George S. Avery

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

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.

Grass Seedling Anatomy: The First Internode of Avena and Triticum

1. A detailed study of the anatomy of the seedling axes of Triticum vulgare and Avena sativa provides evidence for the following interpretations: a. The coleoptile is the first leaf above the single cotyledon; its divergence from the axis marks the coleoptilar node. b. The first internode, whether short (Triticum) or long (Avena), extends from the cotyledonary to the coleoptilar node. It has intermediate root-stem structure with two main endarch collateral bundles and numerous more or less transitional strands. The old term mesocotyl implies that the first internode is part of the cotyledon; this is not the case, and the term should be dropped from the literature. c. The hypocotyl which lies between the cotyledonary plate and the upper limit of primary root structure is so short as to be practically negligible. 2. The vascular relationships of seedling organs are discussed in some detail. The evidence from this study does not support recent theories which would make the epiblast-bearing grasses dicotyledonous or the seedling structure of Gramineae an anomaly among monocotyledons.

Peptidase Activity in the Avena Coleoptile Phytohormone Test Object

1. Avena seedlings were grown in darkness at 25⚬ C. on moist filter paper in preparation dishes. When the coleoptiles were 1.5, 4, 10, 17, and 36 mm. in length, they were uniformly sectioned at 125 or 250 μ on a rotary microtome. Peptidase determinations were made on these segments, and their reduced weights were obtained by use of the gradient tube. Cell counts were made for successive 125-μ segments of coleoptiles 4 and 10 mm. in length. On a per segment basis, peptidase activity, reduced weight, and cell number were found to decrease in progressively older coleoptiles. 2. For any given coleoptile 4 mm. or more in length, enzyme activity per unit weight of tissue, or per cell, was consistently greater at the tip. Correlations between morphological structure and auxin and peptidase gradients in the coleoptile are pointed out.

Comparative Activity of Synthetic Auxins and Derivatives

The activity of indoleacetic, indolebutyric, naphthaleneacetic, dichlorophenoxyacetic, and naphthoxyacetic acids and certain derivatives has been tested on deseeded Avena coleoptiles. Intact as well as the usual decapitated coleoptiles were employed, and both agar and lanolin blocks (and lanolin in unilateral smears) were used as carriers. 1. When tested in agar on decapitated coleoptiles, potassium salts of indoleacetic, indolebutyric, and naphthaleneacetic are as active as the free acids. Esters of indolebutyric are of about the same activity as the acid; esters of indoleacetic are less active than the corresponding acid; and esters of naphthaleneacetic acid are inactive. Indoleacetamide, naphthaleneacetamide, and dichlorophenoxyacetic acid are but slightly active. Naphthoxyacetic, its ethyl ester, and naphthoxypropionic acids are completely inactive (although reported by others to be active in the tomato test). 2. When applied in agar to intact coleoptiles, indoleacetic, indolebutyric, and naphthaleneacetic acids are completely inactive. 3. When blocks of lanolin rather than agar are applied to decapitated coleoptiles, ten to fifty times the concentration of indoleacetic, indolebutyric, and naphthaleneacetic acids must be used to give threshold curvatures. In agar, the relative activities of the acids are 100:5:19, respectively; in lanolin, 100:11:11. 4. When applied in lanolin to intact coleoptiles, indoleacetic, indolebutyric, and naphthaleneacetic acids have the same activity as when applied to decapitated coleoptiles; that is, threshold concentrations are the same whether the coleoptiles are intact or decapitated. 5. Levulinic acid is completely inactive in the Avena test, whether applied in lanolin or agar, and has no complementary effect when applied together with indoleacetic acid.

Growth and Tropic Responses of Excised Avena Coleoptiles in Culture

Introduction If the Avexa coleoptile is removed from the seedling and transferred to a suitable culture medium, to what extent is it able to grow and utilize food and growth hormone artificially supplied to it? To what extent does it possess reserves of food and growth hormone at the time of excision? Is growth hormone really necessary for growth? Most previous experiments involving growth of the Avena coleoptile and its response to various stimuli have been done with the intact seedling, and from this work have come two views: (I) that the tip of the coleoptile is the center of hormone synthesis in the seedling; and (2) that when this tip is decapitated, the upper end of the coleoptile stump, after approximately 2.5 to 3 hours, begins to function as a new physiological tip, producing growth hormone and enabling the stump to renew its growth Evidence is gradually accumulating that these views are in need of revision. For example, SODING (8) discovered that physiological regeneration took place to an equal extent whether one or more millimeters of the coleoptile tip were removed. This suggests that a precursor or even the auxin itself must be coming from another part of the seedling. POHL (6) reported that the coleoptile tip was not the center of hormone synthesis, but that it dispersed substances conveyed to it from the endosperm. POHL also added auxin-a to the seed; it was transported to the coleoptile where it stimulated growth. HEYN (3) found that in cut-off cylinders (segments) phys-

Extent of Auxin-Precursor Hydrolysis in Different Avena Assay Methods

1. It has been stated in the literature that deseeded Avena test plants respond to auxin precursor in 2-6 hours. The evidence presented here, as a result of studies on crude and purified preparations of a maize auxin precursor, shows that curvatures obtained during the 5-hour test period represent free auxin, and not auxin converted from a precursor. 2. The question is raised whether under any conditions deseeded test plants ever convert precursor into auxin, even if employed for long test periods.

Parallelism of Precipitation Reactions and Breeding Results in the Genus Iris. I. Preliminary Study and Correlation with Other Evidence

1. The precipitation reactions between thirty species, varieties, or hybrids are recorded. They represent, in part, six of the twelve sections of the genus Iris. 2. As correlated with breeding evidence, positive precipitation reactions between species usually indicate either a low degree of fertility between such species, or complete interspecific sterility. 3. It appears likely that the presence of a trace indicates too divergent a relationship for a positive reaction of any magnitude. As a rule a trace may be correlated with complete interspecific sterility. 4. A negative reaction between species may indicate close relationship, as shown by successful crosses, or it may indicate extreme divergence between species and be correlated with interspecific sterility. 5. Although decidedly limited, the evidence from gross analyses of storage products and that from microscopic examination of starch grains is in line with the precipitation reactions and the breeding results. As a whole, such evidence supports the present taxonomic grouping, except in the following: a) I. prismatica is divergent from the subsection Sibericae of the Apogon section, as evidenced by gross morphology, breeding results, and precipitation reactions. b) I. pseudacorus, considered in the light of all the available evidence, seems even more distinct from other irises in the Apogon section than its present taxonomic position would indicate. c) I. vinicolor in its precipitation reactions is so similar to the Hexagonae that its inclusion in the group, or with certain members usually assigned to the group, seems warranted. d) I. graminea, in the light of all available evidence, should occupy a rather solitary position. e) The subsection Laevigatae shares the distinctiveness of its individual member, I. pseudacorus. There are positive reactions of varying intensities between the Laevigatae and all other sections and subsections studied, except Vernae. With the Sibericae, however, there is but one positive reaction as contrasted with ten negative reactions, a situation which shows a marked correlation with breeding results, and suggests a comparatively close relationship between the two groups. 6. The precipitation reaction is discussed in the light of chromosome numbers, breeding evidence, and immunological and other phenomena as involved in hybridization and during the ontogeny of hybrid embryos.