Zone Of Cell Division Research Articles

Clonal relationships and cell patterning in the root epidermis of Arabidopsis

ABSTRACT The development of the post-embryonic root epidermis of Arabidopsis thaliana is described. Clonal analysis has identified three sets of initials that give rise to the columella root cap cells, epidermis and lateral root-cap cells, and the cells of the cortex and endodermis respectively. The mature epidermis is composed of two cell types, root hair cells (derived from trichoblasts) and non-hair cells (derived from atrichoblasts). These cells are arranged in sixteen or more discrete files. Each hair cell file overlies the anticlinal (radial) wall of the underlying cortical cells and is separated from the next by one or two non-hair files. The root hair forms as a tip-growing projection from the basal end of the trichoblast i.e. the end nearest the root meristem. The non-hair epidermal cells are significantly longer than the hair forming cells and are located over the outer peri-clinal (tangential) wall of the underlying cortical cells. The size difference between the two cell types is apparent in the cell division zone before hairs form. This suggests that the signals required for the differentiation of the root epidermis function in the meristem itself. Ectopic hairs are present in the ctr1 root epidermis suggesting that a Raf protein kinase may play a role in pattern formation/differentiation in the root epidermis and that ethylene may be a diffusible signal involved in specifying pattern in the root epidermis.

Contributions of the surface of the root tip to the growth of Zea roots in soil

The development of the epidermal layer of roots of Zea is traced from the quiescent centre to the zone where root hairs develop. In the zone of cell division a three layered coat forms on the outside of the epidermal cells consisting of the outer epidermal walls, overlaid by a two-layered pellicle composed of a thick fibrillar inner layer of polysaccharide, and a thin fibrillar outer layer of protein. The epidermal cells divide several times in the same longitudinal file but rarely across a radius to give a new longitudinal file. Thus, the radial walls become much thicker than all but the original transverse walls, and packets of up to 32 daughter cells derived from a single initial may be distinguished. The pellicle develops during these divisions as a continuum over the outer walls of the daughter cells. It is proposed that the pellicle provides a stiffening to the forward end of the root which permits it to penetrate soil without bending. Support for this hypothesis is shown by the Zea mays mutant Ageotropic in which the pellicle is absent, the epidermal surface is disorganized, and which grows crookedly through soil. In the zone of extension growth of normal roots of two Zea species the pellicle thins and disappears. Circumferential strips of the pellicle were peeled off the young epidermal cells and could be stretched to twice their length. This deformation is partly the result of the pellicle stretching and breaking above the attachments of the radial walls. After normal thinning of the pellicle, detachment of the radial walls at their outer ends produces a corrugated surface in the proximal zone of the root tips. In dicotyledons (e.g., soybean), there is no similar pellicle, but a stiff root tip is produced by a long multi-layered root cap, the proximal portion of which covers the elongating epidermal surface.

Leaf development in Lolium temulentum L.: characterization of a slow-to-green mutant.

A nuclear-gene mutation of the C3 grass Lolium temulentum L., which arose following cell suspension culture and plant regeneration, is manifested as delayed and incomplete greening, which occurs from the leaf tip downwards. Many plastids in the mutant exhibit abnormal morphology when examined by transmission electron microscopy; the plastid outer envelope lacks integrity and thylakoids, while still stacked, are spread over a wide area surrounded by diffuse stromal contents. These aberrant plastids can coexist with apparently normal chloroplasts in the same cell of mutant plants. Levels of chlorophyll a and b, and carotenoids, are all lower in the mutants than in normal Lolium temulentum. Leaf length, absolute growth rate, and number of cells per unit length at the leaf base, are greatly reduced (20-30% the normal values) in slow-to-green plants, but relative growth rate, duration of leaf growth, length of cell division zone and proportion of cells dividing are little affected. This novel mutant is a potentially valuable resource for studying interrelationships between photosynthetic function and leaf extension growth in grasses.

