Relationships between root diameter,root length and root branching along lateral roots in adult,field-grown maize

Background and Aims Root diameter, especially apical diameter, plays an important role in root development and function. The variation in diameter between roots, and along roots, affects root structure and thus the root system’s overall foraging performance. However, the effect of diameter variation on root elongation, branching and topological connections has not been examined systematically in a population of high-order roots, nor along the roots, especially for mature plants grown in the field.Methods A method combining both excavation and analysis was applied to extract and quantify root architectural traits of adult, field-grown maize plants. The relationships between root diameter and other root architectural characteristics are analysed for two maize cultivars.Key Results The basal diameter of the lateral roots (orders 1–3) was highly variable. Basal diameter was partly determined by the diameter of the bearing segment. Basal diameter defined a potential root length, but the lengths of most roots fell far short of this. This was explained partly by differences in the pattern of diameter change along roots. Diameter tended to decrease along most roots, with the steepness of the gradient of decrease depending on basal diameter. The longest roots were those that maintained (or sometimes increased) their diameters during elongation. The branching density (cm^–1) of laterals was also determined by the diameter of the bearing segment. However, the location of this bearing segment along the mother root was also involved – intermediate positions were associated with higher densities of laterals.Conclusions The method used here allows us to obtain very detailed records of the geometry and topology of a complex root system. Basal diameter and the pattern of diameter change along a root were associated with its final length. These relationships are especially useful in simulations of root elongation and branching in source–sink models.     

Patterns of variability in the diameter of lateral roots in the banana root system

The relative importance of root system structure, plant carbon status and soil environment in the determination of lateral root diameter remains unclear, and was investigated in this study. Banana (Musa acuminata) plants were grown at various moderate levels of soil compaction in two distinct experiments, in a field experiment (FE) and in a glasshouse experiment (GE). Radiant flux density was 5 times lower in GE. The distribution of root diameter was measured for several root branching orders. Root diameters ranged between 0.09 and 0.52 mm for secondary roots and between 0.06 and 0.27 mm for tertiary roots. A relationship was found between the diameter of the parent bearing root and the median diameter of its laterals, which appears to be valid for a wide range of species. Mean lateral root diameter increased with distance to the base of the root and decreased with branching density [number of lateral roots per unit length of bearing root (cm(-1))]. Typical symptoms of low light availability were observed in GE. In this case, lateral root diameter variability was reduced. Although primary root growth was affected by soil compaction, no effects on lateral root diameter were observed.     

Root system architecture of Quercus pubescens trees growing on different sloping conditions

BACKGROUND AND AIMS: Plant roots' growth direction has important implications for plant development and survival; moreover it plays an effective and vital role in stabilizing weathered soil on a steep slope. The aim of this work was to assess the influence of slope on the architecture of woody root systems. METHODS: Five mature, single-stemmed Quercus pubescens trees growing on a steep slope and five on a shallow slope were excavated to a root diameter of 1 cm. A very precise numeric representation of the geometry and topology of structural root architecture was gained using a low-magnetic-field digitizing device (Fastrak, Polhemus). Several characteristics of root architecture were extracted by macros, including root volume, diameter, length, number, spatial position and branching order. KEY RESULTS: The diameter at breast height (dbh) was the best predictor of the root volume but had no correlation with length and number of roots. The slope affected the root volume for each branching order, and the basal cross-sectional area (CSA), number and length of the first-order roots. Number and length of the second- and third-order laterals were closely related in both conditions, although this relationship was closer in the shallow trees, suggesting the influence of a genetic control. Sloping trees showed a clustering tendency of the first- and second-order lateral roots in the up-slope direction, suggesting that the laterals rather than the taproots provide much of the anchorage. In a steep-slope condition, the taproot tapering was positively correlated with the asymmetry magnitude of first-order roots, indicating compensation between taproot and main lateral roots' clustering tendency. CONCLUSIONS: These results suggest that on a slope, on clayey soils, root asymmetry appears to be a consequence of several environmental factors such as inclination, shallow-slides and soil compactness. In addition, this adaptive growth seems to counteract the turning moment induced by the self-loading forces acting in slope conditions, and as a consequence improves the tree stability.     

