柠条人工幼林细根生长和死亡的季节变化

以晋西北黄土区5年生柠条(Caragana korshinskiiKom.)人工林为研究对象,应用微根管技术对林地100cm土层范围的柠条细根动态进行了观测。以细根根长密度(RLD,mm.cm-3)、生长速率(RLDgr,mm.cm-.3d-1)、死亡速率(RLDdr,mm.cm-.3d-1)和生死之比(Rgd)为基本参数,对生长季(2007年4-9月)柠条细根的生长和死亡特点及其与环境因子(如气温、降雨量、土壤温度、土壤水分等)的关系做了探讨。结果表明:(1)在生长季,柠条细根的平均RLDgr和RLDdr分别为0.1264mm.cm-.3d-1和0.0354mm.cm-.3d-1;(2)下层(50-100cm)细根的RLDgr大于上层(0-50cm);但是下层细根的RLDdr小于上层;(3)柠条细根RLDgr的季节变化趋势为4-7月份迅速增大,8月份达峰值,之后迅速减小;细根RLDdr的季节变化趋势则为4-7月初缓慢增大,之后迅速增大,在生长季末(9月下旬)达到最大;(4)柠条细根Rgd在生长季呈逐渐减小趋势,但是仅季末Rgd1,说明在生长季柠条的细根动态是一个以生长占优势的生死交织过程;(5)RLDgr与气温存在极显著正相关(P0.01),与土壤温度存在显著正相关(P0.05);但是RLDdr与各个环境因子的相关性均不显著(P0.05)。     

柠条人工林幼林与成林细根动态比较研究

以晋西北黄土高原区柠条(Caragana korshinskii)人工林幼林和成林为研究对象,应用微根管技术(Minirhizotron technique)对林地0-100 cm土壤剖面的柠条细根生长动态进行了观测.以2007年生长季(4-9月)观测数据为基础,对两林地的柠条细根生长速率(G,mm cm-3 d-1)和细根死亡速率(M,mm cm-3 d-1)的时空变化格局及其与气温、降水、土壤温度和土壤水分等环境因子的关系进行了研究.结果表明,在年生长季,幼林的G(0.1264 mm cm-3 d-1)和M(0.0354 mm cm-3 d-1)均高于成林(分别为0.0914 mm cm-3 d-1和0.0220 mm cm-3 d-1).在垂直分布上,幼林G出现最大值的土层深度(70-80 cm)较成林(50-60 cm)为深.两林地的G和M具有相似的季节变化特点,即G在4月到7月之间缓慢增大,8月迅速达到峰值,之后迅速减小;M自4月至9月M呈持续增高趋势.配对数据t检验结果显示,幼林与成林的C没有显著差异(P＞0.05),而幼林的M显著高于成林(P＜0.05).Pearson相关系数表明,幼林和成林G的垂直分布与土壤温度和土壤水分的垂直变化没有显著相关性;但是幼林和成林M的垂直分布与土壤温度的相关性显著(幼林地P＜0.01;成林地P＜0.05).在年生长季,幼林G与气温和土壤温度具有显著正相关性(与气温的P＜0.01;与土壤温度的P＜0.05);而成林G与各环境因子的相关性则均不显著(P＞0.05).两林地的M与各环境因子的相关性均不显著(P＞0.05).     

幼龄柠条细根现存量与环境因子的关系

以晋西北黄土高原区柠条(Caragana korshinskii)幼龄人工林为研究对象, 应用微根管技术(Minirhizotron technique)对林地100 cm土层范围的柠条细根生长动态进行了观测。以2007年生长季(5~9月)的根长密度(RLD, mm·cm^-3)数据为基础, 对柠条细根现存量(RLDst, mm·cm^-3)及其与环境因子(≥10 ℃积温、同期土壤积温、积降雨量和土壤水分等)的关系作了研究。结果表明, 40~90 cm土层是柠条细根的主要分布区和生长活跃区, 其细根占细根总量的59.7%。柠条细根现存量的季节变化特征为: 5月至9月上旬RLDst持续增加, 9月下旬RLDst略有降低。柠条细根现存量季节变化与≥10 ℃积温、同期土壤积温和积降雨量均存在极显著正相关关系。     

