After the stems emerge from the "drawn" buds they show a similar attenuation, attaining a length of twice the normal. The excessive elongation of stems is accompanied by variations in the structure and contents of the tissues. The cells are generally longer, while the walls are thinner. In consequence, organs grown in darkness are very weak and easily bent or broken. Growth in darkness is attended by the non-formation of chlorophyll. This is replaced by etiolin, giving the plant a pale, waxy, yellow appearance.
The adaptive elongation is not shown by all species, however. It has been found that stems of beet, hop, dioscorea, and a few others show no adaptations to diminished light. The adaptive modification of stems elongating in darkness is developed from the retarding influence exercised by light upon growth. Thus it is a well-known fact that the action of certain portions of the sun's rays actually impedes or checks the increase in volume known as growth, though it does not influence actual division of the cells to any great extent. When this retarding action is eliminated excessive elongation ensues.
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 laminæ 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 aërial 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, resulting in the formation of tubers. Such structures are occasionally observed in plants grown thickly together.
Vöchting, by a number of most ingenious experiments, has succeeded in producing tubers on any branch of a potato plant by simply inclosing the branch in a small dark chamber. As the result of one experiment the entire main stem springing from a sprouting tuber was converted into a new tuber nearly as large as the first. The entire plant at the close of the experiment had the form of a dumb-bell, with the old tuber as one ball and the new tuber as the other.
The same writer has described important results obtained from a study of the action of light upon the stems of cactus, consisting of a number of flattened internodes. When the growing tips of such plants were allowed to develop in a dark chamber the new internodes grown were cylindrical in form. Such behavior suggests that these plants were originally furnished with cylindrical stems and foliar leaves. The leaves at some time in the history of the plant were found unsuitable, and gradually atrophied, while the stems were flattened and extended to take up their functions.