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.Sumtnary. In the tomato plant water flows through primary xylenm in accordanice with Poiseuille's law. This relation and the analogy between Poiseuille's and Ohm's law were employed to calctulate rates of flow and differences in pressure within vascular bunidles when transpiration rates from individual leaves were known. The resistance of vascular bundles to flow was calculated from a modificationl of Poiseuille's law and from neasurements of vessels in all bundles. The rates of flow in all bundles were derived from a set of simultaneous linear equations of flow, written to correspond with the nature of the vascular network. XValues of the (lifference in pressure associated with flow in bundles were derived from resistances and flow rates in individual bundles. These agreed substantially with values observed in a comparable plant.In large bundles, vessels occur in a frequency distribution that is approximately normal with respect either to the logarithms of their radii or to the fourth power of their radii. 'I'lle largest vessels in a bundle transport most of the water when they are functioning. The tomato plant containis 2 types of vascular bundle. The large bundles of the stem formii a network by joining above each node in combinations of 2 at a time. The small bundles of the stem and petiolar bundles are independent of other bundles from their origins at junctions to their termini. The small bundles offer high resistance to flow, whereas the resistance of large bundles is low. The average conductance of large bundles decreases from the base to the apex of the stem. That of small vascular bundles remains low and more or less constant throughout the plant.Only a small difference in pressure is required to maintain flow in large bundles. For lower leaves, the driving pressure required to move water to the base of a petiole is considerably less than that which moves water through petioles. The difference in pressure that maintains flow increases steadily for successively higher nodes. However, the pressure that drives flow to leaves is not always greater for higher leaves than for intermiediate ones. For the plant examined, the highest leaves required a smaller amount of energy to inove water from the ground than intermediate leaves did. This was also true of thc power expended in moving water to individual leaves.In the large network bundles, significant cross transfer of flowv occurs at junction points from one bundic to another. Because of the interconnections between large bundles. pressure and flow relations are apparently not greatly altered when localized dysfunction occurs in the vessels of larg,e bundles. In small, independent bundles, a localized dysfunction in vessels produces; a signiificant effect on pressure alnd flow relations.WAater moves from one point to another at a rate that is proportional to the difference in potential an(d inversely proportional to the resistance to flow between these points. Gradmann (4). van den Honert (7), and Bonner (2) have each contributed to this concept, alnd rece...

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Pseudomonas phaseolicola (Burkh.) Dowson, a pathogen of bean (Phaseolus vulgaris L.) plants, causes chlorotic haloes in infected host leaves (7). In vitro the pathogen produces an extracellular toxin which when injected into bean leaves also produces chlorotic haloes (2). Whether the symptoms are caused by infection or by injection of toxin, ornithine accumulates in the chlorotic tissues (9). Here we present evidence which indicates that ornithine accumulation is caused by the inhibition of ornithine carbamyl transferase of the host by the toxin.Toxin Preparation. The halo blight toxin was partially purified from the culture filtrate of a virulent strain of P. phaseolicola grown in the YEP medium (9). Five hundred milliliters of the filtrate were concentrated to 20 ml under reduced pressure at 50 C, and the concentrate was desalted with 10 volumes of methanol.The extract was concentrated and the procedure repeated two more times. The final extract was evaporated to dryness and made up to 10 ml with distilled water. Four milliliters of this sample were applied to a 2-X 45-cm column of Sephadex G-10. The column was eluted with distilled water at a flow rate of 17.6 nl/hr (fraction volume, 2.2 ml). An identical batch of the medium, which was not inoculated, was subjected to the same procedure including incubation under the growth conditions, as a control and is designated nongrown. The culture filtrates are designated grown. Aliquots (25 ,ul) from every third fraction collected from the Sephadex column were assayed for halo-inducing activity on young trifoliate leaves of 4-week-old Red Kidney bean plants in a growth chamber (2). The same fractions were diluted 10-fold with distilled water and 80 Iul of the diluted fraction were used to test its inhibitory activity against OCT2 of bean leaves. To determine ve/vo (retention volume/void volume) of the active material, fractions with halo-inducing activity were concentrated to 2 ml and put on an analytical column (90 x 1.2 cm) of Sephadex G-10. The elution rate was 10.8 ml/hr (fraction volume, 1.8 ml).Enzyme Preparation. Acetone powder was prepared from trifoliate leaves of 3 to 4 week-old Red Kidney bean plants as described before (4). Five grams of the acetone powder were stirred for 1 hr in 90 ml of 0.1 M potassium phosphate buffer, pH 7.0, containing 5 X 10-3 M 2-mercaptoethanol. The slurry was cen-