Polarized Growth and Cell Studies in the First Internode and Coleoptile of Avena in Relation to Light and Darkness

Abstract

1. Twenty varieties of three species of Avena were germinated in complete darkness and with preliminary light treatment in the early stages of soaking, followed by growth in darkness. The ratio of internode length to coleoptile length differed markedly in the different varieties. 2. Final length of the first internode of Avena sativa var. Victory grown under a series of different intensities of weak Mazda light varied inversely with the intensity. Similar inhibition of internode elongation in proportion to the intensity of Neon light was found at intensities below 0.1 erg/mm.2/sec. 3. Early stages of germination in Victory oats in light (1000 watt Mazda) and in darkness showed that the internode elongated slightly during early swelling (up to thirty-six hours) in both light and darkness. In darkness further polarized growth occurred in the internode, but in strong light growth ceased early and shifted to the coleoptile. 4. Different amounts of light influenced polarized growth in different organs and tissues in different ways. Very low intensities of light inhibited growth of the first internode but not that of the coleoptile; high intensities inhibited the internode and appreciably shortened the coleoptile (by decreasing the number of cells as well as cell length). In complete absence of light the internode grew extensively, and the coleoptile was somewhat shorter than in plants which received small amounts of light in the early stages of germination. 5. Analyses of cell behavior in the first internode when grown to maturity under different intensities of Mazda light indicated that both cell division and cell enlargement were responsible for polarized growth. 6. During early germination, cell enlargement (up to a certain size) was found to occur in the first internode whether the seedlings were grown in light or darkness; in later development, cell division occurred in darkness and under very low intensities but not in bright light. The size of dividing cells during early germination in darkness was somewhat less than that attained by the cells of internodes grown in light. 7. In darkness, both cell division and cell elongation contributed to growth of the internode in length. The number of internode cells increased from embryo to maturity by 8.5 times and the average cell length increased by 30 times. 8. The effect of light in shortening the first internode of the axis was brought about primarily by inhibition of cell division. It is suggested that the influencing factors are probably concerned with certain substances necessary for cell division, and if so, such substances must be rendered ineffective, or are changed in their path of movement, by very low intensities of light.