MORPHOLOGICAL STUDIES OF THE ROOT OF RED PINE,PINUS RESINOSA I. GROWTH CHARACTERISTICS AND PATTERNS OF BRANCHING

Observations were made of the seasonal root growth behavior under natural conditions and under controlled conditions in plant observation boxes. Under natural conditions root growth conformed to the commonly reported pattern of a surge of growth in the spring, a mid-summer low, and a renewed burst in the fall. Growth of individual roots was cyclic. Growth patterns ordinarily varied according to root diameter and branching and in the plantations were modified by soil moisture conditions. Observations of roots during periods of constant elongation showed that the distance from the root apex to the first lateral root primordium varied directly with growth rate. Laterals did not arise in strict acropetal succession, and lateral root abortion was common, particularly in large-diameter, fast-growing roots. Observations of root initiation in relation to seasonal growth increments and to dormancy structures showed an increase in numbers of laterals on both the proximal and distal portions of a seasonal increment.     

Allometry of root branching and its relationship to root morphological and functional traits in three range grasses

The study of proportional relationships between size, shape, and function of part of or the whole organism is traditionally known as allometry. Examination of correlative changes in the size of interbranch distances (IBDs) at different root orders may help to identify root branching rules. Root morphological and functional characteristics in three range grasses {bluebunch wheatgrass [Pseudoroegneria spicata (Pursh) L?ve], crested wheatgrass [Agropyron desertorum (Fisch. ex Link) Schult.×A. cristatum (L.) Gaert.], and cheatgrass (Bromus tectorum L.)} were examined in response to a soil nutrient gradient. Interbranch distances along the main root axis and the first-order laterals as well as other morphological and allocation root traits were determined. A model of nutrient diffusivity parameterized with root length and root diameter for the three grasses was used to estimate root functional properties (exploitation efficiency and exploitation potential). The results showed a significant negative allometric relationship between the main root axis and first-order lateral IBD (P ≤ 0.05), but only for bluebunch wheatgrass. The main root axis IBD was positively related to the number and length of roots, estimated exploitation efficiency of second-order roots, and specific root length, and was negatively related to estimated exploitation potential of first-order roots. Conversely, crested wheatgrass and cheatgrass, which rely mainly on root proliferation responses, exhibited fewer allometric relationships. Thus, the results suggested that species such as bluebunch wheatgrass, which display slow root growth and architectural root plasticity rather than opportunistic root proliferation and rapid growth, exhibit correlative allometry between the main axis IBD and morphological, allocation, and functional traits of roots.     

Soil penetration resistance, a suitable soil property to account for variations in root elongation and branching

Background and Aims

Root system development is affected by soil conditions. The effects of bulk density, water content and penetration resistance on root development processes were investigated in peach trees.

Methods

Peach tree rootstocks were grown in various soil conditions, combining two bulk densities (1.2 and 1.5?g soil.cm^-3) and three water contents (0.14, 0.17 and 0.20?g.g^-1soil). Root parameters (tip diameter, length of apical unbranched zone, branching density and diameters of main and lateral roots) and plant growth (leaves, branches, trunk, root dry mass) were measured. Root growth processes (elongation, branching) were studied using relationships between root parameters.

Results

The proportion of biomass allocated to each plant compartment was similar whatever the soil conditions. Variations in root development were best explained by the variation in penetration resistance, rather than other soil properties. Increased soil penetration resistance reduced the root elongation rate, especially for thick roots. In addition, the branching pattern was affected. In soil with a high penetration resistance, the root system shape differs from a typical herringbone pattern.

Conclusions

These results allow quantification of the root system plasticity, and improve our understanding of the interactions between root development and soil properties.     

Local NO3- or NH4+ supply modifies the root system architecture of Cedrus atlantica seedlings grown in a split-root device

To study the effects of local nitrate or ammonium supply on the architecture of the Cedrus atlantica root system, cedar seedlings were grown in split-root boxes in a growth chamber. In each box-compartment, roots were fertilized with a solution containing nitrogen, either as nitrate [Ca(NO₃)₂] or ammonium (NH₄Cl), supplied at 0.1 or 5.0 mM. For each seedling, the shoot growth was measured twice a week for 3 months. The root system architecture was also recorded twice a week by tracing the root elongation through the transparent face of the root observation boxes. The apical diameter of the tap-root relay and that of a representative sample of lateral roots were recorded once a month using a monocular magnifier.

