Cessation Of Elongation Research Articles

Axial Growth and Myopia Progression After Discontinuing Soft Multifocal Contact Lens Wear

For myopia control to be beneficial, it would be important that the benefit of treatment (slowed eye growth) is not lost because of faster than normal growth (rebound) after discontinuing treatment. To determine whether there is a loss of treatment effect (rebound) after discontinuing soft multifocal contact lenses in children with myopia. The Bifocal Lenses in Nearsighted Kids 2 (BLINK2) cohort study involved children with myopia (aged 11-17 years at BLINK2 baseline) who completed the BLINK Study randomized clinical trial. Enrollment was from September 2019 through January 2021; follow-up was completed in January 2024. In the BLINK2 Study, all children wore high-add (+2.50 diopter [D]) multifocal soft contact lenses for 2 years and single-vision soft contact lenses during the third year to determine if rebound occurred. High-add multifocal soft contact lenses and single-vision soft contact lenses. Eye length (optical biometry) and refractive error (cycloplegic autorefraction) were measured annually. Of 248 participants enrolled in BLINK2, 235 completed the study. The median age at the baseline visit was 15 years (range, 11-17 years); 146 participants (59%) were female, and 102 (41%) were male. At baseline for BLINK2, mean (SD) axial length and spherical equivalent refractive error were 25.2 (0.9) mm and -3.40 (1.40) D, respectively. After participants switched from multifocal to single-vision contact lenses, axial elongation increased by 0.03 mm per year (95% CI, 0.01 to 0.05) regardless of their original BLINK treatment assignment (P = .81). There was also an increase in myopia progression after switching to single-vision lenses of -0.17 D per year (95% CI, -0.22 to -0.12) that did not depend on the original BLINK treatment assignment (P = .57). There continued to be a difference in axial length and refractive error throughout BLINK2 based on the BLINK Study treatment assignment with the original high-add group having shorter eyes and less myopia than the original medium-add (+1.50 D) and single-vision groups. The BLINK2 Study found no evidence of a loss of treatment effect after discontinuing multifocal contact lenses in older teenagers. These data suggest eye growth and myopia progression returned to faster but age-expected rates and support continuing multifocal lenses until cessation of elongation and progression.

Pectic galactan affects cell wall architecture during secondary cell wall deposition.

β-(1,4)-galactan determines the interactions between different matrix polysaccharides and cellulose during the cessation of cell elongation. Despite recent advances regarding the role of pectic β-(1,4)-galactan neutral side chains in primary cell wall remodelling during growth and cell elongation, little is known about the specific function of this polymer in other developmental processes. We have used transgenic Arabidopsis plants overproducing chickpea βI-Gal β-galactosidase under the 35S CaMV promoter (35S::βI-Gal) with reduced galactan levels in the basal non-elongating floral stem internodes to gain insight into the role of β-(1,4)-galactan in cell wall architecture during the cessation of elongation and the beginning of secondary growth. The loss of galactan mediated by βI-Gal in 35S::βI-Gal plants is accompanied by a reduction in the levels of KOH-extracted xyloglucan and an increase in the levels of xyloglucan released by a cellulose-specific endoglucanase. These variations in cellulose-xyloglucan interactions cause an altered xylan and mannan deposition in the cell wall that in turn results in a deficient lignin deposition. Considering these results, we can state that β-(1,4)-galactan plays a key structural role in the correct organization of the different domains of the cell wall during the cessation of growth and the early events of secondary cell wall development. These findings reinforce the notion that there is a mutual dependence between the different polysaccharides and lignin polymers to form an organized and functional cell wall.

