The control of the cylindrical cell form in plants appears to reside in the orientation of the reinforcing cellulose microfibrils in the side walls. In elongating cells the fibrils are typically transverse. Control of new synthesis of oriented wall texture is shown to be in turn related to the orientation of cytoplasmic elements in the cell periphery. Three properties of these cytoplasmic elements have been deduced from polarization optical properties of treated and normal cell walls. These deduced properties- namely, possession of a long axis and the ability to build microfibrils perpendicular to it, a tendency to cross-bond to make a parallel array, and a sensitivity of this alignment to colchicine-are all well-known properties of mitotic spindle and phragmoplast fibers which form the cross-wall after mitosis. It is proposed that proteins of spindle fiber nature exist in cortical cytoplasm of plant cells and are active in the control of wall texture and cell form.

Abstract. The view that the plant cell grows by the yielding of the cell wall to turgor pressure can be expressed in the equation: rate = cell extensibilitv X turgor. All growth rate responses can in principle be resolved into changes in the 2 latter variables. Extensibility will relate primarily to the yielding properties of the cell wall, turgor primarily to solute uptake or production. Use of this simple relationship in vivo requires that at least 2 of the 3 variables be measured in a growing cell. Extensibility is not amenable to direct measurement. Data on rate and turgor for single Nitella cells can, however, be continuously gathered to permit calculation of extensibility (rate/turgor). Rate is accurately obtained from measurements on time-lapse film. Turgor is estimated in the same cell, to within 0.1 atm or less, by measurement of the ability of the cell to compress gas trapped in the closed end of a capillary the open end of which is in the cell vacuole. The method is independent of osmotic equilibrium. It operates continuously for several days, over a several fold increase in cell length, and has response time of less ithan one minute. Rapid changes in turgor brought on by changes in tonicity of the medium. show that extensibility, as defined above, is not constant but has a value of zero unless the cell has about 80 %o of normal turgor.Because elastic ohanges are small, extensibility relates to growth. Over long periods of treatment in a variety of osmotica the threshold value for extensibility and growth is seen to fall to lower values to permit resumption of growth at reduced turgor. A brief period of rapid growth (5X normal) follows the return to normal turgor. All variables then become normal and the cycle can be repeated. The cell remains essentially at osmotic equilibrium, even while growing at 5X the normal rate. The method has potential for detailed in vivo analyses of "wall softening."The growvth of the plant cell may be regarded. instantaleoulslv at least, as the yielding of the cell wall to the stresses present in it. Tn the Nitella internode and other cells not subject to tissuie tensions, the stresses can be assumiied to be prol)ortional to turgor pressulre. This allows the rate of growtth (R) to be viewed simply as the lpro(luct of the yielding tendency of the cell (Ex, or gross extensibility) and turgor pressure ( T)

Plant cell walls typically consist of crystalline microfibrils embedded in a non-crystalline matrix. The growing cylindrical Nitella cell wall contains microfibrils predominantly oriented in the transverse direction. The present study has shown that the transversely oriented microfibrils are primarily located toward the inner surface of the wall and that, proceeding outward from the inner surface, the wall contains microfibrils of ever poorer transverse orientation, the fibrils being randomly or axially arranged in the outermost regions of the wall. Because cell expansion is primarily in the axial direction, the texture of the fibrillar elements of the wall can be explained by assuming that new microfibrils of transverse orientation are added only at the inner surface of the wall and that they become passively reoriented to the axial direction during cell elongation. The described structure corresponds to that proposed by Roelofsen and Houwink for cells showing "multi-net growth." The demonstration of a continuous gradient of microfibrillar arrangement and its partial quantitative description was accomplished by the analysis, with the polarized light and interference microscopes, of wedge-like torn edges of developing cell walls which were 1 micron or less in optical thickness.

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A small capillary, fused at one end, serves as a micromanometer when the open end is inserted into a large Nitella cell. The cell's ability to compress the gas reveals its turgor pressure directly-save for a small correction due to capillarity. The method gives a lower limit to turgor pressure for the same cell in the normal state. The common method, incipient plasmolysis, gives an upper limit. On Nitella axillaris cells the two methods limit the turgor pressure at 5.1 to 5.7 atmospheres. The manometric method is also applicable to growing cells, where osmotic equilibrium is not present.