AM真菌对紫花苜蓿细根生长及其生物量动态特征的影响

细根对植物功能的发挥和土壤碳库及全球碳循环具有重要意义。采用容器法和微根管法于2013年6~10月整个生长季内对紫花苜蓿的细根生物量、生产以及周转规律进行研究。结果表明:(1)紫花苜蓿活细根现存生物量平均值以接种摩西球囊霉(Gm)处理最高(12.46g·m-2),未接种对照最低(7.31g·m-2),并且活细根现存量在9月中旬达到峰值;死细根现存生物量呈先增加后降低再增加的变化趋势,在整个生长过程中未接种处理高于接种处理,接种根内球囊霉(Gi)处理死细根现存平均生物量(3.11g·m-2)又较接种组其他处理低。(2)苜蓿植株细根生长量以接种幼套球囊霉(Ge)处理最大(0.045 mm·cm-2·d-1),接种Gm处理和未接种对照最低(均为0.027mm·cm-2·d-1);而未接菌植株细根死亡量(0.044mm·cm-2·d-1)显著高于接种植株,接种组又以Gi处理最低(0.021mm·cm-2·d-1)。(3)紫花苜蓿在生长季节内细根生产和死亡的高峰分别出现在8月底和10月份,低谷出现在9月底到10月中旬和6月底到8月;接种地表球囊霉(Gv)后细根现存量和年生长量显著高于对照和接种其他菌种处理,细根的周转以对照组最大,而接种Gv和Gm处理较低。研究发现,通过接种丛植菌根真菌可以提高苜蓿细根生物量,降低细根的死亡,增加细根寿命。     

柠条细根的分布和动态及其与土壤资源有效性的关系

受土壤资源有效性时空异质性的影响,植物细根会表现出明显的垂直分布和季节变化特征。揭示这些特征对认识细根的养分和水分吸收规律,预测C在地下的分配特点具有重要意义。本研究运用Minirhizotron技术对晋西黄土丘陵区30年生柠条（Caragana korshinskii Kom.）人工林0-100cm土层深度范围内细根的密度( FRD, N cm-2)、根长密度( FRLD, mm cm-2)、平均直径(FRDi, mm)和根表面积(FRSA, mm2 cm-2)的垂直分布特征和季节动态进行了一个生长季的观测,并分析了这些参数与土壤温度、水分和有效氮之间的关系。结果表明：(1)FRD、FRLD和FRSA均表现出随土层深度增加而先增大后减小的趋势,以40-60cm土层之值最大(分别占总数的34.3%、35.5%和37.3%);而FRDi随土层深度增加而减小,其最大值为0.31970.0231mm,最小值为0.28840.0109 mm;(2)受土壤资源有效性季节变化的影响,FRD、FRLD和FRSA在不同土层(除0-20cm外)表现出相似的季节动态,即随季节变化而先增大后减小,春季小(分别为0.2204 N cm-2,1.8482 mm cm-2,2.2647 mm2 cm-2)而秋季大(分别为0.5316 N cm-2,4.4046mm cm-2,4.3007mm2 cm-2);FRDi则表现由粗逐渐变细的过程,春季最粗(0.3659mm)而秋季最细(0.2712mm);(3) 各细根参数与土壤温度、水分和有效氮在各土层存在不同程度的相关性。从简单相关分析来看,细根的季节性变化主要受土壤温度和水分的影响,有效氮的影响不明显。FRD、FRLD和FRSA在0-20cm土层主要受土壤水分影响(r=-0.729--0.914, p<0.05),而在20-100cm土层则主要受土壤温度的影响,且显著性随土层加深而增加(r=-0.028-0.832, p<0.05)。各土层细根与土壤有效资源间的相关性反映了细根功能的季节性差异。综合分析表明,各细根参数季节变化的54.0%-98.6%是由土壤温度和水分的交互作用而引起。     