The increase of ammonium or nitrate concentration in the nutrient solution has significantly enhanced the production of lateral roots on the tap-root relay. After 90 days of culture, percentages of short lateral roots obtained with nitrate were higher than those obtained using ammonium. A preferential carbon allocation to the shoots was also obtained with an increasing nitrogen supply. Until the 40th day of culture, the elongation of lateral roots was similar for all treatments and ranged from 0.25 to 0.5 cm day⁻¹. From the 40th day to the 95th day, significant differences were observed between the compared modes and maximum elongation rates were obtained with 5 mM NH₄⁺ (2.18 cm day⁻¹) and 5 mM NO₃⁻ (1.18 cm day⁻¹). Local applications of nitrate and ammonium at a low or a high concentration had local effects on elongation and branching of the root system in the fertilized compartment. Contrasting effects of ammonium and nitrate were observed on the apical diameter of tap-roots and lateral roots. The root-split culture device confirmed that nitrate had local effects on the architecture of the C. atlantica root system.     

Root apical diameter and root elongation rate of rubber seedlings (Hevea brasiliensis) show parallel responses to photoassimilate availability

The hypothesis that root apical diameter may be used to evaluate root growth potential was tested. Temporal variations in the apical diameter of individual roots of rubber seedlings ( Hevea brasiliensis ) were studied together with their elongation patterns, using root observation boxes under controlled conditions. This study confirmed the overall positive correlation between apical diameter and growth rale. Moreover, the two parameters, varied in the same way during the life of a given root. For roots with short growth duration, there was a parallel quick decrease in both apical diameter and elongation rate, whereas roots that grew for longer periods showed synchronous fluctuations for both parameters. Since the mean values for the secondary roots within a root system exhibited the same trends, variations in apical diameter and elongation rates should depend on factors influencing the whole root system. When related to shoot rhythmic growth, both apical diameter and elongation rates were depressed during the periods of leaf growth. These effects were enhanced and/or prolonged by shading, hence reinforcing the hypothesis that this development depends on assimilate availability. Such results can be interpreted in terms of a source-sink relationship within the whole plant by considering the apical diameter, representing the size of the meristem related to the number of rneristematic cells, as an indicator of each root's growth potential.     

Steep,cheap and deep: an ideotype to optimize water and N acquisition by maize root systems

Background

A hypothetical ideotype is presented to optimize water and N acquisition by maize root systems. The overall premise is that soil resource acquisition is optimized by the coincidence of root foraging and resource availability in time and space. Since water and nitrate enter deeper soil strata over time and are initially depleted in surface soil strata, root systems with rapid exploitation of deep soil would optimize water and N capture in most maize production environments.• The ideotype Specific phenes that may contribute to rooting depth in maize include (a) a large diameter primary root with few but long laterals and tolerance of cold soil temperatures, (b) many seminal roots with shallow growth angles, small diameter, many laterals, and long root hairs, or as an alternative, an intermediate number of seminal roots with steep growth angles, large diameter, and few laterals coupled with abundant lateral branching of the initial crown roots, (c) an intermediate number of crown roots with steep growth angles, and few but long laterals, (d) one whorl of brace roots of high occupancy, having a growth angle that is slightly shallower than the growth angle for crown roots, with few but long laterals, (e) low cortical respiratory burden created by abundant cortical aerenchyma, large cortical cell size, an optimal number of cells per cortical file, and accelerated cortical senescence, (f) unresponsiveness of lateral branching to localized resource availability, and (g) low K_m and high V_max for nitrate uptake. Some elements of this ideotype have experimental support, others are hypothetical. Despite differences in N distribution between low-input and commercial maize production, this ideotype is applicable to low-input systems because of the importance of deep rooting for water acquisition. Many features of this ideotype are relevant to other cereal root systems and more generally to root systems of dicotyledonous crops.     

Effect of Main Root Decapitation on Root Branching in Flax Seedlings

Root branching patterns in intact and decapitated flax (Linum usitatissimumL.) roots were compared. The number of initiated primordia in the control and decapitated roots was similar, but decapitated roots produced an increased number of lateral roots owing to an increase in the number of primordia developed into the laterals. It is suggested that the apical meristem influences lateral root development only at the stage of root emergence from the parent root.     