Nodule-Like Structures Induced on the Roots of Wheat Seedlings by the Addition of the Synthetic Auxin 2,4-Dichlorophenoxyacetic Acid and the Effects of Microorganisms

The addition of the synthetic auxin 2,4-dichlorophenoxyacetic acid (2,4-D) at a concentration range of between 5 × 10-5 M and 1 × 10-6M caused nodule-like outgrowths on wheat seedling roots. Concentrations of 2,4-D greater than this range caused stunting and death, while concentrations less than 1 × 10-7 M had no obvious effect on plant growth. At the lower end of the concentration range, 2,4-D had a small inhibiting effect on seedling growth. Time-course studies on the development of the nodule-like outgrowths showed that they developed at the same rate as control lateral roots; there was regionality within the structure, and at least three cell types were present: a cell division zone, amyloplast-containing cells, and highly vacuolated cells. There was no organised vascular system similar to Rhizobium/legume symbiotic nodules, although vascular tissue was found to enter the nodule-like outgrowths. Different strains of Rhizobium, Azospirillurn, Agrobacterium and Escherichia coli were added separately with 2,4-D to the seedling roots, showing different effects on seedling health and on the growth and internal structure of the nodule-like outgrowths. Generally, bacteria caused a less-organised internal structure to the growths and earlier 'senescence'. Some bacteria also caused stunting and death of seedlings at concentrations of 2,4-D that would have no such effect alone. We conclude that these induced nodule-like outgrowths are modified lateral roots with carbon reserves (as starch in amyloplasts) similar to those found in the cortex of roots, and that microorganisms are able to modulate or interfere with the development of these outgrowths.

Photosynthate Partitioning in Basal Zones of Tall Fescue Leaf Blades

Elongating grass leaves have successive zones of cell division, cell elongation, and cell maturation in the basal portion of the blade and are a strong sink for photosynthate. Our objective was to determine dry matter (DM) deposition and partitioning in basal zones of elongating tall fescue (Festuca arundinacea Schreb.) leaf blades. Vegetative tall fescue plants were grown in continuous light (350 micromoles per square meter per second photosynthetic photon flux density) to obtain a constant spatial distribution of elongation growth with time. Content and net deposition rates of water-soluble carbohydrates (WSC) and DM along elongating leaf blades were determined. These data were compared with accumulation of (14)C in the basal zones following leaf-labeling with (14)CO(2). Net deposition of DM was highest in the active cell elongation zone, due mainly to deposition of WSC. The maturation zone, just distal to the elongation zone, accounted for 22% of total net deposition of DM in elongating leaves. However, the spatial profile of (14)C accumulation suggested that the elongation zone and the maturation zone were sinks of equal strength. WSC-free DM accounted for 55% of the total net DM deposition in elongating leaf blades, but only 10% of incoming (14)C-photosynthate accumulated in the water-insoluble fraction (WIF approximately WSC-free DM) after 2 hours. In the maturation zone, more WSC was used for synthesis of WSC-free DM than was imported as recent photosynthate.

Variation in structure and plant regeneration of Agrobacterium rhizogenes transformed and control roots of the potato cv. Bintje

Agrobacterium rhizogenes transformed and control roots of the tetraploid potato cv. Bintje were compared. Transformed roots were obtained after infection by A. rhizogenes 15834 or 1855. Both in leaf and stem segments, more roots were formed at the basal side of the segments, indicative for a polarity in root formation. As compared to control roots the transformed roots are characterized by smaller and more densely stained cells, a zone of cell division, and smaller statoliths. These characteristics are correlated with vigorous growth, high branching incidence and diminished geotropism. The plant regeneration procedure according to Ooms et al. [1] was modified. The transformed roots required less 2,4-D than control roots for the induction of shoot-competent calli. The callus and shoot induction phases were reduced from 8 and 6 weeks to 3 and 3 weeks, respectively. Upon induction, 25%, 58% and 61% of the root clones originating from tuber, stem and leaf, respectively, produced shoots, whereas all of the control roots produced shoots. Shoot outgrowth occurred on liquid MS medium in the absence of hormones.

Use of Osmotic Inhibition in Studies of Shoot Formation in Tobacco Callus Cultures

Tobacco (Nicotiana tabacum) callus was transferred from permissive shoot-forming (SF) to inhibitory conditions and vice versa at different days in culture. Inhibition occurred when high levels (10%, 12%, 15%, w/v) of sucrose were added to the medium. Reciprocal tissue transfers were made at key histological stages of the SF process, and known biochemical and biophysical correlates of the process were examined after the transfers. The earlier in culture the cells were exposed to the permissive SF conditions, the greater was the number of shoots formed. High sucrose levels inhibited all stages of the SF process. Transfer of the SF tissue to the inhibitory conditions caused a reduction of the preferential cell division zones, the disorganization of meristemoids into parenchymatous tissue, and the accumulation of starch. Osmotic adjustment took place in the reciprocal transfers occurring rapidly in the early stages of culture and more slowly in older tissue. The changes in the rate of respiration could not be correlated with the above events. This approach to the study of in vitro organogenesis appears to be useful for examining the mechanism of tissue and organ differentiation.