幼龄柠条细根的空间分布和季节动态

以晋西北黄土高原区5年生柠条(Caragana korshinskii)人工林为研究对象,应用Minirhizotron技术,分别在距茎干水平距离0 cm和50 cm处设点(以下简称为0 cm位点和50 cm位点),对林地0—100 cm土层深度范围内的柠条细根进行了观测。以2009年生长季(4—10月)的细根根长密度(RLD,mm/cm2)和表面积密度(RAD,mm2/cm2)数据为基础,结合同期环境因子(气温、降雨量、土壤温度和土壤含水量等)数据,对0 cm和50 cm两个位点的细根动态特点进行了比较研究。结果表明:(1)两个水平位点的细根垂直分布和季节变化趋势均具有一定差异,主要差异是0 cm位点0—60 cm各土层的RLD均大于50 cm位点,前者各测定期的RLD(RAD)均大于后者。因此,0 cm位点的细根分布量(4.04 mm/cm2和4.67 mm2/cm2)显著大于50 cm位点(3.07 mm/cm2和2.99 mm2/cm2)。(2)就整体(两个位点平均值)而言,RLD(RAD)的垂直分布以40—50cm土层最大,以60—70cm土层最小。RLD(RAD)的季节变化具有由小变大再变小的趋势。年生长季幼龄柠条细根的RLD和RAD总平均值分别为3.55 mm/cm2和3.83 mm2/cm2。(3)就0 cm位点、50 cm位点或整个林地而言,细根RLD的季节变化与气温和土壤温度的季节变化均具有显著正相关性。以上结果表明,幼龄柠条细根的水平分布具有"近主根"特点;RLD的季节变化与温度因子的季节变化具有高度一致性。     

5种温带森林生态系统细根的时间动态及其影响因子

细根(直径≤2 mm)的生长和死亡动态及其影响因子是森林生态系统能量流动和物质循环的重要研究内容,但因受到研究方法的限制而了解甚少.于2010年5-10月采用微根管技术对东北东部山区5种温带森林生态系统的细根生长量(FRP)和死亡量(FRM)进行了动态跟踪测定,并同步测定了土壤温度(Ts)、土壤湿度(Ms)、叶面积指数(LAI)等相关因子.结果表明:不同林型和取样时间的FRP和FRM均差异显著(P＜0.001).杨桦林、硬阔叶林、兴安落叶松林、红松林、蒙古栎林的FRP和FRM分别为:(13.34 ±0.90) μm·cm-2·d-1(平均值±标准误)和(5.02±0.36) μm· cm-2·d-1、(13.04±0.82)μm·cm-2·d-1和(6.85±0.32) μm· cm-2·d-1、(8.74±1.44) μm·cm-2·d-1和(5.05±0.61) μm·cm-2·d-1、(8.02±2.27) μm·cm-2·d-1和(3.88±0.35)μm·em-2·d-1、(7.59±0.82) μm·cm-2·d-1和(3.88±0.61) μm· cm-2·d-1.所有林型生长季期间FRP的时间变化均呈现明显的单峰型,但峰值出现的时间却因林型而异.FRM随生长季的进程而逐渐增加,杨桦林和硬阔叶林FRM在8月初出现峰值,而红松林、兴安落叶松林和蒙古栎林的FRM峰值均出现在生长季末期.Ts、Ms和LAI对FRP和FRM均存在显著的正效应(P＜0.05),3个因子的综合作用对各个林型FRP和FRM变异性的解释率分别达68％和53％以上,表明这些温带森林生态系统细根生长和死亡的时间动态主要受土壤温湿度和叶面积变化的联合影响.     

温度升高对长江口芦苇湿地细根形态和生长的影响

全球变暖对滨海湿地植物细根的影响目前尚不十分清楚。以长江口崇明东滩芦苇(Phragmites australis)湿地为对象,利用开顶式生长箱法进行模拟升温。于2019年5-10月,结合微根管法和根钻法,对0-40 cm土层细根(直径≤2 mm芦苇须根)的总根长、总表面积、比根长、比表面积、平均直径和生物量等指标开展连续观测,并计算其净生长速率和周转速率,分析气温升高对芦苇湿地细根形态特征和生长动态的影响。结果表明:在整个生长季,升温显著提高了0-40 cm土层细根的总根长、总表面积和总生物量,且主要表现在0-20 cm土层,而对比根长、比表面积无显著影响。升温显著增强了0-40 cm土层细根的净生长速率,且使其季节变异性加大。升温显著提高了10-40 cm土层细根的周转速率,但在0-10 cm土层影响不显著。总之,升温显著提高了芦苇湿地细根的总量和生长速率,改变其在土壤中的垂直分布格局,但对其水分和养分吸收效率无显著影响。升温使细根周转速率加快,同时使参与周转的细根总量增加,导致各土层特别是0-20 cm土层根源有机碳输入显著增加,这可能会深刻影响芦苇湿地的土壤碳平衡。     

