88 PLANT PHYSIOLOGY. 



Instead of the ordinary spectroscope, a microspectroscope may be 

 used as described by Detmer, 22. Particularly good colored plates of 

 chlorophyll spectra are given in the Frank and Tschirch diagrams, 

 Nos. XV, XVI. 



The observation of the character of light passing through living leaves 

 is easily practicable and instructive. Various thicknesses of leaves may 

 be placed between the source of light and the spectroscope until the light 

 just disappears. Also an instructive result is given by observing this 

 light by the eye directly. For this should be provided a tube about 20 to 

 30 cm. long and 2 to 3 cm. diameter, closed at one end and provided with 

 a cap just fitting the closed end. It maybe of pasteboard. Holes half 

 the diameter of the tube should be cut in end and cap. Circles of fresh 

 leaves nearly the diameter of the tube may then be placed between end 

 and cap ; then the tube, with the eye at the other end, should be pointed 

 at the sun. No light should leak into the tube, which should be black- 

 ened throughout. Successive layers of leaf should be added until finally 

 no light passes through, and the color of the last vanishing rays should 

 be determined. 



The preceding experiment shows that chlorophyll absorbs 

 certain rays from light, allowing others to pass through it. 

 This suggests that the absorbed rays may be the ones used in 

 the process of Photosynthesis. 



54. Are these light-rays particularly absorbed by Chlorophyll 

 the ones which are active in Photosynthesis? 



Answer by Experiment 27. 



EXPERIMENT 27. This may be tested by allowing the principal 

 rays absorbed by Chlorophyll, i.e., red and blue, to act upon a leaf, 

 and noting the amount of starch formation in comparison with that 

 made under rays not absorbed by Chlorophyll, i.e., green. These 

 rays may most conveniently be applied by use of colored liquids, 

 each of which cuts out all rays except its own color (which can be 

 determined only by use of the spectroscope). The leaves should be 

 prepared for the experiment and the test for starch applied as in 

 Experiment 24. The colored liquids may be made thus : for red, 

 the dye "scarlet"; for blue, ammoniacal sulphate of copper; for 

 green, a carefully tested mixture of potassium chromate and ammo- 

 niacal sulphate of copper. These should be placed in small square 

 bottles which are held by clamps upon the two surfaces of the leaf 

 so that the colors match on the two surfaces ; the leaves thus pre- 

 pared should be exposed to bright but not intense light for a day, 

 and in the evening tested for their starch contents. 



(In preparing the solutions, the spectroscope must be used to deter- 