Use of morphometric analysis for characterization of tobacco root growth in relation to infection byPhytophthora parasitica var.nicotianae

Development of tobacco root systems was characterized under controlled environmental conditions by use of morphometric root analysis. According to the classification scheme of this system, roots terminating in apical meristems are defined as first-order roots. Elements of second-order roots begin where two first-order roots merge, and so forth. Growth of root systems was similar for susceptible and resistant tobacco cultivars in nonautoclaved and autoclaved soils. During 15 days of growth subsequent to transplanting of 2-week-old plants, relative multiplication and extension rates of first-order and second-order roots were constant. Apparent unit extension rates of first-order and second-order root elements increased through 15 days of root system growth. Classification of tobacco root systems by the morphometric scheme provided a useful means of partitioning susceptibility of tissues to infection byPhytophthora parasitica var.nicotianae. Zoospores applied at the tips of first-order roots were most successful in causing infections; 73.3% of the roots inoculated with 16 zoospores per root tip became infected. Percentages of infections after inoculation of first-order root tissues 2 cm behind root tips or after inoculation of second-order roots were 10 and 4.3%, respectively.Florida Agricultural Experiment Station, Journal Series Paper 8106.     

The effect of drought on mycorrhizal production and very fine root system development of Norway spruce under natural and experimental conditions

Mycorrhizal growth rates were measured monthly, using a new method, in two neighouring plots of a natural spruce stand. One of the plots was irrigated while the other suffered from drought during the late summer and autumn months. Drought did not completely stop mycorrhizal growth. It caused a higher rate of root dormancy and a reduced elongation rate of the parent roots but an increased development of new mucorrhizal last order laterals. Thus, the branching density of the very fine root system was increased, even though fewer growing mycorrhizae were found in the non-irrigated plot during the dry period. Similar results were observed in a water-stress experiment with pot-cultures. After rewetting, elongation rate was stimulated and the number of growing mycorrhizae increased rapidly on the non-irrigated plot. Possible relationships between dry weight, distribution and branching density of growing fine root systems are presented.     

Maximum axial growth pressures of the lateral roots of pea and eucalypt

Although lateral roots may contribute significantly towards growth and nourishment of plants, the mechanics of their elongation behaviour in strong soils is not well known. The aim of this study is to report maximum axial growth pressures (p) and maximum elongation rates (E) of the lateral roots of an annual herbaceous plant (pea) and a woody perennial (eucalypt). As such measurements have not been reported previously, measurements of P and E for lateral roots were compared with the primary roots of pea for which reports are widespread. Values of P were estimated from the measured maximum values of axial force and root diameter on single, intact roots of seedlings in the laboratory. Additional measurements of both P and E were made for the lateral roots of pea when the growth of the remaining root axes was stopped (with removal of tips) to determine the overall effects of root-growth-inhibition on P and E of single roots.Values of P and E for lateral roots of pea were significantly greater than those for the lateral roots of eucalypt. Although root diameter for the primary roots of pea were similar to those for the lateral roots of eucalypt, the former exerted nearly twice as much pressure as the latter. The lateral roots of pea elongated significantly slower than the primary roots; however, P of lateral roots was significantly lower than the primary roots when elongation of all other roots was inhibited during the measurements. Production and/or development of lateral roots increased when elongation of the remaining roots (both primary and lateral roots) of pea seedlings was restricted due to the removal of root tips and exposure of one of the lateral roots to high strength. In general, maximum axial force exerted by primary and lateral roots was similar for roots of <1 mm diameter. However, primary roots exerted greater maximum axial force than the lateral roots when root diameter was >1 mm. As axial pressure of lateral roots was independent of root diameter, thickening of root tips is less likely to assist penetration of lateral roots in strong soils.     

Why are laterals less affected than main axes by homogeneous unfavourable physical conditions? A model-based hypothesis

When plants develop in strong soils, growth of the root system is generally depressed. However, branching and elongation of branches are often less affected than growth of the main axes, whenever the whole root system encounters even-impeded conditions. On the basis of a model simulating root growth and architecture as related to assimilate availability, we propose a simple hypothesis to explain such behaviour. In the model, growth of each root depends on its own elongation potential, which is estimated by its apical diameter. The potential elongation rate–apical diameter relationship is the same for all the roots of the system and is described by a monomolecular function. Our hypothesis is that the effect of soil strength can be simulated by introducing an impedance factor in the definition of root maximum potential elongation rate, common to the whole root system. When such impedance factor is applied, it affects more the potential of larger roots (main axes) than that of thinner roots (secondary and tertiary branches). Simulations provided in high impedance conditions led to root systems characterised by short taproots, whereas growth of secondary roots was unaffected and growth of tertiary roots was enhanced. Actual branching density was also higher, although branching rules have been unchanged. Such simulated systems where similar to that observed in strong soils. Friction laws or pore size can be involved in the larger reduction of the potential growth of main axes. Moreover, when growth of main axes is restricted, assimilate availability becomes higher for branches and that could explain that their growth could be increased in a homogeneous strong soil. This revised version was published online in June 2006 with corrections to the Cover Date.     

