Slatis has raised a single valid point. The chromosome-per-cell distributions displayed in our put numbers on our observations. We do recognize the implications of such damage in our experimental cell populations, if the observations were significantly elevated above control values. We cannot say that they were, or that they were not.We feel that Slatis' attempt to show chromosome damage by pointing to differences which are the result of random sampling error is not a matter of a one-tailed or a two-tailed test.We agree that the pertinent statistical tests, at least for data in our Table 2, should be one-tailed. In fact, our conclusions were based on one-tailed ttests, the full results of which were not tabulated. The confidence limits given with our data were for the convenience of readers who wished to inform themselves of the magnitude of random sampling errors associated with the percentages in the Abscisic Acid: A New Name for Abscisin II (Dormin) Abscisin II was the name given to the second of two abscission-accelerating substances isolated (1) from cotton fruit. The same substance was subsequently isolated (2) from sycamore leaves as the result of a search for a "dormin" [an endogenous substance inducing dormancy (3)]. Since then, the substance has been identified in a large number of higher plants. The structure of abscisin II has been determined and confirmed by synthesis (4); structure and correct absolute configuration (5) are shown by the insert.

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The need to control abscission in order to increase the quality and yields of agriculturally useful plants has been recognized for centuries. Hand thin ning of fruits to minimize alternate bearing characteristics in some tree crops, and to improve the size and quality of fruits in others, has only within the past 20 years been replaced by the use of chemicals. The reduction in labor costs has been tremendous. Crane (27) has recently reviewed the current information on the chemical control of fruiting, and the reader is referred to this citation for further information on the subject. In the area of field crops there are numerous cases where the removal of leaves would greatly facilitate harvesting. Unfortunately, at the present stage of our knowledge, economi cally successful chemical methods have been evolved in only a few cases. In the most successful application, defoliation of cotton in preparation for mechanical harvesting, the chemicals and methods in use are often somewhat less effective than desired. In spite of this, over 7,000,000 acres of cotton are defoliated annually in the United States alone. Recognition of the need to apply chemical defoliation practices to other crops, to control patterns of fruit-set, and to improve quality and yields offer pressing needs for the con tinued intensive effort to understand the physiological and biochemical factors involved in the control of the abscission process.Four recent reviews (2, 3, 42, 83) have presented a comprehensive survey of the literature on abscission. This article wi!! be limited, insofar as possible, to recent reports and how they seem to have altered or changed our past concepts. The use of the term auxin will be restricted to endogenous growth promoting substances active in the Avena or similar bioassays. The abscission zone will be considered as the histologically distinct region at the base of an abscising organ and the term separation layer, the transverse layer of cells where separation is effected.Auxin and abscission.-Auxin has been recognized as playing a significant role in abscission since 1933 (48). In recent reviews, Addicott (3), Jacobs (42), and Rubinstein & Leopold (83) have each assigned to auxin a specific and differing regulator� role in the abscission process. The observed effects of auxin-like substances on abscission of explants (excised abscission zones) and the correlative effects demonstrated by mutilation experiments with whole plants form the major basis for the separate concepts.Addicott and co-workers based their interpretation of the role of auxin in control of abscission chiefly on the observations that 3-indoleacetic acid (IAA) applied to petiole stumps (distal) of explants retarded abscission and 1 The survey of literature perta ining to this review was completed in August 1965. 295 Annu. Rev. Plant. Physiol. 1966.17:295-314. Downloaded from www.annualreviews.org Access provided by University of California -San Francisco UCSF on 11/27/14. For personal use only. Quick links to online content Further ANNUAL REVIEWS

The loss of organic substances from cotton (Gossypium hirsutum L.) MATERIALS AND METHODS The seeds of Gossypium hirsutum L. used were from a selfpollinated M-8 genetic selection, a colchicine-doubled haploid from a Deltapine cultivar. Seed coats were removed to reduce germination variability due to differential permeability of seed coats to water.Seeds were germinated in gerniination paper rolls wet with distilled water. After sufficient germination time at 31 C to produce 2 cm long radicles (usually 24-26 hr), uniform seedlings were selected from the paper rolls, surface-sterilized with 0.1 % hypochlorite solution, rinsed in sterile water, and placed in stainless steel racks in Petri dishes with 1 cm of the seedling axis immersed in sterile solution. Each rack contained 20 seedlings; five racks were used for each treatment (five replications). The treatments were conducted in darkness. After completion of the assigned treatment (see below), the liquid in each dish was filtered, and the water was removed by freeze-drying. Preliminary assays and thin layer chromatographic studies showed that much of the dried residue consisted of carbohydrates and amino acids; these materials were, therefore, assayed and used as a measure of solute loss. The dried residue was redissolved in a small volume of 60 % methanol, and the solution was filtered and assayed for carbohydrates by the anthrone method (20). Ninhydrin-positive substances were determined by the method of Moore and Stein (14).The treatments applied to seedlings follow. RESULTS AND DISCUSSION Effect of Chilling, pH, and Anaerobic Conditions on Solute Loss. Low temperature induced large increases of solute loss from radicles (Fig. 1). The quantity of sugars and amino acids lost from roots held at 5 C increased with duration of chilling time. Thin layer chromatographs of the carbohydrates in the solution indicated that glucose, fructose, and sucrose were lost from roots at 5 C, whereas roots at 31 C lost only sucrose. Similar but more complex patterns were found on chromatograms 53 www.plantphysiol.org on April 27, 2019 -Published by Downloaded from