柠条细根的空间分布特征及其季节动态

以晋西北黄土区30年生柠条(Caragana korshinskii Kom.)人工林为研究对象,2007年应用Minirhizotron技术,分别在距茎干水平距离0、50、100 cm处设点,对林地0-100 cm土层深度范围内的柠条细根空间分布及其生长季的动态进行了研究。结果表明:(1)生长季柠条细根根长密度(RLD)总平均值为1.3423 mm/cm²。在水平方向上,距茎干水平距离50 cm处分布最多(1.5369 mm/cm²),其次为0 cm处(1.3855 mm/cm²), 100cm处分布最少(1.1044 mm/cm²)。在垂直深度上,各土层RLD平均值大小顺序为40-60 cm>60-80 cm>20-40 cm>0-20 cm>80-100 cm;(2)在0-100 cm土层范围内,月平均RLD在生长季的波动范围为0.4405 2.1040 mm/cm²,其中9月份最多,4月份最少;RLD在5个土层深度3个水平距离处随季节变化均表现先增加后减少的趋势,且不同空间位置RLD峰值变化均在秋季(8 10月份)波动。细根的这种时空分布差异,可能主要受林下土壤资源空间异质性及其季节性变化的影响,但也不排除其它因素的影响(如真菌,植食性昆虫)。     

不同林龄柠条细根现存量比较

采用微根管技术(Minirhizotron technique)对晋西北黄土丘陵区幼林(5a)与成林(30a)柠条(Caragana korshinskii)细根动态进行了为期5a的原位观测。基于2008—2011年的观测数据,对两林龄柠条不同土层细根现存量动态进行了比较研究,并探讨了两林龄柠条细根现存量与不同年际间水热条件的差异。结果表明:在0—100 cm土壤剖面,柠条幼林与成林细根现存量的峰值均位于50 cm土层以下,成林细根现存量峰值位于50—60 cm土层,幼林细根现存量峰值则从观测期初的90—100 cm土层到观测期末的80—90 cm土层。各观测年内,两林地各土层每年生长季初(3—4月)会出现细根现存量的积累;30—100 cm土层中,幼林细根最大现存量出现时间均较成林早,而生长季末(9—10月),所有土层幼林细根现存量下降均较成林快。柠条细根现存量的垂直分布主要受土壤水分影响,季节变化受温度的影响更大,年际间细根现存量的差异主要是由于年降雨量变化;幼林细根现存量受降水、土壤水分、土壤温度等的影响比成林大。     

Influence of storage methods on corn root length measurements

This study evaluated the changes in root length, mass, and diameter after air drying and rehydration of corn (Zea mays L.) root samples. For corn roots washed from soil, rehydrated root length was not reduced when compared with fresh root length, but rehydrated root mass was reduced to about half of fresh root mass, and rehydrated root diameter was approximately 75% of fresh diameter. Three storage methods (air dried, 70% ethanol, and 5% formaldehyde solution) were also compared for corn roots grown in moist paper towels. Although root mass and diameter were significantly reduced by air drying, root length was not altered by any of the treatments.     

Applications of functional classification methods for tree fine root biomass estimation: Advancements and synthesis

The morphology of fine root branching of woody plants is highly variable in their forms and functions. In the past two decades, researchers have increasingly recognized that the root-diameter-based method, using an arbitrary size of root diameter, failed to precisely characterize the physiological and ecological processes involved in finest roots. The number of publications using root-order-based approaches has increased regardless the fact that root trait-measurements based on root order are time-consuming and labor-intensive. A new approach—root functional classification method—was proposed and had been applied in the literature. The functional classification of fine roots separates roots of < 2 mm to absorptive and transport pools, making it more feasible for studies on root biomass and turnover. This new concept redefines fine root guild and has great potentials for future studies. Our literature review of the topic indicates that less is known about the inter-specific differences in estimates of biomass of absorptive and/or transport roots, with a large variation of absorptive roots on global scale. In addition, our review emphasizes the importance in: a) precision estimating of the absorptive biomass of fine roots, and b) proper definition of the range of the transport roots within and among forest ecosystems. Finally, after compare the strengths and weaknesses of the functional classification method, we propose several specific suggestions to improve the applications of this approach.     

