134 ABSORPTION-SPECTRA [CH. 



green-houses when assimilation is going on, and why it is 

 important to employ glass of a certain degree of trans- 

 parency to the red-orange rays ; why plants which flourish 

 in full sunshine become " drawn " and " sickly " in the 

 shade, and how important a part plants play in preventing 

 the undue accumulation of carbon-dioxide in the atmo- 

 spheric ocean, and in counterbalancing the loss of oxygen 

 always going on owing to the tendency of this active 

 element to combine with other bodies in Nature. 



The large surface of the leaf, increased still more by 

 the enormous intercellular excavations, and the thin semi- 

 transparent texture, enable the chlorophyll-cells to obtain 

 access to light and air to a degree impossible with most 

 other organs ; and we have seen that the normal mean 

 position of the leaf, attained owing to its reactions during 

 and after growth, is that of best exposure to the sun's 

 rays, while direct observations have shown that the light 

 passes into and some rays even through the mesophyll. 

 The absorption-spectrum of a thin leaf, in fact, which is 

 not essentially different from that of suitable solutions of 

 chlorophyll itself, shows that the light chiefly absorbed is 

 the red-orange, between the lines B and C, and to a less 

 extent the blue and violet from a region in the neighbour- 

 hood of the line F to the ultra-violet, while the yellow and 

 green are transmitted. 



That it is the red-orange rays which are essentially 

 concerned in carbon-assimilation seems to result therefore 

 not only from experiments in which the number of 

 oxygen bubbles are counted, and especially from experi- 

 ments under coloured shades, &c, but also from the fact 

 that it is just these rays which are so powerfully held 

 back in the chlorophyll of the leaf. 



It has been found, for instance, that green plants 

 rapidly died of inanition in light which had passed through 