Dynamics of growth, proliferation and differentiation of wheat root cells exposed to a high nickel concentration

The dynamics of root growth, proliferation of initial cells of the root cap, rhizodermis, and central metaxylem, as well as structural changes in the cells induced by a 72-h exposure to a high (0.1 mM) concentration of NiSO4 were studied in 3-day-old wheat (Triticum aestivum L.) seedlings. In the roots of control plants, we observed a 12-h rhythm of changes in the length of the cells that completed elongating. Upon the treatment with nickel, this effect was negated, and a considerable reduction in the root length increment was observed in 12 h. In 24 h, root growth essentially ceased. Cell elongation was suppressed acropetally, and the cells, whose elongation was over, became shorter. In the meristem and apical part of the elongation zone, slow cell growth continued during the second and even third days. Autoradiography showed that the earliest effect of nickel on the processes of root morphogenesis observed in 6 h was a suppression of cell transition to DNA synthesis. The cells, where DNA synthesis has already started or which were in other stages of the cycle, continued to pass slowly through the cycle and completed it. Sister cells formed as a result of division subsequently left the cycle in the phase G1 and transited to dormancy. It was found that the main mechanism of cell proliferation cessation was the suppression of cell transition to DNA synthesis. In the cells elongating when exposed to nickel, tissue-specific changes in the nucleus structure were observed (chromatolysis in the rhizodermis and cortex, pycnosis in the endodermis, a disturbance of the nucleus structure in the central metaxylem). These disorders were only observed after cessation of elongation. Root incubation in 0.1 mM nickel solution did not affect the onset of cell differentiation in the xylem and metaphloem and shifted its beginning to the root tip. However, in 24 h the initiation and growth of root hairs were suppressed. It was concluded that tissue-specific nickel-induced changes in the nucleus structure in the elongating cells do not cause the cessation of root growth, although point to nickel toxic effect on the cells in the course of elongation.

Involvement of the ethylene response pathway in dormancy induction in chrysanthemum

Temperature plays a significant role in the annual cycling between growth and dormancy of the herbaceous perennial chrysanthemum (Chrysanthemum morifolium Ramat.). After exposure to high summer temperatures, cool temperature triggers dormancy. The cessation of flowering and rosette formation by the cessation of elongation are characteristic of dormant plants, and can be stimulated by exogenous ethylene. Thus, the ethylene response pathway may be involved in temperature-induced dormancy of chrysanthemum. Transgenic chrysanthemums expressing a mutated ethylene receptor gene were used to assess this involvement. The transgenic lines showed reduced ethylene sensitivity: ethylene causes leaf yellowing in wild-type chrysanthemums, but leaves remained green in the transgenic lines. Extension growth and flowering of wild-type and transgenic lines varied between temperatures: at 20 °C, the transgenic lines showed the same stem elongation and flowering as the wild type; at cooler temperatures, the wild type formed rosettes with an inability to flower and entered dormancy, but some transgenic lines continued to elongate and flower. This supports the involvement of the ethylene response pathway in the temperature-induced dormancy of chrysanthemum. At the highest dosage of ethephon, an ethylene-releasing agent, wild-type plants formed rosettes with an inability to flower and became dormant, but one transgenic line did not. This confirms that dormancy is induced via the ethylene response pathway.

Imaging of the Yellow Cameleon 3.6 indicator reveals that elevations in cytosolic Ca2+ follow oscillating increases in growth in root hairs of Arabidopsis.

