PHO TOS YN THESIS. 8 9 



tcrmine that t/iey are made of just suc/i a density that tJu full bottle 

 transmits its own color only, and as much of that as possible.) 



Colored glass would seem available for this purpose, but the spectro- 

 scope shows that none of the colors obtainable are pure ; the eye is an 

 entirely unsafe guide to the quality of colors, as it does not resolve mix- 

 tures. Probably the liquids could be arranged in some better way than 

 the above, which gives only fair results. Gelatine plates are better than 

 glass, for the colors are obtainable nearly pure for red and green, though 

 the blue is poor. (The gelatine known as No. 073 gives a perfect red, 

 while 052 gives a fair blue. No good green is available.) Attempts 

 made by my students to dye sheets of colorless gelatine by use of the 

 above-mentioned colors have not been successful. The ideal method of 

 applying colors to a leaf for this purpose would be one in which both red 

 and blue light are supplied together to the same piece of leaf, which ob- 

 viously cannot be done by solutions containing both colors, nor by use of 

 two colors of gelatine sheets placed together. (See Note 7 of Addenda.) 



Another method of testing the effect of colored rays and other exter- 

 nal conditions upon photosynthesis is that of counting the oxygen bub- 

 bles given off by water-plants placed under the special conditions (see 

 Darwin and Acton, 36), either in the spectrum itself, or behind colored 

 screens. But it is hardly practicable here. On the use of pure-color 

 screens, consult the paper by Pennington, in Contributions from the Bo- 

 tanical Laboratory of the University of Pennsylvania, 1, 203. 



Also, for this purpose the pure-color chamber later described (page 108) 

 should be available, by the use of small seedlings grown low in small pots, 

 but I have not yet thoroughly tested it in this way. 



You have learned (Section 49) that starch contains carbon. 

 The only known source of supply of this element for most 

 plants is the CO., of the atmosphere, though small quantities 

 may be absorbed by some plants in compounds from the soil. 



55. Is atmospheric C0 2 necessary for Photosynthesis? 

 Answer by Experiment 28. 



EXPERIMENT 28. This may be settled by determining whether 

 starch-formation can go on in an atmosphere deprived of CO 2 . Pre- 

 pare two large bottles with wide mouths, and rubber stoppers through 

 which chloride of calcium tubes have been fitted (Fig. 18). Cut under 

 water two small shoots, holding two or three leaves each, from a plant 

 prepared as for Experiment 24, and place these in vials nearly full of 

 water. Place shoots and vials in the bottles, and on the bottom of 

 one put a quarter-inch depth of soda lime (a powerful CO 2 absorber) 

 and fill the tube of the same with the soda lime. In the other (the 

 control experiment) place fine sawdust in place of the soda lime, to 