Diurnal Growth of Tall Fescue Leaf Blades

Tall fescue leaf blades elongate at near constant rates during most of the light and dark periods of the diurnal cycle, with the dark rate being higher by 60 to 65%. Our objective was to determine relationships among diurnal rates of leaf elongation, deposition of water and deposition of dry matter (DM) into the elongation zone. Two separate experiments were conducted, both with a 15-hour photoperiod and constant 21 degrees C at the growth zone. Increased rates of leaf elongation in darkness were due to proportionally increased rates of elongation of 4-millimeter segments of the elongation zone. Length of the total elongation zone was 30 millimeters in both light and darkness. The spatial distribution of water contents in the elongation zone varied little during the diurnal cycle. Thus, dark stimulation of leaf elongation rate (+65%) and of water deposition (+77%) into elongation zones were similar. Water content per unit leaf length increased by 50% between the basal and distal limits of the elongation zone, indicating that tissue also grew in the lateral and vertical dimensions. Longitudinal growth of tissue, however, allowed 5 to 7 times more water deposition into the elongation zone than growth in cross-sectional area. This relationship was similar in light and darkness. In both light and darkness net rates of DM deposition (microgram per millimeter leaf length per hour) increased from the zone of cell division towards the region of most active elongation, 10 to 15 millimeters from the ligule, then decreased towards the distal end of the elongation zone. Net DM deposition rates (microgram per hour) integrated over the 30-millimeter elongation zone were similar during light and darkness. Thus, DM in the elongation zone was diluted during darkness as a result of increased water deposition. Net DM deposition rates at and above the distal end of the elongation zone were clearly positive during the light, but were close to zero or negative in darkness. Thus, DM deposition into the elongation zone and the adjacent recently expanded tissue was differentially affected in the diurnal cycle, DM deposition occurred in both tissues in light, but was restricted to the elongation zone in darkness.

Growth Rates and Carbohydrate Fluxes within the Elongation Zone of Tall Fescue Leaf Blades

Investigations were performed to better understand the carbon economy in the elongation zone of tall fescue leaf blades. Plants were grown at constant 21 degrees C and continuous 300 micromoles per square meter per second photosynthetic photon flux density where leaf elongation was steady for several days. Elongation occurred in the basal 20 mm of the blade (0-20 millimeters above the ligule) and was maximum at 9 to 12 millimeters. Eight 3-millimeter long segments were sampled along the length of the elongation zone and analyzed for water-soluble carbohydrates. Sucrose concentration was high in the zone of cell division (0-6 millimeters) whereas monosaccharide concentration was high at and distal to the location where cell elongation terminated (20 millimeters). Fructan concentration increased in the basal part, then remained constant at about 85% of the total mass of water-soluble carbohydrates through the remainder of the elongation zone. Data on spatial distribution of growth velocities and substance contents (e.g. microgram fructan per millimeter leaf length) were used to calculate local net rates of substance deposition (i.e. excess rates of substance synthesis and/or import over substance degradation and/or export) and local rates of sucrose import. Rates of sucrose import and net deposition of fructan were positively associated with local elongation rate, whereas net rates of sucrose deposition were high in the zone of cell division and those of monosaccharide were high near the termination of elongation. At the location of most active elongation imported sucrose (29.5 milligrams per square decimeter per hour) was used largely for synthesis of structural components (52%) and fructan (41%).

Specific Leaf Weight in Zones of Cell Division, Elongation and Maturation in Tall Fescue Leaf Blades