In tip-growing cells, the tip-high Ca(2+) gradient is thought to regulate the activity of components of the growth machinery, including the cytoskeleton, Ca(2+)-dependent regulatory proteins, and the secretory apparatus. In pollen tubes, both the Ca(2+) gradient and cell elongation show oscillatory behavior, reinforcing the link between the two. We report that in growing root hairs of Arabidopsis (Arabidopsis thaliana), an oscillating tip-focused Ca(2+) gradient can be resolved through imaging of a cytosolically expressed Yellow Cameleon 3.6 fluorescence resonance energy transfer-based Ca(2+) sensor. Both elongation of the root hairs and the associated tip-focused Ca(2+) gradient show a similar dynamic character, oscillating with a frequency of 2 to 4 min(-1). Cross-correlation analysis indicates that the Ca(2+) oscillations lag the growth oscillations by 5.3 +/- 0.3 s. However, growth never completely stops, even during the slow cycle of an oscillation, and the concomitant tip Ca(2+) level is always slightly elevated compared with the resting Ca(2+) concentration along the distal shaft, behind the growing tip. Artificially increasing Ca(2+) using the Ca(2+) ionophore A23187 leads to immediate cessation of elongation and thickening of the apical cell wall. In contrast, dissipating the Ca(2+) gradient using either the Ca(2+) channel blocker La(3+) or the Ca(2+) chelator EGTA is accompanied by an increase in the rate of cell expansion and eventual bursting of the root hair tip. These observations are consistent with a model in which the maximal oscillatory increase in cytosolic Ca(2+) is triggered by cell expansion associated with tip growth and plays a role in the subsequent restriction of growth.

Extensional deformation, stress relaxation and necking failure of viscoelastic filaments

We investigate the transient viscoelastic behavior of weakly strain-hardening fluids in filament stretching devices during uniaxial elongation and following the cessation of stretching. The numerical results are compared with experimental observations on a concentrated shear-thinning polystyrene solution which is well characterized by a multi-mode Giesekus model. The finite element computations incorporate the effects of viscoelasticity, surface tension, and fluid inertia and the time-dependent moving-boundary problem is solved using the code POLYFLOW. A detailed comparison of multi-mode computations with single-mode solution is presented in order to examine the differences in the predicted viscoelastic behavior and the role of the fluid relaxation spectrum. The evolution in the transient Trouton ratio at different deformation rates is compared with experimental measurements and with the theoretical predictions of ideal homogeneous uniaxial elongation. Simulations of the filament stretching device using the multi-mode viscoelastic model demonstrate a significant improvement in the agreement between the predicted and observed extensional viscosity at short times. The computed Trouton ratio is also in good agreement with theoretical expectations for ideal homogeneous uniaxial extension, despite the strongly non-homogeneous viscoelastic necking of the fluid column observed during elongation in the filament stretching device. Following the cessation of elongation, numerical simulations predict an interesting and complex evolution in the kinematics of the fluid filament. Initially the tensile stresses in the column relax in the non-linear form predicted theoretically, indicating that filament stretching devices can be used to monitor transient extensional stress relaxation, provided that the evolution of the tensile force at the end-plate and the filament radius at the mid-plane are carefully measured. However, at longer times after cessation of stretching, the local extension rate at the axial mid-plane begins to increase rapidly, leading to a `necking failure' that is greatly accelerated compared to that expected in a corresponding Newtonian filament. The calculations show that this unstable necking is not driven solely by the surface tension but also by the viscoelasticity of the fluid, and is coupled with significant elastic recoil of the material near the end-plates. The rate of necking in the column is a sensitive function of the extensional viscosity predicted by the constitutive model, in particular the magnitude and the rate of strain-hardening that occurs during uniaxial elongation. This phenomenon can also be simply and accurately described by an appropriate set of coupled one-dimensional thin filament equations that use the finite element computations to provide a suitable initial condition for the axial distribution of the polymeric stresses in the filament.

Photoregulation ofcot-1,a Kinase-Encoding Gene Involved in Hyphal Growth inNeurospora crassa

Blue light plays a key role as an environmental signal in the regulation of growth and development of fungi and plants. Here we demonstrate that inNeurospora crassahyphae branch more frequently in cultures grown in light. Previous studies had identifiedcot-1as a gene that controls apical hyphal cell elongation. In thecot-1mutant, cessation of elongation is accompanied by hyperbranching. Here we demonstrate that thecot-1gene encodes two transcript species of about 2100 nt (cot-1(s)) and about 2400 nt (cot-1(l)) in length and that the ratio of both transcript species abundance is photoregulated. The origin of the difference betweencot-1(l) andcot-1(s) was localized to the 5′ end of thecot-1transcripts, suggesting that two COT1 isoforms with different activities may be formed. Both light effects, on branching and oncot-1expression, were dependent on functionalwc-1andwc-2gene products. In addition to light,l-sorbose comprises another environmental cue that controls hyphal branching inN. crassa.In the presence ofl-sorbose, photoregulation ofcot-1was blocked, suggesting the involvement of alternative and potentially interdependent signaling pathways for the regulation of hyphal elongation/branching.

