraphs I have given much care 

 to the form of their presentation, for the reason that this particu- 

 lar topic illustrates exceptionally well the principal method of 

 scientific procedure in the acquisition of new knowledge. First, 

 in the given problem, to observe all the facts that the militant 

 eye can discover : next to compare and marshal the data thus won 



