Relation of Light to Growth of Plants.

Plants grown in varying intensities of light from full sun to darkness show characteristic and well-known differences in growth and development. Much of this effect is frequently summed up in the phrase, "light retards growth," which has become almost axiomatic in plant physiology and has been used to explain certain types of phototropism as well as the varying growth rates of etiolated and unetiolated plants, etc. Like many other physiological axioms, the inhibiting effect of light was popularized by SACHS (20) who performed some of the early experiments indicating that plants may make most of their growth in darkness. SACHS recognized the several secondary effects of light on growth, but thought it directly inhibiting through some action on the growing regions. Numerous other workers have considered light inhibiting for growth. PRANTL (18), POPP (15) and McCAL.A, WEIR and NEATBY (13) agree, at least in part, with SACHS. MASON (12) and Popp and BROWN (16) have stressed the action of ultraviolet rays in checking growth. MASON found that date frond elongation stopped soon after sunrise at Indio, California, and was not resumed until near sunset. Growth was resumed within a few minutes if the plants were covered during the day, but could be stopped under the cover or at night by the radiation of a quartz mercury arc. Tropical workers, BROWN and TRELEASE (2), COSTER (3), OSMASTON (14), PORTERFIELD (17) and TRELEASE (21), are generally agreed that daytime checks of plant growth are caused by internal water deficits in insolated plants rather than by any direct action of light. It might be assumed that the lower percentage of ultraviolet in the sunlight of the humid tropics accounted for their observations if it were not that LLOYD (9) was unable to find any direct effect of sunlight upon the growth of Eriogonum nudum at the Desert Laboratory at Tucson. LOoMIS (11), working with potted maize plants in the greenhouse, concluded that the elongation of this plant was controlled by the temperature and the internal water supply of the plant and was not directly affected by light. Temperature is normally positively correlated with radiation, but water supplies within the meristematic tissues tend to be negatively correlated. The greenhouse glass would reduce the ultraviolet of sunlight by screening out the shorter and more active wavelengths. PRESCOTT (19), however, found that maize in the field in Egypt also made its greatest growth after sunrise in the morning and around sunset, with a midday drop that was accentuated by declining soil moisture percentages. There