SCIENCE 73(1899): 566-567 (MAY 22, 1931)
GROWTH OF PLANTS UNDER CONTINUOUS LIGHT
University of Colorado
J. Adams, Amer. Jour. Bot., 12: 398, 1925.
R. B. Harvey, Bot. Gaz., 74: 447, 1922.
STUDIES carried on by the writer since 1926 with plants illuminated both day and night seem of sufficient interest to report upon briefly at this time. Others who have tried somewhat similar although not exactly the same experiments1 seem not to have secured just the results which have been so apparent in my work. Preliminary accounts of my studies were made at meetings of the Southwestern Division of the American Association for the Advancement of Science in Santa Fe, 1927, and in Flagstaff, 1928. The experiments are being continued, and a full account will be published at a later time.
Plants, chiefly annuals, have been grown in the greenhouse under natural light in the daytime and, in addition, during both day and night they have had the light of two 100-watt Mazda lamps suspended above the bench at a distance of four feet-the lamps provided with an overhead reflector. Controls, shielded from the artificial light, are growing in the same room of the greenhouse on the same bench at a distance of about ten feet. A total of nearly one hundred species have been worked with, some of them 'during two or more seasons if first results seemed doubtful. The list includes common garden vegetables, grains, weeds, native herbs and garden ornamentals.
In general, the experimental plants are taller than the controls at all times during the entire growth period, this increased height being due to elongation of internodes. Frequently the experimental plants are slender-stemmed and have a decumbent habit. Flowering is usually hastened under continuous light but in a few species is completely inhibited. Plants of some species reach full adult stature, come to blossom, and produce fruit and seed while the check plants are still in the rosette stage close to the ground.
The root system in plants of the experimental series is invariably less extensive than that of the controls; roots are smaller, shorter, and have fewer branches. Thickened taproots do not develop.
Leaves of plants under continuous illumination often show no modification but in a considerable proportion of the species studied they have smaller, thinner blades, and often longer petioles. Leaves of monocotyledons tend to be very much lengthened as do the sinuately deeply-cleft leaves of certain Hydrophyllaceae. Reduction in leaf size is especially noticeable in certain members of the chickweed family, and this with the thin stems and greatly lengthened internodes and frequent paleness of color gives a suggestion of etiolation. But only a few species are sufficiently pale or show the leaves so much reduced as to make the similarity to etiolated plants very pronounced. The greatly lengthened internodes are, however, a practically constant feature.
Internally, the stems of plants grown under continuous light show a thinner cortex, less vascular tissue (especially phloem), and a relatively larger pith than the controls. Leaf-blades in cross-section look as if derived from plants grown in the shade, usually having a single layer of palisade and with more and larger intercellular spaces than the plants grown under ordinary greenhouse conditions. Leaf cells are smaller, hence the leaves are thinner. Roots of the plants of the experimental series show slight development of phloem but otherwise are of usual structure, except that as previously noted they are small and short and with few branches.