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Simunary. Pea epicotyls (PisuniI satikl'u,,, cv. Alaska) were enclosed in chamibers in which their elongation was restricted by means of a foam neoprene stopper or by a medium of glass beads. These treatmentii increase(l evolution of ethylene and restllted in reduced length and increased dianmeter of both the internodes and the cells of the internodes. These responses increase(d with increasinig degrees of restriction. A time-sequenice study of the emergence of epicotyls through 90 mm of glass beads showed that all accelerated evolution of ethylene precede(d a reduction in elongation. As the epicotyls elongated through the glass bead mediumii and less resistance was encounitered, evolution of ethylenie decline(l and rapid elonigatioi was resumed.

Summiarv. The produiction of ethylene by etiolated pea epicotyls (Pisuim sativum11 L., cv. Alaska) is confined to the plumule and plumular hook portion of the epicotyl, and occurs at a rate of about 6 pLlkg-l.hr-1. Such a rate is sufficient to give physiologically active concentrations of ethylene within the tissue. Exposure of etiolated seedlings to a single dose of red light caused a transient decrease in ethylenc produiction and a corresponding increase in plumuilar expansion. Far-red irradiation following the red light treatment decreased the red effect to the level achieved by the far-red alone, suiggesting that the ethylene produiction mechanism is controlled by phytochrome and thuis that the ethylene intervenes as a regulator in the phytochrome control of pltimuilar expansion.A relationship between the production of ethylene an(l the inhibition of plumular expansion in etiolatedI pea epicotyls can be 'deduced from several recent observations. First, the rate of ethylene productioin in the growing epicotyl does not increase in proportion to the increasing mass or ntumber of cells (5)

The effects of a series of concentrations of ethylene (10, 20, 40, to 10,240 nl,'i) on elongation, diameter, and geotropism of the stems and roots of etiolated seedlings of Pisuin sativum L., Arachis hypogea L., Phaseolus vulgaris L., and Gossypiurn hirsutum L. were measured or observed. Of the 24 possible responses, 4 were unaffected at the concentrations used, 5 were affected slightly, and the remaining responses exhibited a 14-fold range of apparent half-maximum concentration dependencies (i.e. 95 nl/l for the effect on pea epicotyl geotropism to 1350 nl/l for the promotion of cotton hypocotyl diameter MATERIALS AND METHODSIt has been reported that a number of different effects of ethylene have similar concentration dependencies and thus possibly the same primary binding site (2, 4). Further, they observed that a known metal binder, carbon monoxide, replaces ethylene in all its actions at a concentration of several hundred nl/ 1, and that competition between CO2 and ethylene has been established for essentially all actions of ethylene. From these observations it has been suggested that the primary binding site for ethylene action in plants is a metal-containing receptor having a limited access of approach with a Km of 6 X 10'" M (2, 4).There is sufficient published evidence to raise questions about the universality of these conclusions, especially regarding the single concentration dependency. It was noted by the earliest workers that different plant species varied in their sensitivity to ethylene (6,7,11,12 Seeds of peas (Pislim sativumn L., cv. Alaska) and beans (Phaseolus vulgaris L., cv. Burpee's Stringless Green Pod) were imbibed for 6 hr in aerated distilled H20 at 25 C, and seeds of peanuts (Arachis hypogea L., Spanish type, cv. Starr) and cotton (Gossypium hirsutum L., cv. Stoneville 213) were similarly imbibed for 1 hr. The seeds were then placed between moistened layers of white paper toweling which were sandwiched between layers of 1.25-cm thick, open celled urethane foam which had been washed thoroughly with distilled HP and had been drained. A set of seven such layers (with 15 seeds/layer) was held upright in a 30 cm x 10 cm X 25 cm black, opaque plastic container, which was placed in a dark germination cabinet at 25 C. Each container of seeds was supplied with a flow of 8 1/hr of humidified air which had been passed through a 60 cm X 6 cm diameter column of vermiculite and celite (4: 1 v/v) moistened with a saturated solution of potassium permanganate to remove all traces (<2.0 nl/l) of unsaturated hydrocarbons.After a specified number of days the seedlings were transferred to the treatment chambers. The chambers consisted of two 30.5 x 45.7 cm sheets of glass separated by a 1.25-cm thick rubber gasket (which was glued just inside the perimeter of the back sheet of glass). The back sheet of glass was attached to a plywood backing and the front sheet was held in place by a frame which was bolted to the plywood back.

Previous attempts to model steady state Munch pressure flow in phloem (Christy and Ferrier. [19731. Plant Physiol. 52: 531-538; and Ferrier et al. [19741. Plant Physiol. 54: 589-600) lack sufficient equations, and results were produced which do not represent correct mathematical solutions. Additional equations for the present closed form model were derived by assuming that unloading of a given solute is dependent upon the concentration of that solute in the sieve tube elements. Examples As no unique steady state mathematical solution could be found, the above models were used to approximate steady state sieve tube transport by a judicious choice of initial concentrations and convergence criteria, in a quasi-time-dependent iteration procedure. It is unlikely that such inconsistencies occur in real phloem, and thus, another equation must exist which would provide a unique, closed form solution at a given loading rate.The above problem did not arise in the mechanical analogue model of Eschrich et al. (4). In this system, a given, initial amount of sugar was placed within an open or closed tubular, semipermeable membrane. In the absence of continuous loading and/or unloading of solute, their mathematical model correctly predicted the velocity of the solute front, the lack of equilibrium in the open tube, and the equilibrium condition when the front reached the end of the closed tube. Therefore, the necessary additional equations for a closed form mathematical model of steady state phloem translocation may lie with the assumptions regarding the loading and/or unloading of solutes. These considerations can best be illustrated by examining the existing equations as they apply to all of the individual elements of a sieve tube.REVIEW OF GENERAL THEORY The standard water potential terminology (9,15,16) Water potentials of the xylem-apoplast continuum will be denoted by the subscript x, and those of the sieve tube elements as i. The flux of water through the peripheral membrane (J,i, see Table I for units) of the ith sieve element would be a function of the water potential difference, and the hydraulic conductivity (La) and reflection coefficient (c-) of membranes, i