The behavior of leaves in illuminations below the normal depends upon the relation of these organs to the storage structures of the plant as well as upon other factors, and many types are dependent upon their own activity for plastic material necessary for growth.
It is to be said in general that leaves of dicotyledonous plants are incapable of full development in darkness, though to this rule there are many exceptions. Thus the leaves of the beet develop normally, or nearly so, in darkness.
On the other hand, leaves of monocotyledonous plants attain normal size in darkness, especially those with straight or curved parallel venation. Some, as the iris, swamp marigold, and onion, attain a greater length in darkness than in light. Here, as in stems, cell division is not modified, but the growth of the individual cell is increased.
The growth of leaves in darkness may be easily observed if the underground perennial stems of common mandrake are placed in a dark chamber before the growth of the leaf buds has begun. The leaves are peltate, and in the bud are folded about the end of the petiole after the manner of an umbrella. Usually this umbrella expands as soon as it has pushed upward and become free from the soil, attaining a diameter of twenty-five to forty centimetres when outspread. In darkness, however, it refuses to unfold, the laminae are pale yellow and retain the crumpled form of the bud, and as the petiole shows an exaggerated elongation the organ takes on the appearance of a very small parasol on a very long handle. The imperfect development of leaves and the rapid decay of aerial organs deprived of sunlight leads to the conclusion that the action of light is necessary to the health and normal activity of these organs, and the light therefore exercises a tonic influence upon vegetation.
Many species of plants are so plastic and capable of such ready response to variations in external conditions that they undergo distinct morphological changes in response to variations in the intensity of the light. The common potato is an example of this fact. The edible tubers are simply thickened stems, and the plant has the habit of storing starch in any stems not acted upon by the light. The branches arising from the base of the main stem are generally underneath the surface of the soil, and afford the proper conditions for tuber formation. Sugar is constructed in the leaves, carried down the length of the stem, and deposited in the underground branches as starch. Space is made for the accumulating store by the multiplication of the thin-walled cells of the pith. If any of the upper branches should become shaded, they become at once the focus of converging streams of sugar, and similar enlargement ensues, re-