Apparatus was developed to permit simultaneous study of the effects of two of the factors regarded as responsible for the poor growth of roots in some compacted soils. The other factors were purposely minimized by the use of glass beads and tap water as the substrate and sterilized, freshly germinated corn seedlings as the plant material. The factors investigated were mechanical impedance and oxygen supply to the root. Although the mechanical impedance imposed could not be precisely stated, the control of the soil atmosphere was excellent. It was found that the rate of growth of unimpeded roots declines when the oxygen content falls below 10% and that the growth of impeded roots is seriously reduced under these conditions. Microscopic examination of roots grown under conditions of severe mechanical restraint have not yet revealed reliable symptoms of impedance other than a distortion of the normal shape.

T HE pressure exerted 3 and work performed by roots during growth are important considerations in evaluating the effect of soil physical properties on plant growth. Data on experiments to measure such pressures appear in a paper by W. Pfeffer, "Druck und Arbeitsleistung durch Wachsende Pflanzen" published in Abhandlungen der Koniglich Sachsischen Gesellschaft der Wissenschaften, 33:235-474, 1893. Since this paper is absent from American soils literature the following review is presented to add to the data and to stimulate further interest in such measurements.The basic principle of Pfeffer's measurements of root pressure was to encase a portion of the root of a seedling plant in a plaster of Paris block in order to provide a base against which the growing root could exert pressure to move a second block that was cast around the exposed tip or side of the root. Movement of the second block would compress a calibrated spring providing a measure of root elongation or expansion and the pressure exerted by the root. Figure 1 shows the arrangement of Pfeffer's apparatus. Measur·ement of Axial Root Growth PressureTo determine the axial pressures of growing roots with this arrangement, (figure 1 (a)), a pot P filled with moist excelsior M, was fastened to a stand S. The lower portion of the root was embedded in a plaster of Paris block A which was anchored securely by the plaster of Paris collar, which extended into the pot through the bottom hole, as shown by the white oval area in the pot. The root tip R extended through block A into a shallow hole •in a second plaster of Paris block B. Block B was firmly secured to a glass disk D which was in turn connected with the stand S through compression spring F. The adjustment of screws E permitted the adjustment of the initial tension in the spring F. Two indicator needles I were mounted on the spring to permit a direct measurement of the compressed spring with the aid of a cathetometer.To prepare a plant for axial pressure measurement, a seedling was placed in the pot filled with excelsior with the root extended 15 to 30 mm. through the bottom hole. The pot was then inverted and a paper cylinder with a radius of about 1 em. placed around the root. The height of the cylinder was cut so that approximately 5 to 8 mm. of the root still extended above the cylinder. A plaster of Paris mixture was poured unto the cylinder, flowing down into the pot through the bottom hole forming a collar within the pot to bind block A to the pot. Then a glass plate, with a hole comparable to the size of the root, was covered with wax paper and pressed over the root tip, permitting the tip to extend upward through the glass and paper. When block A was hard, the glass and paper were removed and replaced by a very thin piece of paper similarly provided with a hole. The plaster of Paris block B was poured on top of this paper, thus separating blocks A and B by a very small gap G, corresponding to the thickness of the thin paper. Results of measurement of axial root growth pressures secured by this me...

Purpose Increased use of Linac‐based stereotactic radiosurgery (SRS), which requires highly noncoplanar gantry trajectories, necessitates the development of efficient and accurate methods of collision detection during the treatment planning process. This work outlines the development and clinical implementation of a patient‐specific computed tomography (CT) contour‐based solution that utilizes Eclipse Scripting to ensure maximum integration with clinical workflow. Methods The collision detection application uses triangle mesh structures of the gantry and couch, in addition to the body contour of the patient taken during CT simulation, to virtually simulate patient treatments. Collision detection is performed using Binary Tree Hierarchy detection methods. Algorithm accuracy was first validated for simple cuboidal geometry using a calibration phantom and then extended to an anthropomorphic phantom simulation by comparing the measured minimum distance between structures to the predicted minimum distance for all allowable orientations. The collision space was tested at couch angles every 15° from 90 to 270 with the gantry incremented by 5° through the maximum trajectory. Receiver operating characteristic curve analysis was used to assess algorithm sensitivity and accuracy for predicting collision events. Following extensive validation, the application was implemented clinically for all SRS patients. Results The application was able to predict minimum distances between structures to within 3 cm. A safety margin of 1.5 cm was sufficient to achieve 100% sensitivity for all test cases. Accuracy obtained was 94.2% with the 5 cm clinical safety margin with 100% true positive collision detection. A total of 88 noncoplanar SRS patients have been currently tested using the application with one collision detected and no undetected collisions occurring. The average time for collision testing per patient was 2 min 58 s. Conclusions A collision detection application utilizing patient CT contours was developed and successfully clinically implemented. This application allows collisions to be detected early during the planning process, avoiding patient delays and unnecessary resource utilization if detected during delivery.

The influence of drying on the magnitudes of soil strength parameters C and ϕ was determined for three cohesive soils. Shear strength measurements were made using a modified laboratory triaxial apparatus for three different soil types on samples prepared in the laboratory. All samples of a given soil type were molded at the same soil water content and to the same bulk density. Samples were dried slowly to soil water contents between the 1/3‐bar soil water suction level and air dryness.The criterion used to determine shear was either the development of a definite failure plane or an increase in sample diameter of 0.25 mm (0.010 in), whichever occurred first. Volume change of the sample and the applied axial and radial stresses were measured and recorded continuously throughout a test cycle. Mohr's circles were constructed and the parameters C and ϕ were determined for all samples. The parameters C and ϕ both increased with drying for these soils. Additional studies will be necessary to completely describe the soil shear strength‐drying relationship, or to determine the mechanisms of action which prevail.