Root Regeneration and Plant Water Status of Pinus radiata D. Don Seedlings Transplanted to Different Soil Temperatures

The regeneration and configuration of new roots on transplanted8 month old Pinus radiata D. Don seedlings were measured inrelation to soil temperature and time after transplanting. Theeffects of root wrenching and nutrition on root regenerationwere compared. Low soil temperature adversely affected the initiationand elongation of new roots. The branching and morphology ofthe new roots were also influenced by soil temperature. Theprocesses of initiation and elongation are greatly retardedbelow a critical root temperature between 11 and 14 ?C. Wrenchingand fertilization stimulated new root growth at low temperatures.There is a close interrelationship between the constituent partsof the network which forms the new root system. The first-orderlaterals on transplants are critical as a basic framework forextension and production of second-order laterals which formthe major part of the regenerating root system. Needle water potential was closely related to the new root growth.The water stress experienced by the transplants for severalweeks after planting was due primarily to the suppressive effectof low soil temperature on root regeneration. Wrenched plantswere less water-stressed than unwrenched plants. Field measurements confirmed the finding from controlled experimentsthat, in southern Australia, the low soil temperature in theplanting season imposes a major restriction on early root regenerationwhich in turn inflicts water stress in transplants. This needsto be considered during the planning of planting and fertilization.     

Nutrient Supply and the Growth of the Seminal Root System in Barley: I. THE EFFECT OF NITRATE CONCENTRATION ON THE GROWTH OF AXES AND LATERALS

A method was devised by which different zones along a singleseminal axis of an intact plant could be exposed for extendedperiods to contrasting concentrations of nitrate (either 0.01or 1.0 mM) in continuous flow, the supply of all other nutrientsbeing favourable throughout. The concentration of nitrate wasfound to exert a direct and strictly localized effect upon thegrowth of lateral roots which, depending upon the supply ofassimilates from the shoot, resulted in marked modificationsto the form of the root system. Zones receiving 1.0 mM nitrateshowed an increase in the number and extension rate of bothfirst- and second-order laterals, associated with a preferentialaccumulation of dry matter, compared with zones in 0.01 mM nitrate.The average number of laterals (both first and second order)per cm of parent root was 4.4 in the presence of 1.0 mM nitrateand 2.2 in 0.01 mM. The average extension rates of first-orderlaterals were 0.61 and 0.26 cm d^–1 and second-order laterals,0.10 and 0.05 cm d^–1 for nitrate concentrations of 1.0and 0.01 mM respectively. The precise numbers and extensionrates of laterals in any one zone were affected, however, bythe rate of growth of laterals in other parts of the root system.In contrast, the extension rates of axes were little affectedby the concentration of nitrate to which their apical meristemswere exposed and approached 2.0 cm d^–1 provided the plantswere not nitrogen-starved. The significance of these resultsto the physiology of root growth and soil-plant relations isdiscussed.     

Growth and Development of Roots of Grapevine (Vitis vinifera L.) in Relation to Water Uptake from Soil

Developmental patterns of lateral roots and their vascular differentiationwere investigated for Vitis vinifera L. cv. Shiraz to assessthe likely contribution of lateral roots to total water uptakeof plants subjected to different irrigation regimes. Correlationanalyses showed a significant positive correlation between mainroot diameter and the diameter of first order lateral rootsof well-watered plants, but in water-stressed plants the twowere not significantly correlated. The correlations betweendiameters of first order lateral roots and the diameters ofmain roots were greater than correlations between the lengthsof first order laterals and the diameters of main roots. Thesuberised surface area of well-watered main roots increasedfrom 4% of total surface area at 0·25 cm to 100% at 10cm from the tip, whereas that of stressed plants increased from15% at 0·25 cm to 100% at 5 cm from the tip. In all treatmentsthe highest linear density of first order laterals was about7 laterals cm^-1 of main root. More than 50% of first order lateralshad diameters less than 0·05 cm, and more than 90% ofthem had lengths less than 5 cm. Calculations of axial resistancesbased on xylem diameter measurements suggest that the axialresistances of root segments may not be uniform along rootsas is often assumed in models of water uptake. Water flow intothe main roots via the lateral root pathway is likely to bemuch smaller than that via the direct radial flow pathway asonly about 1% of surface area of main roots is directly occupiedby lateral roots, leaving the other 99% of main root surfacearea available for the direct radial flow pathway.Copyright1994, 1999 Academic Press Axial resistance, grapevine (Vitis vinifera L. cv. Shiraz) roots, root diameter, root length, xylem vessels     