骨形成不全症に対するＢａｉｌｅｙ‐Ｄｕｂｏｗ Ｒｏｄｓの使用経験

We reviewed our experience of multiple osteotomies with realignment and intramedullary rod fixation of deformed bones in 3 patients with osteogenesis imperfecta. There were one 7-year-old boy and two 6-year-old girls who were twins. The average duration of follow-up was 4.3 years. A further five operations were needed for treatment of complications which included one of femoral non-union, three of anterior placement of the rods and bony angular deformities in the distal part of the tibia, and one of proximal migration of the proximal end of the femoral rod. Satisfactory revisions for these had been carried out.Another complication was cessation of elongation in six rods due to slight bending of the rods from 2.5 to 3 years postoperatively. No patients suffered a fracture after rodding. Prior to surgery none of the subjects had been able to walk. After surgery the boy was able to walk with braces and the twins walked using braces and walkers at the time of review. Dispite a high rate of complications, Bailey-Dubow rods are considered to be a useful treatment for growing children suffering from osteogenesis imperfecta.

Activity of Phenylalanine Ammonia‐Lyase, Tyrosine Ammonia‐Lyase, and Cinnamyl Alcohol Dehydrogenase in the Maize Stalk

The relation between the activities of lignification enzymes and deposition of cell‐wall components during progressive stages of development has not been demonstrated. In this study, phenylalanine ammonialyase (PAL), tyrosine ammonia‐lyase (TALI, and cinnamyl alcohohNADPH dehydrogenase (CAD) activities were determined in maize (Zea maize L.) internodes at several development stages, and the activities were correlated with cellulose, hemicellulose, and lignin concentrations. Growth‐chamber grown maize was harvested at the fourteenth leaf stage, enzyme activities were quantified, and cell‐wall components were determined by the detergent system of fiber fractionation from the lower, middle, and upper sections of Internodes 7 through 14. Internode 7 was the second internode above the groundline. Cell‐wall maturation proceeded basipetally within each internode or the stalk, and appeared to be regulated by a coordinated and basipetal sequence of enzyme activities. Activity of PAL and TAL was highest in internodes undergoing rapid elongation and differentiation (Internodes 14‐13) and progressively diminished in Internodes 11 through 7, which were maturing following cessation of elongation. CAD activity was lowest in upper Internodes 14 through 12, but progressively increased in internodes that were halting elongation (Internodes 11‐7) and were commencing cell‐wall fiber deposition. The degree of lignification increased as CAD activity increased; hemicellulose, cellulose, and lignin depositions correlated strongly with CAD activity rather than with PAL and TAL. CAD activity appears committed to lignification specifically, and to secondary cell‐wall formation, in general, whereas PAL and TAL activities appear to have no direct regulatory role.

Episodic whole plant growth patterns in Ligustrum

Episodic growth of Ligustrum japonicum Thunb. plants was determined by measuring total shoot and root fresh weight nondestructively. Episodic growth patterns were apparent in shoot elongation, shoot and root fresh weight as a percent of total fresh weight, shoot and root relative growth rates (RGRs and RGRr) and two‐dimensional measurements of the root system. Increases in root growth and initiation of lateral root branching were coincident with changes in percent of total fresh weight in the root and RGRr. The rate of fresh weight gain of roots, shoots and the whole plant increased continuously throughout two experiments and thus episodic growth patterns were not apparent from these values. Alternating episodes of shoot and root growth, which is shown by percent fresh weight allocation and RGR, did not directly correspond to shoot elongation and cessation of elongation. Continuous, nondestructive measurement of total shoot and root growth reveals important changes in growth which may be obscured by other measurement techniques.

