458 CHEMICAL DISCOVERY AND INVENTION 



interior through the small openings, called stomata, which exist 

 for the most part on the lower surface of the leaf, and these 

 minute pores when fully open do not exceed 1 or 2 per cent of 

 the total area of the leaf surface. The astonishing result has 

 been arrived at that in a fully active leaf the atmospheric carbon 

 dioxide is taken up at least fifty times as fast as it would have 

 passed into a series of small openings of equal size with the 

 stomata, if these had been filled with a strong solution of caustic 

 alkali. 



The changes which go on in the interior of the leaf are asso- 

 ciated in a mysterious way with the green colouring matter, 

 chlorophyll, which plays the part of a sieve or filter of the sun's 

 rays, stopping some and allowing others to proceed. 



From the researches of C. A. TimiriazefT, Professor of Botany 

 in the University of Moscow, it has been shown that the reduction 

 of carbon dioxide as well as the production of starch is due to 

 the rays which are absorbed by the chlorophyll. As the chloro- 

 phyll transmits chiefly green light the part which is stopped lies 

 chiefly in the red, and on passing through a spectroscope the 

 light which is transmitted is seen to have a dark band between 

 the lines B and C of the solar spectrum. The blue and violet 

 rays produce very little effect. 



It is, however, not known with certainty what compound is 

 the first result of the decomposition of the carbon dioxide, 

 though it is commonly assumed that formaldehyde is the initial 

 product. Its formation can be expressed by the simple equation 



C0 2 +H 2 0=CH 2 0+0 2 . 



This accounts for the elimination of the equal volume of oxygen 

 which is known to be the other product. Formaldehyde is 

 inimical to living organisms, and if it accumulated to any 

 appreciable extent in a living leaf would speedily put an end to 

 all vital processes within ; in other words, it would kill the proto- 

 plasm by which it is supposed to have been produced. But 

 while the formation and existence of formaldehyde in minute 

 quantity appears to have been demonstrated in leaves exposed 

 to light, 1 the greater part of it disappears immediately in conse- 

 quence of condensation into some kind of carbohydrate, it may 

 be a form of glucose, or, as appears more probable, starch, which 

 remains stored up for use in the growth of the plant. It is not 



1 Usher and Priestley, Proc. Roy. Soc., 77, B, 369. 



