Rural health networks in Florida.

This article describes the development of rural health networks in Florida, which has adopted formal policies to support these networks. First, the history and content of the relevant legislation are described. The current networks are identified and their development to date summarized. Finally, a detailed case study is employed to outline the steps taken to establish one network.

An acoustic approach to diver navigation.

Three experiments were carried out to assess the capability of divers to localize acoustic signals underwater and to navigate by them. In the first experiment, divers attempted to determine the correct distance to several underwater transducers projecting acoustic stimuli on a horizontal plane. The second experiment consisted of two related studies where the diver/subjects attempted to discover which of many possible stimuli would produce the most robust perception of underwater "sound movement," or the Underwater Auditory Phi Phenomenon (UAPP). A third experiment consisted of navigational swims by divers; the acoustic stimuli utilized were based on those identified in prior experiments as the most preferred. The results demonstrated that divers are able to discriminate among signals emanating from acoustic sources at various distances underwater and to do so at levels well above chance. Second, divers judged 500-Hz square waves to be the signal which best facilitated an acceptable UAPP; thermal noise and 1-kHz square waves followed in that order. However, these differences were only slight and, in practice, divers maintained that the noise signal was the most useful. Third, it was found that divers apparently can effectively navigate by means of auditory signals alone--at least within certain limits. Finally, a significant decrease in the discrimination abilities of divers for frequencies above 6 kHz suggests that intensity cues may not be as robust as time-of-arrival information with respect to underwater sound localization.

The design and calibration of a low speed windtunnel for studying the foliar deposition and uptake of aerosols.

A 2.75 m-long (9 ft.) windtunnel facility to deliver highly toxic aerosols to the foliage of terrestrical plants under safe and well-characterized conditions is described. Flow velocities of 0.1 to 28 cm/sec in the 930 cm2 (1 ft2) cross-section experimental area are attainable. Calibration of this facility using a 198Au-labelled colloidal gold aerosol is presented.

Temperature effects on radial propagation of water potential in cotton stem bark.

Low temperature affects the lateral movement of water across the xylem-phloem boundary in intact cotton stems. There is a reduction in the effective diffusion coefficient relating free energy flux to water potential gradients with an associated increase in resistance to water flow. Detached phloem and excised leaves do not show this effect of low temperature. Experiments on stem section halves indicate that the effect is probably associated with the cambial region.

Rehydration versus Growth-induced Water Uptake in Plant Tissues.

Experiments show that the rate of water uptake by living tissues external to mature xylem of cotton stems (Gossypium hirsutum L. Auburn 7-683) is very similar to the corresponding curves for leaf tissue. In both cases one obtains a two-phase curve with phase I corresponding to passive rehydration and phase II pertaining to active growth.A theory of water movement in plant tissue first proposed by Philip allows one to make a more rigorous distinction than made previously between phase I and phase II. This theory is applied explicitly to water uptake by leaf disks and results in a simple expression for the time required for phase I completion. Because the time required varies as the square of the disk radius, it is essential to use a standad disk size in water uptake studies of a particular tissue.Additional analysis indicates that clear temporal distinction cannot be made between phase I and phase II. Different portions of the leaf disk rehydrate at significantly different rates, resulting in a grey zone with phase I and phase II occurring simultaneously in different parts of the disk.

Stem diameter in relation to plant water status.

An instrument containing a linear variable differential transformer was constructed to obtain continuous, nondestructive measurements of both short term changes in stem diameter and long term growth. In cotton plants, stem diameter, leaf water potential, and leaf relative water content are all closely related to net radiation at the top of the canopy. Leaves from the east and west sides of a plant show slight, but consistent differences in diurnal water potential patterns.Stem diameter and leaf water potential are not related by a single-valued function, since there is a diurnal hysteresis between the two, and growth causes an increase in diameter each night. However, the instrument can be used to monitor long term stem diameter growth.

Leaf water balance during oscillation of stomatal aperture.

Continuous transpiration and beta-guage measurements were made on cotton plants, the stomatal apertures of which had been induced to oscillate, thus allowing a water balance to be made, and leaf potential to be measured as functions of time.Analyses showed phase differences between the water entering and leaving the leaf. Also, from the phase relationship between the flow into the leaf and the water potential in the leaf it was shown that the water potential in the xylem of the plant also oscillated. This is proposed as a necessary condition for the stomates of all the leaves of a plant to oscillate in phase.It is convenient to describe the dynamics of the phenomena using a simple electrical analogue, and the usefulness and limitations of the model are discussed.

Diurnal pattern of water potential in woody plants.

The dynamic relationship between the rates of water loss and uptake controls plant water status. Marked diurnal variations in water potential of both leaves and fruit occurred in all plants studied. Variations in water status during the day were most clearly related to changes in evaporative demand of the air and were different for the east and west sides of a tree. At night, the plant water potential reflected the soil moisture status.Changes in the water potential of pear fruit were correlated with changes in fruit diameter. Since water loss from fruit occurred mostly through the pedicel into the xylem of the tree, the fruit could be used as a crude gauge of xylem water potential, which also showed dramatic changes during the day.

Effects of salt secretion on psychrometric determinations of water potential of cotton leaves.

Thermocouple psychrometers gave lower estimates of water potential of cotton leaves than did a pressure chamber. This difference was considerable for turgid leaves, but progressively decreased for leaves with lower water potentials and fell to zero at water potentials below about -10 bars. The conductivity of washings from cotton leaves removed from the psychrometric equilibration chambers was related to the magnitude of this discrepancy in water potential, indicating that the discrepancy is due to salts on the leaf surface which make the psychrometric estimates too low. This error, which may be as great as 400 to 500%, cannot be eliminated by washing the leaves because salts may be secreted during the equilibration period. Therefore, a thermocouple psychrometer is not suitable for measuring the water potential of cotton leaves when it is above about -10 bars.

Phosphorus transport to the xylem and its regulation by water flow.

The effects of water flow on phosphorus uptake by roots and on its subsequent translocation to shoots were separated by giving short-term pulses of (32)P-labelled nutrient to intact tomato plants. At the end of a 5 min pulse, all the (32)P taken up by the plants was confined to the roots. Only about half of this (32)P was later translocated to shoots; there was very little translocation after 4 hours.Experiments after long-term labelling showed that only a small part of the total P in the root is readily translocated to shoots. This P appears to be in part of the symplast and contributes about 75% of the P transported to the xylem sap. The rest is presumably derived by leakage from vacuoles.A slow rate of water flow reduced both uptake into the symplast and the translocation to the shoots of P which had already been absorbed by the roots. This was conclusively demonstrated by giving a (32)P pulse before reducing the rate of water flow; (32)P not translocated to shoots was partly retained by the roots and partly lost to the external solution. Water flow also accelerates transport to the xylem of previously-absorbed P in excised roots.It is concluded that the major effect of water flow on phosphorus transport to shoots occurs after phosphorus uptake by the roots, probably during radial transport to the xylem.

Removal of salt from xylem sap by leaves and stems of guttating plants.

Although root pressure and guttation presumably result from a high concentration of salt in the root xylem, the guttation fluid is very dilute. Measurements of the osmotic potential of the guttation liquid and of exudates at various levels in guttating plants indicate that salt is removed from the xylem in the upper part of plants, particularly in the leaves. The concentration of salt solutions forced through individual leaves by an artificial root pressure has no influence on the osmotic potential of the guttation fluid. This suggests that leaves play an important role in removing salt from the xylem of guttating plants.