MODIFICATION OF MECHANICAL STRESS EFFECTS ON ELONGATION OF DARK-GROWN SOYBEAN BY Ca2+ AND K+

A single episode of thigmic (contact rubbing) stress applied to the apical hook of a dark-grown soybean hypocotyl results in a two-phase response: an initial rapid (s) cessation of elongation followed by a long (h) recovery. Increased Ca2+ in the growth matrix from 1 to 5 mM increased the average pre-stress elongation rate from 22 μm min-1 to 40 μm min-1 further increases in Ca2+ did not further enhance elongation rate, There was no effect of nutritional status on the initial rapid response to a single episode of thigmic stress; all seedlings stopped elongating within seconds regardless of nutrient status. However, seedlings cultured with higher levels of Ca2+ recovered their pre-stress rate more quickly. Correlation of enhanced recovery with enhanced root development was observed for seedlings grown in 5 mM Ca2+. Inclusion of 1.5, or 10 mM K+ in the growth matrix further reduced recovery time from ≥6 h to ≤3h. K+ effects may be due to the ability of seedlings to adjust osmotically. It previously has been shown that seedlings have significantly less K+ in their elongation zone following stress, and the osmotic potential of the expressed sap is lower than that of controls. Physiological and biophysical changes that occur in response to mechanical perturbations have potential for modification by nutritional status.This project was supported in part by NASA grant NAG10-0093.

Early Selection in Pinus ponderosa: Compromises Between Growth Potential and Growth Rhythm in Developing Breeding Strategies

Abstract Shoot elongation of seedlings from 13 wind-pollinated families in each of 3 natural populations was described in years 2 and 5 by a function that allowed calculation of the start, rate, duration, and cessation of elongation. Additive genetic variances for these variables and for height in year 5 were pronounced, heritabilities were high, and expected responses to selection were large. The age-age genetic correlation for each trait was strong, as were the genetic correlations for 5-year height with all traits except the start of shoot elongation. Expected responses to selection suggested that (1) genetic gains in growth can be pronounced; (2) strong selection for increased growth is accompanied by a change in growth rhythm to that typical of unselected populations from much milder environments; (3) selecting for the rate of shoot elongation tends to control the correlated responses in growth rhythm while providing about 70% of gains expected from direct selection on height; and (4) selecting in the greenhouse in year 2 is nearly as effective as selecting the field in year 5. While breeders might choose among strategies that either control correlated responses or optimize gains in growth potential, developing a program that is capable of realizing the expected gains requires an understanding of the system of genetic variability. For. Sci 38(3):661-677.

Cytological and physiological studies of the effects of IBP on Pyricularia oryzae (II): Effects on apical cells of single hyphae

AbstractThe effects of IBP (S‐benzyl O,O‐diisopropyl phosphorothioate) on tips of single hyphae of Pyricularia oryzae were investigated by interference contrast microscopy. Labelling hyphae with calcofluor white followed by IBP treatment revealed that elongation of apices of almost all hyphae at the colony margin was inhibited after treatment for 4 h. Successive observations on single hyphae of an IBP‐sensitive isolate indicated that apical cells stopped elongating approximately 10 min after the onset of treatment with 2 μg IBP ml−l. Small vacuoles appeared after 50 min; later they increased in number and size, and coalesced, finally producing a chain‐like arrangement of vacuoles in the cytoplasm. When hyphae were treated with 10 μg IBP ml−1, cessation of elongation and vacuolation occurred earlier than when treated with 2 μg ml−1. Apical cells of hyphae of an IBP‐tolerant isolate appeared unaltered even when treated with 10 μg ml−1. These results indicate that a major effect of IBP is to inhibit specifically the growth of apical cells of the IBP‐sensitive isolate.

