An investigation of microtubule organization and functions in living Drosophila embryos by injection of a fluorescently labeled antibody against tyrosinated alpha-tubulin

Rhodamine-labeled monoclonal antibodies, which react with tyrosinated alpha-tubulin (clone YL 1/2; Kilmartin, J. V., B. Wright, and C. Milstein, 1982, J. Cell Biol., 93:576-582) and label microtubules in vivo (Wehland, J., M. C. Willingham, and I. Sandoval, 1983, J. Cell Biol., 97:1467-1475) were microinjected into syncytial stage Drosophila embryos. At 1 mg/ml antibody concentration, the microtubule arrays of the surface caps became labeled by YL 1/2 but normal development was found to continue. The results are compared with the data from fixed material particularly with regard to interphase microtubules, centrosome separation, and spindle and midbody formation. At 5 mg/ml antibody concentration the microtubules took up larger quantities of antibodies and clumped around the nuclei. Nuclei with clumped microtubules lost their position in the surface layer and moved into the interior. As a result, the F-actin cap meshwork associated with such nuclei either failed to form or subsided. It is concluded that microtubule activity is required to maintain the nuclei in the surface layer and organize the F-actin meshwork of the caps.     

Electron microscopy of vesicular-arbuscular mycorrhizae of yellow poplar. IV. Host-endophyte interactions during arbuscular deterioration 1,2.

Scanning electron stereoscopy and transmission electron microscopy were used to correlate morphological alterations and cytological phenomena associated with deterioration of arbuscules in yellow poplar mycorrhizae. Arbuscular degradation was initiated at the tips of the finest branches and progressed basipetally. Cytoplasm in arbuscular hyphae progressively deteriorated and was followed by collapse of the fungal walls. Degraded portions of the arbuscules aggregated into clumps comprised of host wall material and the distorted fungal walls. Host nuclei, abundant mitochondria, and proplastids were closely associated with arbuscular branches undergoing cytoplasmic deterioration and with clumped portions of the arbuscule which contained degraded hyphal branches. Most of the arbuscules observed had deteriorated to the clumped stage. Some cortical cells contained several clumped arbuscules and nearly mature, intact arbuscules which indicated that reinfection occurs even as degradative phenomena are in progress. It is suggested that substantial quantities of mineral nutrients may be made available to the host via degradation of fungal cytoplasm in the arbuscular hyphae preceding aggregation of degraded hyphae into discrete clumps.     

Extended chromatin fibres: crystallinity, molecular order and reactivity to concanavalin-A

The present study was undertaken to test the reproducible formation of the extended chromatin fibres (ECF), beads and superbeads, and detect molecular order and crystallinity by optical anisotropies in those structures. Chicken erythrocyte smears and mouse liver cell imprints were treated with 2.0-3.0 m NaCl solution in 1% Triton-X100 vertically prior to staining with 0.025% toluidine blue at pH 4. Detection of birefringence and colours of abnormal dispersion of birefringence (ADB) following toluidine blue binding to DNA revealed that the DNA molecular order and crystallinity in decondensed and condensed chromatin are preserved after ECF formation. The tests for Con-A binding to mannose/glucose residues of glycoproteins was confirmed within nuclei, and in the ECF, beads and superbeads. ECF formation was not regular. Clumped cells did not show ECF, although chromatin mobility was observed within the nuclei. Electron microscopy demonstrated that after treatment of nuclei with 0.77 m NaCl ECF always spread from the nuclei, in clumped nuclei the fibres aggregated instead of spreading.     

不同密度条件下芨芨草空间格局对环境胁迫的响应

基于小尺度上植物间相互作用与空间格局的高度相关性,选用可避免环境异质性影响的K2点格局函数,研究了沙枣-芨芨草群落中63个芨芨草样方(3密度条件×3生境条件×7重复)中芨芨草种群在小尺度下(0—0.5 m)的空间格局,探讨了在3种密度条件(高、中、低)及3种生境条件下(冠盖区、过渡区、空旷区),芨芨草种群空间格局对土壤理化性质胁迫(盐分、有机质及容重)程度的响应。研究结果表明,就土壤盐分,养分含量及土壤容重而言,土壤理化性质胁迫程度沿冠盖区、过渡区和空旷区增加的趋势。相应地,芨芨草种群呈聚集分布的样方比例在沿冠盖区(6/21)较低,而在过渡区(11/21)和空旷区(11/21)较高。然而,在不同密度条件下,芨芨草种群空间格局对土壤性质胁迫的响应不同。在低密度条件下,芨芨草种群在冠盖区多数为聚集分布(4/7),在过渡区和空旷区全部为聚集分布;中密度条件下,芨芨草聚集分布样方比例沿冠盖区-过渡区-空旷区梯度增加(分别为2/7,3/7,4/7),但低于同等胁迫条件下低密度样方中聚集分布数量;高密度条件下,在各胁迫条件下,除了一个过渡区高密度样方,其余芨芨草种群均为随机分布。总体上,随着环境胁迫增强,在中低密度下,芨芨草种群趋向于聚集分布;但在高密度下,芨芨草种群均以随机分布为主。此外,芨芨草种群空间格局随密度变化趋势比随土壤理化性质胁迫梯度变化趋势更加明显,可能表明相对于土壤理化性质胁迫程度,芨芨草种群密度对其空间格局影响更大。因此,在考虑芨芨草种群空间格局对环境胁迫的响应时,应当考虑种群密度因素。     