The Developmental Anatomy of the Seminal Root of Wheat

Wheat embryo usually gives rise to five seminal roots in matured caryopsls, although, the sixth root might develop in some cases. The first one is known as the primary root. Primary root emerged early, and its primodium was distinctly originated from the proembryo and could be gradually identified as three layers of initials. Lateral seminal roots emerged later from the embryonic axis in pairs, and originated from the surrouding cells of the procambium. Differentiation of lateral roots was much more vigorous than that of the first seminal root (primary root), and, its mother cells of metaxylem vessel appeared soon, Lateral seminal roots usually had more metaxylem vessels. In short, only the first root is the primary root, the lateral seminal roots are adventitious in nature, since their structures are similar to those of other adventitious roots.     

Root strength,growth, and rates of decay: root reinforcement changes of two tree species and their contribution to slope stability

Information on live root-wood strength, rates of root decay and root growth of both radiata pine (Pinus radiata D. Don) and kanuka (Kunzea ericoides (A. Rich.) Joy Thomps. var. ericoides) are combined to form a generalized conceptual model of changes in nett root reinforcement. The model provides an initial opportunity to rank the plant species having specific below-ground rooting habits that can be used to control erosion, and when linked with extreme flood probability can be used to indicate the risk of a storm likely to cause slope instability in the period between clear-felling and regrowth. Erosion-susceptible slopes planted 1 year after clearfelling in radiata pine at 1250 stems ha^-1 regain root site-occupancy in 4.7 years, an interval during which there is an 80% chance of experiencing an extreme flood. Similarly for radiata planted at 800 and 400 stems ha^-1, root site-occupancy is regained in 5.6 and 7.5 years, and the probability of occurrence of an extreme event within these periods is 85 and 90%, respectively. For erosion-susceptible slopes on which kanuka has become established, the probability of a significant event within the 2.8 years prior to root site-occupancy is 60%. Slopes felled of radiata pine are potentially more vulnerable to the stresses promoting slope instability, at least in the earlier years. This revised version was published online in June 2006 with corrections to the Cover Date.     

植物生根的分子机理研究进展

随着无性系育种在农林业上的广泛应用,其生根难的问题显得尤为突出。通过查阅相关文献,本文探讨了影响植物根发育的分子机理。从植物激素,主要从生长素调控植物根发育的分子机理;细胞周期相关基因影响植物根发育和与细胞壁形成相关的基因影响植物根发育等方面叙述了目前关于植物根发育的研究进展。并提出了解决研究生根分子机理材料难选择问题的方法。     

Development and validation of a model to describe root length density of maize from root counts on soil profiles

Root length density (RLD) is an important determinant of crop water and nutrient acquisition, but is difficult to measure in the field. On a soil profile, in-situ counts of root impacts per unit surface on soil profiles (NI) can be used to calculate RLD if crop-specific parameters for preferential root orientation (anisotropy) are known. An improved method for field determinations of RLD was developed and validated for maize at sites in Côte d'Ivoire and Burkina Faso. Root anisotropy was measured with cubes of undisturbed soil with 0.1 m sidelength, based on NI observed on three planes oriented perpendicularly to each other. RLD was also measured for the enclosed volume. Repetition of such measurements enabled estimation of the robustness across sites of empirical and geometric models for the relationship between RLD and NI:RLD = NI CO, with CO being the coefficient of root orientation, theoretically equals 2 for an isotropic distribution. Root systems were found to be nearly isotropic, except near the root front (0.3 to 0.5 m), where roots had a preferentially orthotropic orientation. Measured RLD was generally about 50% larger than RLD calculated from observed NI and CO, indicating that at least one of the measurement techniques had a systematic error. The ratio between measured and calculated RLD (CE), which ranged from 0.8 to 2, increased with the age of the plants and decreased with soil depth. CE was therefore introduced as an additional coefficient, resulting in RLD = NI CO CE. The empirical value for CO CE was between 2 and 5. The empirical coefficients CO and CE were the same for the sites in Cote d'Ivoire (oxisol with an iron pan at 0.6 to 0.9 m) and Burkina Faso (alfisol with an iron pan at 0.4 to 0.8 m). The model was validated with independent data sets at both sites, and gave satisfactory predictions of RLD on the basis of NI obtained from single soil planes, which can be easily measured in the field.     

