PHOTOSYNTHESIS 



in 



i crt).X6 cm. in size, 

 of the spectroscope, to 



proa 



Fig. 28. — Arrangement op 

 vials upon a glass 

 plate to form a pure- 

 COLOR screen; Xf. 



The heaviest lines represent 

 tin-foil. 



Experiment. — In four good vials, some 

 place pure-color solutions made up, by aid 

 match the black bands of the chlorophyl 

 spectrum as closely as possible, viz., red 

 from the dye "scarlet," (orange^yellow seems 

 unattainable in one liquid), green from a 

 mixture of ammoniacal sulphate of copper 

 with potassium chromate, and blue-violet 

 from ammoniacal sulphate of copper. Add, 

 as a control, a vial of distilled water. Place 

 the corked vials side by side on a glass plate 

 (Fig. 28), separated from one another and 

 from the plate by tin-foil, except that under 

 each vial is left an open slit 3 mm. wide and 

 the length of the vial; on the under side of 

 the plate add tin-foil with exactly matching 

 slits, and then place the plate on the leaf of 

 a plant kept previously a day or two in 

 darkness, following the normal light-screen 

 method (page 87). Expose the leaf to 

 bright light for two or three hours, then 

 blanch it and apply the iodine test. 



Precautions. The solutions must be made up with the spectroscope, 

 and should be such that each transmits the full intensity 01" its own light, 

 but no other rays. The vials should be not quite full, and to prevent the 

 forcing out of the corks under the sun's heat, there should be a fine thread 

 between each cork and neck, which permits some adjustment of pressure. 

 The vials may be attached to the tin-foil (which is itself gummed to the 

 glass) by drops of shellac; and it is well to surround them by wooden strips 

 as in the figure (Fig. 28). The colors may be preserved in their vials for 

 future use. Several sources of error are present, i.e., the difficulty of 

 making the colors spectroscopically accurate, the different heating effect 

 of the different colors tending to promote translocation in different degrees, 

 and a very probable more rapid translocation from under the blue than 

 from under the red. The form of the vials concentrates the light upon the 

 leaf, hence partially compensating the great absorption. 



Other Methods. This important subject may be approached in any 

 one of several ways. One of the best known is that used by Sachs, in which 

 double-walled glass bell jars, filled between the walls with colored liquids, 

 are placed over similar plants, the amounts of photosynthate being after- 

 wards determined by the starch test. The jars are expensive and the method 

 somewhat slow and cumbersome, and it introduces the individual-difference 

 error inseparable from the use of different plants. In place of the bell jars, 

 MacDougal (124) uses concentric glass cylinders. A very different method 

 consists in the counting of the oxygen bubbles given off by water-plants 

 placed behind colored screens or in different parts of a spectrum; it is some- 