Patterns of change in specific leaf weight (SLW), water-soluble carbohydrate (WSC) content and leaf width were used to delineate the region of secondary cell wall accumulation, and determine the rate of increase in structural material along a developing leaf blade of tall fescue (Festuca arundinacea Schreb.). Structural specific leaf weight (SSLW) was determined by subtracting WSC mass from dry weight to emphasize structural material. Because meristematic activity, cell elongation, and cellular maturation are arranged successively in the grass leaf, these patterns represent a developmental sequence through which each segment of the leaf blade passes. Patterns were generally similar for two genotypes, one selected for high (HYT) and the other for low (LYT) yield per tiller, for a single genotype grown at 17 or 25 °C, and for two field-grown populations which differed in leaf area expansion rate (LAER). In all three studies, the elongation zone of the developing leaf had 31 to 39 per cent WSC on a dry weight basis. The LYT genotype had a higher SLW at all stages of development when grown at 17 than at 25 °C, due to greater WSC accumulation. At 20 °C, the HYT genotype had a higher SLW all along the elongating leaf blade than the LYT genotype. This difference was due to a difference in SSLW, while WSC content was similar. The LER was 64 per cent higher in the high population than the low, but elongation zones were similar in WSC. In all cases, SSLW was high in the meristematic region, lowest near the distal end of the cell elongation zone, then increased linearly as tissue matured. Rate of increase in SSLW was 8.5 and 5.2 g m−2 d−1 for the HYT and LYT genotypes, respectively, and 7.6 and 6.7 g m−2 d−1 for the high and low LAER populations, respectively.

Anatomical analysis of growth and developmental patterns in the internode of deepwater rice

Submergence of the stem induces rapid internodal elongation in deepwater rice (Oryza sativa L. cv. "Habiganj Aman II"). A comparative anatomical study of internodes isolated from airgrown and partially submerged rice plants was undertaken to localize and characterize regions of growth and differentiation in rice stems. Longitudinal sections were examined by light and scanning-electron microscopy. Based on cell-size analysis, three zones of internodal development were recognized: a zone of cell division and elongation at the base of the internode, designated the intercalary meristem (IM); a zone of cell elongation without concomitant cell division; and a zone of cell differentiation where neither cell division nor elongation occur. The primary effects of submergence on internodal development were a threefold increase in the number of cells per cell file resulting from a decrease in the cell-cycle time from 24 to 7 h within the IM; an expansion of the cell-elongation zone from 5 to 15 mm leading to a threefold greater final cell length; and a suppression of tissue differentiation as indicated by reduced chlorophyll content and a lack of secondary wall formation in xylem and cortical sclerenchyma. These data indicate that growth of deepwater-rice internoes involves a balance between elongation and differentiation of the stem. Submergence shifts this balance in favor of growth.

Cytogenesis in the developing retina of the cat.

Cytogenesis in the cat retina was studied using tritiated (3H) thymidine autoradiography and nuclear stains. Three zones of cell division were recognized. In the first zone cell cleavage occurs at the outer limiting membrane. The distribution of these mitotic figures is uniform as early as an embryonic age of 29 days (E29) until E50. At about E50 a "cold spot", made apparent by the absence of mitotic figures, is evident at the site of the developing area centralis. This spreads to encompass the whole retina by postnatal day 10 (P10). A second zone of cell division was recognized by the presence in the developing inner nuclear layer of 3H-thymidine labelled nuclei which do not migrate to the outer limiting membrane (ventricular surface) to divide. Some of these labelled nuclei are located in regions of the retina where cytogenesis at the ventricular surface has ceased. A third zone was observed in the optic nerve fibre/ganglion cell layers from about E54 until beyond the first postnatal month. This activity gives rise to vascular endothelial cells in the nerve fibre/ganglion cell layers. Once established, the developing vascular cells invade the inner plexiform and inner nuclear layers to form the deeper capillary net.

Interpolation of microtubules into cortical arrays during cell elongation and differentiation in roots of Azolla pinnata.

Longitudinal sections of roots of Azolla pinnata R. Br. were prepared for electron microscopy so that cortical microtubules could be counted along the longitudinal walls in cell files in the endodermis, pericycle, and inner and outer cortex, and in sieve and xylem elements. With the exception of the xylem, where there are no transverse cell divisions, each file of cells commences with its initial cell and then possesses a zone of concomitant cell expansion and transverse cell division, followed, after completion of the divisions, by a zone of terminal cell differentiation. The cells augment their population of cortical microtubules as they elongate and divide, showing a net increase of up to 0.6 micron of polymerized microtubule length per min. Two main sub-processes were found: (i) When a longitudinal wall is first formed it is supplied with a higher number of microtubules per unit length of wall than it will have later, when it is being expanded. This initial quota becomes diluted as the second sub-process commences. (ii) The cells interpolate new microtubules at a rate which is characteristic of the cell, and, in the endodermis, of the face of the cell, while the cell elongates. Most cell types thus maintain a set density of cortical microtubules while they elongate and divide. Comparisons of endodermal cells in untreated controls, and roots that had been treated with colchicine, low temperature, or high pressure indicate that the initial quota of microtubules, and the later interpolations, and differentially sensitive to microtuble perturbations. Three types of behaviour, all related to changes in the cell walls, were noted as cortex, xylem and sieve element cells entered their respective phases of cell differentiation. The cortical cells expanded in all dimensions, and the interpolation of microtubules diminished or ceased. The sieve elements continued to elongate, and interpolated at a high rate, reaching unusually high densities of microtubules when the cell walls were being thickened. During this period a net increase of 2.0 micron of polymerized microtubule length per min was calculated. Thereafter interpolation ceased and the density of microtubules declined. The sample applied to developing xylem except that, because wall-thickening is localized rather than widespread, the rise and subsequent fall in the density of microtubules was less marked. The data are discussed in relation to the participation of microtubules in wall deposition and to the hypothesis that cortical microtubules arise in discrete zones along the edges of cells.