Biosynthesis of Extensin during Normal and Light-Inhibited Elongation of Radish Hypocotyls

Summary Both intact and excised radish hypocotyls elongate at a rate of 0.6 to 0.7 mm/hr in the dark. Light causes a rapid and marked inhibition of the elongation rate. This light-induced inhibition of hypocotyl elongation is readily reversed by αα-dipyridyl, a chemical which chelates ferrous ions and inhibits hydroxyproline biosynthesis. In dark-grown intact plants the cessation of hypocotyl elongation is followed by the deposition of protein-bound hydroxyproline (extensin) in the cell wall. However, the light-induced cessation of elongation is not accompanied by an enhanced synthesis of cell wall hydroxyproline or the deposition of extensin in the cell wall. The results suggest that an αα-dipyridyl enhancement of elongation does not necessarily imply that there is a causal relationship between extensin deposition and the cessation of elongation, as suggested by other investigators. The effect of αα-dipyridyl on elongation may be unrelated to its inhibition of hydroxyproline biosynthesis.

Peroxidase changes during the cessation of elongation in Pisum sativum stems

The cessation of cell elongation in intact P. sativum epicotyls is accompanied by an increase in both soluble and cell wall peroxidases. These pero

Hydroxyproline-rich cell wall protein (extensin): Biosynthesis and accumulation in growing pea epicotyls

Plant cell walls contain a glycoprotein component rich in the otherwise rare amino acid hydroxyproline. We examined the synthesis and accumulation of wall hydroxyproline during different states of elongation growth in pea epicotyls. Light-grown peas contained more wall hydroxyproline than their taller, dark-grown counterparts. When elongation was studied by marking growing stems in situ, there was a marked accumulation of wall hydroxyproline coincident with the cessation of elongation. Dividing and elongating regions of the epicotyl showed less wall hydroxyproline than did regions where elongation was no longer occurring. Hydroxyproline biosynthesis was examined by incubation of excised sections of tissues in various growth states in 14C-proline. The extent of conversion of these residues to 14C-hydroxyproline served as a measure of the rate of hydroxyproline synthesis. This rate was highest in tissues which had ceased elongation. The low rate of hydroxyproline synthesis in dividing and elongating cells was probably not due to the inability to hydroxylate peptidyl proline or to secrete proteins. These data show a positive correlation between the synthesis and accumulation of cell wall hydroxyproline and the cessation of cell elongation in pea epicotyls.

Lösliche Saccharase bei normalem und durch Gibberellin stimuliertem Längenwachstum von Zwergerbseninternodien

The activity of soluble invertase and its relation to growth were studied in dwarf pea plants treated with gibberellic acid (GA3) and in water treated controls. Invertase activity increased with elongation in the individual internodes, reached a maximum and later declined, meaning that invertase of internodes is undergoing continous turnover during elongation (fig. 1, 2). GA3 treatment increased both elongation and invertase activity to the same extent (fig. 3, 4) without altering the kinetics of the two processes (fig. 4). The maximum of invertase activity per g fresh weight preceded that of elongation rate (fig. 4a) of the internodes of the two treatments. But when the enzyme activity was calculated to the protein of the internode, the maxima of activity and elongation rate were reached at the same time (fig. 4 b). The invertase activity per internode reached its maximum only upon cessation of elongation (fig. 5). The results were discussed with regard to the causal relationship between invertase and growth.

