Q. Why are leaves a light green in spring?
A. Because the young leaves of spring have more sap than carbon; and, therefore, the yellow of the green prevails.
Q. Why are leaves a yellowish brown in autumn?
A. Because the carbon of the leaves is dying away, and the yellow tinge of the tissue and falling sap prevails over the blue.
Q. Why are plants a pale yellow when kept in the dark?
A. Solar light is essential for the production of carbon; and as plants kept in the dark lose their carbon, they lose the blue colour which should convert their yellow sap to green.
Q. Why are potatoes yellow?
A. Potatoes are grown underground, and, therefore, contain very little carbon (or blue colour); hence the yellow sap of the potato is not converted to green by carbon.
Q. Why are potatoes (which grow exposed to the air and light) green?
A. Because the sun-light increases their carbon; which (mingling with the yellow sap) turns the potato green.