The formation of muscle scars in articulate brachiopods

Distinct impressions (muscle scars) commonly mark the sites of attachment of muscle bases on the interiors of both brachial and pedicle valves of articulate brachiopods. Such impressions are formed as a result of modifications in the fine structure and secretory behaviour of outer epithelial cells (responsible for shell secretion) to which the muscles become attached. Muscles are not attached directly to outer epithelium but are separated from it by a very thin layer of connective tissue. In outer epithelial cells underlying muscle bases the two most apparent changes in cell morphology, compared with epithelial cells unaffected by muscle attachment, are a reduction in optimum cell dimensions and the development of a fibrillar supporting framework (tonofibrils) which extends between the inner and outer plasma membranes. As each muscle base enlarges during ontogeny, outer epithelial cells in the path of the advancing muscle base are progressively overlapped. Below the arcuate anterior margin of the muscle base cell division takes place in the outer epithelium. Changes in secretory behaviour are equally drastic. Around the arcuate anterior margins of scars the standard secondary shell mosaic is replaced by a much more irregular deposit (myotest). Evidence of a zone of vigorous cell division within the associated outer epithelium can be seen in the way that smooth spatulate terminal faces, once they become incorporated within the periphery of scars, split up into two or three minor lobate branches. In addition cell division is accompanied by an increase in organic exudation and a corresponding decrease in mineral secretion. Whereas normal shell secretion involves the exudation of a single organic membrane across an arcuate zone in the anterior part of each secreting plasmalemma and calcite secretion across the remaining surface area, epithelial cells underlying muscle bases exude two thin triple unit membranes which ensheath each individual myotest fibre. The calcite secretory zone is restricted to a narrow central zone on each secreting plasmalemma and the second organic membrane is exuded over the posterior sector and continues posteriorly to join up with the anterior membrane in the cell behind. In some scars, adjacent fibres may amalgamate to produce a localized laminar fabric. In at least one rhynchonellid genus, Notosaria nigricans , mineral secretion under muscle bases may periodically cease with the development of a relatively thick organic layer. Within a species the morphology of exposed parts of fibres in any one muscle scar tends to be fairly consistent and generally each muscle base imparts to the underlying shell a characteristic fabric which can be readily distinguished from that of neighbouring scars. In an initial survey of a number of unrelated genera ultrastructural modifications within homologous muscle scars were found to exhibit a fair degree of similarity. The fact that homologous muscles perform basically the same function would account for this structural similarity. In general it is evident that disruptions and modifications of the standard secretory regime associated with the emplacement of muscle bases arise in response to a need for stronger adhesion between the shell and outer epithelium. Such additional adhesion as is required is facilitated by the widespread exudation of organic membranes to which each outer epithelial cell can remain firmly attached by strong hemidesmosomal connections. The dense framework of tonofibrils extending between the inner and outer plasma membranes serves to brace the cells internally against external stresses applied during muscle contraction. The recognition of myotest fabrics in fossil genera which are closely comparable with the myotest fabrics of living articulates would seem to indicate that similar localized disruptions in secretory behaviour occurred also in the geological past.