THE EFFECT OF ABSCISIC ACID ON STEM ELONGATION AND CORRELATIVE INHIBITION

SummaryThe mature length of the second internode (which was undergoing rapid vacuolation at the time of treatment with ABA) was not appreciably affected by doses of ABA below 20 μg per plant although the rate of elongation was depressed for the first 2 days after treatment, the depression being as much as 50% in tall varieties. Doses of ABA in excess of 20 μg not only depressed elongation rate immediately, but resulted in shorter internode length at maturity. Etiolated seedlings showed a stimulation of elongation rate and mature length in the dwarf ‘Meteor’, hut no statistically significant differences in the tall variety ‘The Pilot’ with doses below 20 μg ABA. Higher doses of ABA caused immediate cessation of elongation in vacuolating internodes, but only postponed the onset of vacuolation in younger internodes. Lighted plants also showed this postponement of the onset of vacuolation in the younger internodes at higher doses; in both lighted and etiolated seedlings the normal growth rate was resumed after a delay of between 3 and 7 days.Decapitation reduced elongation rate during the subsequent 48 hours either with or without ABA. In the absence of ABA there was a subsequent recovery in growth rate of the dwarf variety so that the final length of the second internode was the same for both intact and decapitated plants. But decapitation of the tall variety produced a marked reduction in mature length of the second internode, as it did in all ABA treated plants both dwarf and tall, intact and decapitated. The only significant interaction between decapitation and ABA on elongation rate occurred in the tall variety: during the first 24 hours all doses of ABA produced approximately the same big reduction in elongation rate in intact plants but had no statistically significant effect on decapitated plants; during the second day only the higher doses of ABA reduced the elongation rate of intact plants while all three levels produced the same big reduction in elongation rate of the decapitated plants. In the dwarf variety there was no significant interaction between the decapitation and ABA effects. Lateral bud outgrowth in decapitated plants was inhibited by ABA, and 100 μg per plant achieved inhibition comparable to that produced by the apex in intact plants. Hence ABA may possibly be the inhibitory substance demonstrated to be responsible for the phenomena of correlative inhibition.

Elongation of Stem Internodes in the Japanese Morning Glory (Pharbitis nil) in Relation to Auxin Destruction

AbstractThe relationship between auxin destruction and stem internode elongation was investigated in the vines of the Japanese morning glory (Pharbitis nil Choisy). In young plants an age‐dependent gradient was demonstrated in which the decreasing rate of elongation of older internodes correlated with an increasing ability of such tissue to destroy indoleacetic acid. Fragments of tissue from old internodes when incubated with indoleacetic acid (IAA), destroyed the hormone immediately and rapidly; in contrast, young, rapidly elongating internode tissue destroyed IAA only after a lag of several hours. In older plants the gradient was more erratic towards the middle of the plant but old and young tissue behaved as in young plants, i.e., old internodes destroyed IAA rapidly whereas young internodes did not. It appears reasonable to conclude that cessation of elongation in maturing internodes is brought about by developing an internal environment in which auxin is rapidly destroyed.

Root Development of Native Plants Under Three Grazing Intensities

The purpose of this study was to determine the effects of various intensities of grazing upon roots and root systems of plants native to the ponderosa pine zone of the Colorado Front Range. It was designed to ascertain the root characteristics of certain species of the plant community and any changes in root distribution induced by 17 years of moderate and heavy grazing by cattle. The cumulative effects of such grazing are also illustrated. Most studies concerning the effects of plant defoliation show that any cropping (clipping or grazing) reduces plant growth, especially root growth. The initial root and rhizome response to defoliation is the cessation of elongation (Parker and Sampson 1931, Crider 1955). Subsequent reactions to continued defoliation are reduction in root numbers and branching (Jacques 1937, Albertson, Riegel, and Launchbaugh 1953), root diameter (Biswell and Weaver 1933), and depth of soil penetration (Ruby and Young 1953, Cook, Stoddart, and Kinsinger 1958). The amount of reduction is directly related to the severity and frequency of defoliation (Graber 1931, Albertson et al. 1953, Thaine 1954). In general the degree of defoliation is more detrimental to root growth than frequency of defoliation. Crider (1955) showed that apical growth of grass roots stopped within 24 hr after the removal of 40% or more of the foliage in one operation. The time required for roots to resume growth varied directly with the degree of foliage removal. The consequences of overgrazing are apparently carried over from season to season. WVeaver