Control of root growth: effects of carbohydrates on the extension, branching and rate of respiration of different fractions of wheat roots

Rates of extension, numbers of laterals and rates of respiration were measured in different fractions of wheat ( Triticum aestivum L. cv. Alexandria) roots following changes in carbohydrate supply. The supply of carbohydrate was varied by selective pruning and exogenously fed sugars. Pruning shoots to a single leaf (leaf-pruning) reduced the rate of O₂ uptake by intact roots. Rates were not stimulated by shortterm feeding of sucrose (25 m M ), but were stimulated by the uncoupler p -trifluoro-methoxy(carbonylcyanide)phenylhydrazone (FCCP). Feeding glucose to roots of leaf-pruned and non-pruned plants for 16–24 h increased the rate of O₂ uptake. It is concluded that respiration is under fine control by adenylates and coarse control by carbohydrate supply, with carbohydrates regulating directly the rate of some energy consuming process(es). These energy consuming processes are located in growing tissue fractions. Feeding glucose to leaf-pruned and non-pruned plants increased rates of O₂ uptake in seminal root tips, the zone of developing lateral primordia and mature root sections with elongating laterals, but had no effect on mature sections from which the laterals had been excised. Leaf-pruning reduced the extension rate of seminal axes and first-order laterals when measured over 24 h. Feeding glucose to roots from the time of pruning increased the rate, but did not fully restore it to control values. Pruning roots to a single seminal axis (root-pruning) and feeding glucose to non-pruned plants had no effect on the extension rate of the seminal axis or its laterals over this time period, although rates were increased by root-pruning when measured over 3 days. The number of lateral root primordia was reduced by leaf-pruning and increased by root-pruning and feeding glucose. The results are discussed in terms of the role of carbohydrates in the control of root growth and branching.     

水曲柳和落叶松不同根序之间细根直径的变异研究

细根直径大小和根序高低对细根寿命和周转估计具有重要的影响,研究不同根序之间的直径变异对认识细根直径与根序的关系具有重要意义。该文根据Pregitzer等(2002)提供的方法,研究了位于东北林业大学帽儿山实验林场尖砬沟森林培育实验站17年生水曲柳(Fraxinus mandshurica)和落叶松(Larix gmelinii)人工林细根1~5级根序的平均直径的变化、直径的最小值和最大值范围、直径的变异系数。结果表明,水曲柳和落叶松细根直径<2 mm时,包含5个根序,随着根序由小到大的增加,细根直径也在增大。各根序平均直径之间,存在较大的差异。在同一根序内,细根直径范围很大,水曲柳和落叶松一级根最小直径均<0.20 mm,最大直径分别<0.50 mm(水曲柳)和<0.70 mm(落叶松)左右。2~3级根序直径最小值在0.20~0.30 mm之间,最大值≤1.0 mm。5级根直径最小值<1.0 mm,最大值超过2.0 mm。随着根序等级增加,直径变异系数增大。一级根序的直径平均变异系数<10%,2~3级根序直径平均变异系数在10%~15%左右,4~5级根序直径的平均变异系数在20%~30%之间。因此,在细根寿命与周转研究过程中,必须同时考虑直径和根序对细根的寿命估计的影响。     

Dependence of Root Cell Growth and Division on Root Diameter

Primary roots of 98 species from different families of monocotyledonous and dicotyledonous plants and adventitious roots obtained from bulbs and rhizomes of 24 monocot species were studied. Root growth rate, root diameter, length of the meristem and elongation zones, number of meristematic cells in a file of cortical cells, and length of fully elongated cells were evaluated in each species after the onset of steady growth. The mitotic cycle duration and relative cell elongation rate were calculated. In all species, the meristem length was approximately equal to two root diameters. When comparing different species, the rate of root growth increased with a larger root diameter. This was due to an increase in the number of meristematic cells in a row and, to a lesser degree, to a greater length of fully elongated cells. The duration of the mitotic cycle and the relative cell elongation rate did not correlate with the root diameter. It is suggested that the meristem size depends on the level of nutrient inflow from upper tissues, and is thereby controlled during further growth.