Vegetative Anatomy of Cultivated Grapes--A Review

The anatomy of vegetative structures of cultivated grapevines, as described in selected literature, is presented, with emphasis on development of established vines of <i>Vitis vinifera</i>. Other topics briefly considered are the anatomy of propagules, muscadine grapes, tetraploid vines, phylloxera galls, mineral-deficient vines, and injury of leaves by ozone and 2,4-D. The terms used to describe grapevine morphology and anatomy are defined in a glossary. A growing root shows zones of cell division, cell elongation, and tissue differentiation. Vascular structure is similar in the root and the stem, although the mitotic activity of vascular and cork cambia is more irregular. Root cortex is often invaded by endotrophic mycorrhiza. The stem primary vascular system is composed of leaf, bud, and tendril traces. Procambium and phloem differentiate acropetally, and xylem differentiates in both directions from the nodes. During the first growing season of the stem, a cambium produces secondary xylem and phloem and extends rays. Xylem consists of large scalariform vessels surrounded by thick-walled pitted parenchyma and large masses of living septate fibers. Phloem consists of blocks of living septate fibers alternating with blocks of sieve elements, companion cells, and parenchyma. Ray cells are pitted and thick-walled in the xylem but thin-walled in the phloem. Cells of the stem and root store starch, which is greatly depleted when shoots begin to grow in the spring. Phellogen forms from the living cells of the nonconducting primary phloem in the first year in stems of <i>Euvitis</i> Planch. (in the subepidermal layer in <i>Muscadinia</i> Planch.), and then successively each year in the nonconducting secondary phloem. It cuts off all the tissues lying outside of it, and these are shed with the periderm. In spring, sieve elements formed the previous year are revitalized and function as conduits until new sieve elements are formed from the cambium. Each foliage leaf on a growing shoot has a precocious axillary bud consisting of a single prophyll and a variable number of leaves. This develops into the summer lateral branch. The four vascular traces of this axillary bud diverge from the traces of the subjacent tendril or bud, not from those of the subtending leaf. The bud in the prophyll of the summer lateral develops into the primary bud with 2-3 prophylls, a few leaves, and then several leaves opposed by clusters or tendrils in a generally regular pattern. Formed in the axils of the two basal prophylls of the primary shoot are the secondary and tertiary buds, respectively. These three buds, surrounded by the prophyll of the summer lateral, enter dormancy at the end of the growing season and constitute the compound bud or eye of the mature cane. The next season the primary bud develops into the fruiting shoot. The secondary and tertiary buds may develop into shoots or remain latent for extended periods. A tendril arises as a primordium on the apical meristem opposite that of a leaf. Two of its vascular traces are connected with those of the bud below it, and two with those of the leaf above it. It functions first as a hydathode and later as a twining organ. At each node, five traces to the leaf leave the ring of stem vascular bundles through separate gaps. The traces divide and anastomose to form the complicated network of palmate venation in the leaf, each tooth ending in a hydathode. Blade tissues of a mature leaf, which are differentiated from six layers of meristematic cells, comprise the upper epidermis, one layer of palisade cells, three layers of spongy mesophyll cells, and the lower epidermis with stomates.

Influence of cell-wall thickness on cell division: electron microscopic study with Bacillus cereus.

Bacillus cereus that had been exposed to chloramphenicol for 0 and 90 min synthesized walls whose thickness increased with time. These cells were washed and resuspended in fresh growth medium. Cell division was examined by electron microscopy. In untreated cells, a slight invagination of the cytoplasmic membrane with deposit of cross-wall material marked the onset of cell division. The cross wall grew inward until the septum was about [Formula: see text] completed. This was followed by splitting and bifurcation of the outer edge of the cross wall, which in turn separated the peripheral wall scars. Cross-wall penetration with further splitting continued until the daughter cells were completely partitioned. The peripheral wall scars that were located at the junction of peripheral and end walls at the time of cell separation indicated that the zone of cell division was probably not a region of active cell elongation. In cells treated with chloramphenicol for 90 min, cross-wall initiation and completion all occurred beneath a thickened peripheral wall. The circumferential portion of the peripheral wall at the zone of cell division appeared to be responsible for the hindrance of cell separation. This thick layer subsequently severed at three or four sites to allow the separation of daughter cells. For cells that had initiated cross-wall invagination at the time when chloramphenicol was added, accumulation of a large amount of cross-wall material at the zone of cell division was observed. Upon removal of the antibiotic, cells abandoned the old and initiated a new dividing site. For cells that had already completed cross-wall formation at the time of chloramphenicol treatment, the increase in wall thickness did not appear to interfere with cell separation. Models for the different stages of cell division in all these bacteria were presented.

Ultrastructural effects of testosterone on epiphyseal cartilage.

