DIV. ii PHYSIOLOGY 249 



causing there an increase of the pressure, and this is the cause of the appearance of 

 bubbles of gas at every wounded surface. 



The foundations of our knowledge of the assimilation of carbon 

 dioxide by the green plant were laid, in the end of the eighteenth 

 and beginning of the nineteenth centuries, by PRIESTLEY, INGEN- 

 HOUSS, SENEBIER, and TH. DE SAUSSURE. The discovery is of 

 extraordinary significance, for THE FORMATION OF ORGANIC MATERIAL 



FROM CARBON DIOXIDE BY THE GREEN PLANT IS THE PROCESS WHICH 

 KKNDERS POSSIBLE THE LIFE OF ALL OTHER ORGANISMS AND IN 

 PARTICULAR OF ANIMALS UPON THE EARTH (cf. p. 255). 



By means of the gas-bubble method it is easy to bring proof of 

 the statement made above that only the green parts of plants, and 

 these only in light, are able to assimilate C0 2 . Thus the stream 

 of bubbles from an Elodea which goes on briskly at a brightly-lit 

 window becomes slower as the plant is brought into the middle 

 of the room, and ultimately ceases when the intensity of the light is 



400 



FIG. 248. Absorption spectrum of chlorophyll according to GR. KRAUS. The Fraimhofer lines 

 (B, C, etc.) are indicated above and the wave-lengths (700 /t^-400 /&/*) below. The black and 

 shaded regions are those where the light is absorbed or weakened. 



still such as to allow our eyes to read. Within certain limits 

 assimilation increases in proportion to the intensity of the light. 

 Similar experiments may be carried out using artificial sources of 

 light. They show that all the methods of illumination in common 

 use may be effective in the assimilation of C0 2 . The rays of different 

 wave-length are by no means of equal use in assimilation. 



The ultra-red and ultra-violet rays have very little effect, and the assimilatory 

 activity is almost entirely limited to the rays of a wave-length from about 0'4 p 

 to '8 ft which are perceived by our eyes. "Within these limits light of a wave- 

 length of about 0'68 /* has undoubtedly the greatest effect ; this is the wave-length 

 at which the maximum absorption of light by chlorophyll occurs (Fig. 248). In 

 other regions of the spectrum also, according to URSPRUXG, there is a correspondence 

 between the absorption of light and assimilatiou. It is true that the assimilation 

 in blue and violet light is not so great as the absorption bands would suggest ; 

 according to UKSPUUXG this depends on secondary causes. 



Since sunlight is in nature an indispensable factor in C0 2 assimila- 

 tion it becomes at once clear why certain organs of the plant, the 

 foliage leaves, have a flat expanded shape. Their large surface fits 



