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The question of how rotation on a horizontal axis clinostat removes plants from the influence of the gravitational stimulus is answered. It is shown that appropriate horizontal axis clinostat rotation restricts the fall of intracellular particles to a quasi-circular path such that the position of the particle remains virtually stationary within cells. The displacement of the path of fall, due to centrifugal force, is then considered, and a method of determining the optimal rotation rate is developed from physical principles. This method selects the rotation rate which minimizes the volume of cytoplasm through which particles pass under the joint influence of centrifugal and gravitational forces. With the recognition that single axis clinostats are ineffective with large plants or for long experiments, a new type of clinostat is proposed on which intracellular conditions can be rendered virtually identical to those of plants in satellite free fall regardless of plant size or duration of experiment.It is shown that most low gravity biological responses can be studied using clinostats with only occasional satellite free fall experiments for verification. It is further inferred that most of these responses can be effectively and economically studied by computer simulation. The existence of a lower as well as an upper limit for the effective rate of horizontal clinostat rotation was also recognized. If plants were rotated too slowly, they tended to show curvature in response to the successive stimuli imparted during rotation. It was concluded that a rotation rate of 1 revolution in four times the latency period (the period of stimulation required to produce curvature in a plant at rest) or for some cases in twice this period produced no curvature. Ultimately a revolution rate of 2 revolutions per hour was recommended as minimal, based on the reasoning that with latency periods as short as 15 min, this rate did not expose a plant to more than 7.5 min of stimulation in any given quadrant of rotation.This now historic reference concluded with a discussion of the appropriate manner of attaching plants to a clinostat and suggested that attachment should be such that the longitudinal axis of the plant should be normal (perpendicular) to the horizontal rotational axis of the clinostat.These empirically derived tenets set forth so long ago state concisely and lucidly the problem facing any worker employing the clinostat as a research instrument. Over the ensuing 66 years, discussions concerning the conditions of clinostat rotation necessary to eliminate the unidirectional stimulus of gravity have appeared (9,17,18,22) and have generally substantiated these early observations.In this manuscript, the mechanism by which horizontal-axis clinostats nullify the geotropic stimulus is explained, and the effect of clinostat-generated, centrifugal force is re-examined.

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Suuz;nary. Using appropriate clinostait rotation nethods, it has been shown that increases -in root growth and geotropic curvature of oat coleoptiles are related 'to and ex.plained (within the limits of auxin economy) by increased rates of planit respiration imparted by nullification of the gravitational stimulus. Increased respiration with decreased graVitational stimulus, in turn, is explained by increased uniformity in intracellular distribution of metabolically active graviprecipitable particles.Oat seedlings grown with nullification of the gravitational stimulus show a more uniform distriibution of graviprecilpitable protoplasmic inclusions. Respiration involves reactions between particles of sufficient mass to be precipitated in 1 X g unidirectional force field and particles of such size that they are distributed throughout the protoplasm due to thermal energy. When plants are grown without nullification of the gravitational stimulus, graviprecipitable protoplasmic inclus-ions tend 'to accunmulate in the lower portions of the cells. Respiration rates are consequently limited due to a relatively low concentration of the smatller palrticles in the region of higlh concentration of the larger particles. Distribution of the larger particles throulghout the protoplasm negates the reaction raite liniiting effects of these initracellular concentration gradients.It has been shown using starclh particles to index graviprecipitation of intracellular particles that enhancement of respiration by nulliffication of the gravitational stimulus may be induced, maintained, discontinued, and reindutced by treatments which concomitantly result in the more uniiform distribiition of graviprecipitable protopl;asmic inclusions, the maintenance of this uni form distribution, the discontinuation of this uniform distribution anid its reinduotion.The degree of uniformity of intracellular particle distribution in response to treatments only partially nullifying the gravitational stimulus is shown wholily consonant with growth responses of roots and coleoptiles when the growth is measured as the expression of increased respiration in the absence of auxin limitation.The findings collectively indicate that gravity sensing by plants is broadly based on the physicochemical relationship between particle distribution and the expression of respiratory metabolism as growth.Oat seedlings grown on horizontal-axis clinostats showv increased root 'growth and a greater curvature in response to unidirectional geotropic sti'mulation than either plants grown on a vertical-axis clinostat or those grown vertically without rotation (3).

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