82 PLANT PHYSIOLOGY 



One of the optical properties of a substance of the first impor- 

 tance is its power of differential absorption of light. This is 

 investigated by means of the spectroscope, the principle of which 

 the student must now thoroughly understand. It happens, 

 as will later fully appear, that this property in chlorophyl is of 

 fundamental physiological importance, which necessitates here 

 this inquiry: 



What is the absorption spectrum presented by chlorophyl ? 



To determine this it is simply necessary to examine spectroscop- 

 ically the light passed through chlorophyl, either in the living leaf, or, 

 more conveniently, in solution, and to compare this with the spectrum 

 of unaltered light. 



Experiment. Prepare a solution of chlorophyl freshly in the dark 

 by the method of the preceding section, using the most available of 

 the materials listed below. Place it in three vials, preferably flat- 

 sided (or in small Soyka flasks) ; support these in front of the slit of 

 the spectroscope, which is directed towards a convenient white light, 

 such as incandescent, electric, or Welsbach; examine the spectrum 

 in various thicknesses in comparison with an unaltered spectrum 

 formed by the comparison prism. Make sure, by way of control, 

 whether any of the observed effect is due to the solvent. Chart the 

 chlorophyl and the pure spectrum in colors, using a scale to ensure 

 correct proportions and position, and add the usual notation. 



Materials. These are fully discussed, from the present point of view, 

 with an account of methods and precautions, by Miss Eckerson in the 

 Botanical Gazette, 40, 1905, 302. As there recorded, the most typical spectra, 

 in order of excellence, are yielded by Primula obconica, Radish, Horse Bean, 

 Abutilon young leaf, seedling Oats, Cestrum elegans young leaf, Poinsettia 

 young leaf, Tomato, Chinese Primrose, and Castor Bean. A more com- 

 plex spectrum, with additional bands indicative of incipient decomposition, 

 is shown by acid leaves of a yellow-green color, such as Pelargonium species, 

 Begonia coccinea, Oxalis Bowiei. 



Spectroscopes. The absorption bands in the chlorophyl spectrum are 

 so prominent that any spectroscope, however simple, shows them clearly. 

 Very convenient for their examination is the Kirchoff and Bunsen student 

 form, costing about %o.oo. It is greatly improved for the purpose if there 

 is added a proper support for the vials, together with an adjustable mirror 

 for lighting the comparison prism, and another for the scale, thus permitting 

 the entire instrument to be used with one electric or other light. The com- 

 plete arrangement is shown diagrammatically by Fig. 15. Much more com- 

 pact, and nearly as efficient, though lacking the advantage that the principle 

 can be readily shown, is the direct-vision spectroscope. One of these, espe- 

 cially prepared for student use, is among my normal apparatus, and is figured 

 herewith (Fig. 16). It is a Browning instrument with comparison prism, 



