PHOTOSYNTHESIS 85 



The Light from Dense Solutions. The color of the transmitted light just 

 before it is finally extinguished by increasing thickness of solution of chloro- 

 phyl is of great interest in connection with its spectrum. It may be observed 

 by use of a very dense solution, made up from dark- 

 green leaves placed either in a row of vials from 

 which side light is cut off, or in a wide crystal- 

 lizing dish. It should also be viewed without the 

 spectroscope, for which purpose it may be placed in a 

 long test-tube wrapped with black paper and held over 

 a light-reflecting mirror. The same phenomenon can 

 be observed in living leaves by use of the diaphanoscope 

 of Sachs (Gesammelte Abhandlungen, r, 169), a simple 

 instrument readily adapted from concentric cardboard ' _ 



OSCOPE WITH 



cylinders blackened inside and arranged to hold discs of '„ . . 



1 j. i_ u j_ - r re- \ LEAr DISCS ^ po " 



leaf, as shown by the accompanymg figure (Fig. 17). sitiqn m L0NG1 . 



The eye is placed at the open end, with the discs directed ' 



i l ■ 1.^ i- i_* Vn_ 1 l" J u 1J l. TUDINAL SECTION; 



towards a bright light. The longer cylinder should be 

 about the length of clear vision, viz., 25-30 cm. 



Colored Foliage Leaves. Many of these, such as Coleus, etc., seemingly 

 red, may be shown to possess chlorophyl by placing them in hot water or 

 alcohol; and this is true, also, of some brown and red seaweeds. 



The student will of course make sure by review, or other- 

 wise, of his knowledge of the physical significance of the black 

 bands .in relation to light energy. The impbrtance of the sub- 

 ject will be evident later. 



Turning for a time from the properties of chlorophyl to its 

 distribution, if the student will observe its places of occurrence 

 through those plants which possess it, he will see that it occurs 

 only in the parts exposed to light, and moreover is most abun- 

 dant where the light is greatest. This implies some very close 

 connection between the use or other meaning of chlorophyl and 

 the presence of light. To test the existence of such a connec- 

 tion, the natural first step is to observe the effect of withholding 

 the light from green tissues, and thus is presented the following 

 problem : 



What differences develop between similar green tissues kept 



■ for some time respectively in and away from the light ? 



To determine this it is only necessary to screen a green tissue from 

 light, leaving a control portion exposed; but the screening must be 

 done in such a way that the screened and unscreened portions are 

 under precisely the same conditions except as to light, are not seri- 