Although experimental and clinical experience indicates that large doses of testosterone lead to premature cessation of growth, the exact mechanism and precise site of action of this hormone on the growth apparatus of long bones remain unknown. In this study, plateaued male rats were injected with supraphysiologic doses of testosterone to observe the submicroscopic effects on the various zones of the epiphyseal cartilage. In the zone of cell division there were increased numbers of dividing cells. The maturing cells accumulated larger amounts of secretory products at earlier stages of their life cycle, and appeared to undergo a more abrupt hypertrophy. In the zone of prehypertrophy, the interterritorial matrix contained foci of early and premature calcification and thicker and longer collagen fibers than at comparable levels in controls. It is concluded that in intact animals, even large doses of testosterone initially cause a stimulation of chondrocyte proliferation, prior to promoting maturation processes.

Penetration and infection of alfalfa roots by Phytophthora megasperma and the pathological anatomy of infected roots

Penetration of the fine roots of Vernal alfalfa by Phytophthora megasperma was investigated in solution culture. The zoospores encysted on the root tips around the zone of cell division and cell extension and directly penetrated the host by a small infection peg. No appressoria were observed, but the flattened base of the cyst adhered to the host and appeared to serve the same function. Inside the root the hyphae were inter- and intra-cellular. In naturally infested and artificially inoculated field soils most of the fine roots were destroyed and large lesions appeared on the tap root. The latter originated from infection of the spongy-phellem cells formed around the base of the fine lateral roots. Restricted lesion development on the tap root was associated with the formation of wound periderm around the infection court. No wound periderm was formed when the vascular system was infected and only minor damage occurred when invasion was restricted to the cortex. Root damage reduced growth rates in older plants and severe infection produced yellow foliage, premature defoliation, and wilting.

Metabolism and Binding of 14C-Maleic Hydrazide

Maleic hydrazide (MH) is taken up by corn and pea seedling roots and bound to some material which is insoluble in 80% ethanol or 5% trichloroacetic acid. (14)C-MH is stable metabolically; chromatography of the 80% ethanol-soluble (14)C from treated corn roots and tobacco pith gives no indication of degradation. Very little (14)C-MH is bound in the zone of cell division (where MH acts to inhibit root elongation) or even in the region of cell enlargement in corn roots and most is bound 1 or more centimeters behind the tip. Likewise, very little MH is bound in corn coleoptile and tobacco pith sections. About 90% of (14)C-MH bound in corn roots is associated with large particles which may be cell wall fragments. The binding is blocked by azide and dinitrophenol, indicating a requirement for metabolic energy; however, inhibitors of protein synthesis (chloramphenicol, puromycin, cycloheximide) and DNA synthesis (fluorodeoxyuridine) do not inhibit binding. Only very small amounts of MH are bound in root homogenates, providing further evidence that the binding process is active. Once the MH is bound in the roots, the complex is stable for at least 1 week. Treatment with 2-aminoethanol releases MH.

Phytochrome in etiolated annual rye

Phytochrome per fresh weight in the apical 0.5 cm of the 1.5-cm coleoptile was twice that in the basal 0.5 cm; phytochrome per dry weight and phytochrome per DNA did not vary significantly from the base to the apex. Ratios of phytochrome to fresh or dry weight in the 6-cm coleoptile were nearly constant in the basal 5 cm, but increased in the two apical 0.5-cm sections. Phytochrome per DNA in the 6-cm coleoptile did not vary significantly in the apical 4 cm, but decreased by one-half in the basal 2 cm.In the 5-cm first leaf, phytochrome per fresh or dry weight was greatest in the basal 0.5 cm (zone of cell division). Phytochrome per DNA in the leaf decreased from apex to base; the phytochrome per DNA in the basal 0.5 cm (zone of cell division) was one-third as high as in the apical 0.5 cm.Pytochrome per cell was greater in all parts of the 6-cm coleoptile compared to the 1.5-cm coleoptile, although no cell division had occurred during this elongation. Phytochrome per cell was higher in the sections of the first leaf wihch were growing primarily by cell expansion rather than by cell division. Phytochrome expressed per unit dry weight was more constant than any of the other parameters studied.It was concluded (1) that the phytochrome content of all coleoptile cells is the same at any given age (length); (2) that the phytochrome content of all nondividing leaf cells is equal at any given age (but lower than that of coleoptile cells); (3) that the phytochrome content of all cells increases in direct proportion to their expansion (increase in dry weight).