196 POPULAR SCIENCE MONTHLY. 



The behavior of leaves in illuminations below the normal de- 

 pends upon the relation of these organs to the storage structures of 

 the plant as well as upon other factors, and many types are de- 

 pendent 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 ap- 

 pearance of a very small parasol on a very long handle. The imper- 

 fect 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 dis- 

 tinct 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 un- 

 derneath 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 multi- 

 plication 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- 



