Fig. 115.
Radish seedlings grown in the
dark, long, slender, not green.
Fig. 116.
Radish seedlings grown in the
light, shorter, stouter, and green
in color. Growth retarded by light.
262. Growth in the absence of light.—Plants grown in the dark are subject to a number of changes. The stems are often longer, more slender and weaker since they contain a larger amount of water in proportion to building material which the plant obtains from carbohydrates manufactured in the light. On many plants the leaves are very small when grown in the dark.
263. Influence of light on direction of growth.—While we are growing seedlings, the pots or boxes of some of them should be placed so that the plants will have a one-sided illumination. This can be done by placing them near an open window, in a room with a one-sided illumination, or they may be placed in a box closed on all sides but one which is facing the window or light. In 12-24 hours, or even in a much shorter time in some cases, the stems of the seedlings will be directed toward the source of light. This influence exerted by the rays of light is heliotropism, a turning influenced by the sun or sunlight.
Fig. 117.
Seedling of castor-oil bean,
before and after a one-sided illumination.
Fig. 118.
Dark chamber with opening at one side to
show heliotropism. (After Schleichert.)
264. Diaheliotropism.—Horizontal leaves and shoots are diaheliotropic as well as diageotropic. The general direction which leaves assume under this influence is that of placing them with the upper surface perpendicular to the rays of light which fall upon them. Leaves, then, exposed to the brightly lighted sky are, in general, horizontal. This position is taken in direct response to the stimulus of light. The leaves of plants with a one-sided illumination, as can be seen by trial, are turned with their upper surfaces toward the source of light, or perpendicular to the incidence of the light rays. In this way light overcomes for the time being the direction which growth gives to the leaves. The so-called “sleep” of plants is of course not sleep, though the leaves “nod,” or hang downward, in many cases. There are many plants in which we can note this drooping of the leaves at nightfall, and in order to prove that it is not determined by the time of day we can resort to a well-known experiment to induce this condition during the day. The plant which has been used to illustrate this is the sunflower. Some of these plants, which were grown in a box, when they were about 35 cm high were covered for nearly two days, so that the light was excluded. At midday on the second day the box was removed, and the leaves on the covered plants are well represented by [fig. 119], which was made from one of them. The leaves of the other plants in the box which were not covered were horizontal, as shown by [fig. 120]. Now on leaving these plants, which had exhibited induced “sleep” movements, exposed to the light they gradually assumed the horizontal position again.