Circadian rhythms in stomatal aperture and in stomatal conductance have been observed previously. Here we investigate circadian rhythms in apertures that persist in functionally isolated guard cells in epidermal peels of Vicia faba, and we compare these rhythms with rhythms in stomatal conductance in attached leaves. Functionally isolated guard cells kept in constant light display a rhythmic change in aperture superimposed on a continuous opening trend. The rhythm free-runs with a period of about 22 hours and is temperature compensated between 20 and 300C. Functionally isolated guard cell pairs are therefore capable of sustaining a true circadian rhythm without interaction with mesophyll cells. Stomatal conductance in whole leaves displays a more robust rhythm, also temperature-compensated, and with a period similar to that observed for the rhythm in stomatal aperture in epidermal peels. When analyzed individually, some stomata in epidermal peels showed a robust rhythm for several days while others showed little rhythmicity or damped out rapidly. Rhythmic periods may vary between individual stomata, and this may lead to desynchronization within the population. Stomata function to control CO2 uptake and transpirational water loss, and they are closely regulated by such factors as light (for review see 23, 28), RH (for review see 22), and carbon dioxide concentration (for review see 17). In addition to environmental factors, stomata are controlled by an endogenous circadian clock. This control may appear as a rhythmic change in aperture under constant conditions (15,21) or as a rhythmic change in sensitivity to some environmental factor; light has been studied most frequently (4, 5, 15). Here we are concerned only with the first type of endogenous control: simple, free-running rhythmicity in stomatal aperture. A wide variety of plant species show such free-

Joint laxity was quantified by measuring the distance from the thumb tip to the forearm during passive apposition in 500 normal Southern Chinese women. Joint laxity was found to have a normal distribution throughout the population and to decrease with age. When 109 Chinese girls with idiopathic adolescent THE JOURNAL OF BONE AND JOINT SURGERY REFERENCES Beighton P, Solomon L, Soskolne CL. Articular mobility in an African population. Ann Rheum Dis 1973:32:413-8. Beighton PH, Horan Fl'. Dominant inheritance in familial generalised articular hypermobility. J Bone Joint Surg [Br] 1970:52-B:l45-7. Bird HA. A clinical review of the hyperlaxity ofjoints with particular reference to osteoarthrosis. Eng Med l986;lS:8l-5. Carter C, Wilkinson J. Persistentjoint laxity and congenital dislocation of the hip. J Bone Joint Surg [Br) 1964;46-B :40-5. Dickson RA, Archer IA. Biomechanics ofspinal deformity. J Bone Joint Surg [Br) l986;68-B:682. Ennek.ing WF, Harrington P. Pathological changes in scoliosis. J Bone Joint Surg [Am] l969;Sl-A:165-84. Johns PJ, Wright V. Relative Sanderson MC, Francis MJO, Smith R. Skin collagen in scoliosis. J Bone Joint Surg [Br] l978;60-B:29l.

Measurements of stomatal aperture in epidermal peels are used in a variety of physiological studies, but variability between stomates often hinders experimentation. We mount epidermal peels of Vicia faba over a microscopic grid that enables us to map the positions of individual stomates and thus measure the same individual stomates repeatedly, using a simple digitizing system. Repeated measurements of the same population of 15 stomates show much lower variability than measurements of different populations either on the same peel or on different peels.individual stomates and making repeated measurements on the same individuals. We compare reproducibility in population means for repeated measurements on the same population with that for measurements on different populations. We also describe an inexpensive image analysis system for digitizing, collecting, and organizing data on stomatal aperture.In addition to reducing variability in population averages, studying individual stomates allows one to assess differences in stomatal responses within a population (2, 3, 6); our method is especially useful in such a study because it enables one to return repeatedly to the same stomates even if they are in different fields of view or on different peels.Stomates on leaves regulate the diffusion of CO2 into the leaf for photosynthesis and the diffusion of H20 out of the leaf for transpiration. The

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