14 NATURE OF CHLOROPHYLL 



absence of chlorophyll in the plastids. Similarly green plants 

 lose their chlorophyll when placed in the dark. This is the 

 principle employed in the blanching of celery. The stalks are 

 surrounded by earth or the light is excluded from them by some 

 other means; chlorophyll disappears from the plastids and fails 

 to develop in the new cells that are formed in the dark. Plants 

 in which the chlorophyll is not developed are said to be etiolated. 

 The plastids are in the cells, however, for if an etiolated seed- 

 ling is removed from the dark to the light the chlorophyll will 

 begin to appear in a few hours. The greening of potatoes when 

 removed from the ground and left in a strong light is a familiar 

 example of this. It is evident that the formation of chlorophyll 

 is usually closely connected with the action of light. Another 

 important point regarding the influence of light is the fact that 

 such a food as starch is not usually found in the leaf in the 

 absence of light. This may be easily verified by cutting the 

 initials of your name in rather large letters in a strip of black 

 paper or smooth tin-foil and fastening the strip in the early 

 morning to the upper surface of a well-sunned leaf of a starch 

 forming plant. After a few hours remove the leaf from the plant 

 and place it in alcohol. This will dissolve the chlorophyll and 

 the leaf will become quite white in a few hours. Now place the 

 leaf in a tincture of iodine which turns starch to a blue or blue- 

 black color. Wherever the light was excluded from the leaf 

 there is only a pale yellow color but all portions of the sunned 

 leaf show an abundance of starch as revealed by the blue or blue- 

 black coloration. Consequently the letters appear black on a 

 'yellow background which marks the limits of the strip of paper. 

 No starch can be detected as a rule in leaves taken from plants 

 that have been growing in the dark for twenty-four hours. 

 Plastids that do not contain chlorophyll, examples of which are 

 found in the cells forming the white bands or blotches in many 

 variegated plants, do not form starch in the light. It is evident 

 from these facts that the chlorophyll and light co-operate in 

 some way in the formation of starch. It is known that chloro- 

 phyll absorbs a certain portion of the sunlight. If a beam of 

 sunlight is caused to pass through a prism it is separated into 