不同分类系统下油松幼苗根系特征的差异与联系

植物根序和径级不仅反映细根的形态结构, 而且能反映根系的一些生理特征, 如细根寿命和周转等。该文以二年生油松(Pinus tabulaeformis)幼苗根系为研究对象, 系统比较了根序分类方法和径级分类方法在描述根系特征上的优缺点, 探索了两者之间的内在联系。结果表明: 二年生油松幼苗最多可包括6级根序, 直径的变化范围为0.169-3.877 mm。按根序划分, I-VI级根序的总根长和总根表面积主要集中在前3级根序, 这3级根序的根占总根长的78.77%和总根表面积的62.72%。前3级根序的比根长是后3级根序比根长的1.3-3.0倍, 比根面积是后3级比根面积的1.0-1.5倍。按常用的径级(以0.5、1.0、1.5和2.0 mm为阈值)划分方法, 油松幼苗大部分根系直径≤1.5 mm, 此区间细根的根长和根表面积占总根长的93.76%和总根表面积的84.35%。直径≤1.5 mm的细根平均比根长是>1.5 mm细根比根长的3-7倍, 比根面积的1.5-3.0倍。由于油松根序和径级之间有显著的指数关系, 依据径级最大程度反映根序的原则, 提出了新的径级划分方法, 即以0.4、0.8、1.3和2.0 mm为阈值对油松幼苗根系径级重新进行划分。此时, 上述区间可分别包括I级、II级、III级、IV级、V级根序中根尖数的93.22%、86.37%、75.96%、70.47%和76.67%。同时也可分别涵盖各径级根长的89.34%-70.83%、根面积的86.01%-76.12%以及体积的87.73%-76.12%。此时, 根系不同径级与根序之间可以建立起良好的对应关系。这些结果表明, 通过合理划分径级区间可以较好地反映根序 特征。     

岷江干旱河谷25种植物一年生植株根系功能性状及相互关系

相同条件下相同生长期的植物根系生长与适应策略及其差异性还不清楚。因此,采集岷江干旱河谷地区25种乡土植物(木本15/草本10种)的种子于2009年3月播种在同一干旱环境中,9月测定了1年生植株的最大根深(RDmax)、根幅(RW)与根生物量(RB),计算了总根长(TRL)、比根长(SRL)及细/粗根生物量比(RBf/c),分析了它们之间的关系,进行了根系功能组划分。结果表明:1)25种植物1年生植株RDmax与RW变异较小,总变异率为14.9%和20.7%;TRL和SRL变异相对较大,分别为28.5%和34.7%,草本植物SRL明显大于木本植物;RB和RBf/c种间变异较大,总变异率分别为50.1%和70.5%;2)25种植物的RDmax、RW、RB和TRL间呈显著正相关关系,表明根系较深的物种RW较大,TRL和RB也较高;SRL与RDmax呈极显著负相关关系,与RBf/c呈极显著正相关关系,表明根系垂直分布较浅的物种细根发达,SRL较大;3)主成分分析显示,25种植物可分为3个功能组:第1组具有较大RDmax、RW和RB,资源利用持续时间较长;第2组具有较大TRL、SRL和RBf/c,资源利用效率较高;第3组根系功能性状没有一致的突出特点,可能通过降低自身生理机能适应生存条件。综合分析表明,岷江干旱河谷区25种植物1年生植株根系的功能性状变异明显,可塑性大,历经长期自然选择压力而形成了不同的环境适应策略,但生长型并不必然表达出1年生植株根系功能性状的差异性。     

Relationship between root elongation rate and diameter and duration of growth of lateral roots of maize

The objective of this work was to describe the relationship between elongation rate and diameter of maize roots and to estimate the length and growth duration of lateral roots of maize. Diameters and elongation rates of roots were measuredin situ on plants grown 5 weeks in small rhizotrons under greenhouse conditions. At the end of the experimental period the roots were harvested and diameters of axile and lateral roots were measured. The frequency distribution of diameters of harvested roots was bimodal with a minimum at 0.6 mm; 97% of axile roots were larger than this value and 98% of the lateral roots were smaller. Root elongation per day increased as diameter increased but the slope of the relationship with lateral roots was about 2.5 times that with axile roots when separate linear regressions were fitted to the two populations. The length of lateral roots found on axillary roots between the base and about 30 cm from the apex was approximately 2.2 cm. All of the data was consistent with the hypothesis that the lateral roots grew for about 2.5 days and then ceased growing. The axillary roots continued to grow throughout the experimental period at a rate of about 3 cm day⁻¹. Contribution from the Department of Agronomy, New York State College of Agriculture and Life Sciences, Cornell University, Ithaca, NY 14853. Agronomy paper No. 1661. This research is part of the program of the Center for Root-Soil Research.