Influence of food distribution on behavior in captive bongos,Taurotragus euryceros: An experimental investigation

A feeding experiment was conducted to test if Bongo antelopes, being selective feeders searching for clumped quality food, show similar behavior changes as primates under similar conditions. One group of 3 females and 3 calves of Dvur Kralove Zoo was either fed clumped (all food, i.e., leaves and branches of 4 species of tree plus rye grass in one heap), or the same amount distributed over as many heaps as animals present. No increase in agonistic interactions under clumped conditions was found, but non-agonistic and sociopositive patterns increased significantly. Intake of low quality food increased under clumped conditions. Social distances and enclosure use were not significantly different. These findings are in accordance with predictions from models developed for primate feeding strategies, and suggest that selectively feeding antelopes possess behavioral mechanisms for tension-reduction similar to “contest-type” primates. Zoo Biol 16:237–245, 1997. © 1997 Wiley-Liss, Inc.     

Differentiation of interphase domains of the homologous human X-chromosomes

In accordance with molecular biology data reported elsewhere, homologous interphase X-chromosome territories differ greatly in the abundance of inactive condensed chromatin. On the other hand, a three dimensional FISH (3D FISH) method has revealed that domains of both inactive and active X-chromosome have similar volumes and similar maximum section areas (Smax). To solve this contradiction, we examined differences between homologous human interphase X-chromosome territories using two dimensional FISH (2D FISH) preparations of clustered PHA-stimulated lymphocytes. For obtaining such preparations, we developed a new technique to avoid a stage of hypotonic treatment of living cells, since this treatment levels the chromatin compactness degree. According to our 2D FISH data, the mean ratios of Smax for larger and smaller homologous X-chromosomes, calculated for individual flattened nuclei, were 1.83 +/- 0.08 and 2.02 +/- 0.09, respectively, for clumped cells and groups of loosely associated and separated lymphocytes. In comparison, the same ratio calculated for individual 3D nuclei of PHA-stimulated lymphocytes was 1.38 +/- 0.05 (Falk et al., 2002). Our findings give evidence for enrichment of inactive X-chromosomes by low stretchable condensed chromatin. In addition, these findings show that an active X is enriched by a high stretchable form of chromatin, whose content may distinctly vary from cell to cell.     

Ultrastructural effects of lethal freezing on brain,muscle and Malpighian tubules from freeze-tolerant larvae of the gall fly,Eurosta solidaginis

In preparation for winter low temperatures, larvae of the gall fly, Eurosta solidaginis, accumulate the cryoprotectants glycerol, sorbitol, and trehalose. The fat body cells of these freeze-tolerant larvae can survive intracellular freezing to -80 degrees C for 48 h even though no whole larvae survive this treatment. We hypothesized that some other tissue was more susceptible to freezing and therefore may be responsible for larval death. This paper compares the ultrastructure of brain, muscle, and Malpighian tubules between non-lethally frozen and lethally frozen freeze-tolerant larvae. The nuclei of cortical brain cells from lethally frozen larvae exhibited clumped chromatin and nuclear membranes with occasional expansions or 'blebs' of the intermembranous space, while the cytoplasm contained swollen spheres of endoplasmic reticulum. In contrast, non-lethally frozen brain contained nuclei with evenly dispersed chromatin, smooth nuclear membranes and a cytoplasm free of swollen endoplasmic reticulum. Muscle tissue of lethally frozen larvae contained disrupted myofilaments surrounding the Z-line in comparison to non-lethally frozen muscle which had myofilaments extending all the way to the Z-line. Alterations of Malpighian tubule cells from lethally frozen larvae included an extracted cytoplasm with swollen and rounded mitochondria. In contrast, Malpighian tubule cells from non-lethally frozen larvae had a more concentrated cytoplasm with many rod-shaped mitochondria. Results show alterations to all three tissue types due to lethal freezing. The brain tissue contained the most observable alterations and therefore may be the most susceptible to lethal freeze damage.     

A rapid procedure for isolating hemopoietic cell nuclei

A new method for isolating cell nuclei is described which involves freezing and thawing cells in 2% Tween 40, then gentle homogenization to release nuclei, followed by immediate microcentrifugation through 50% sucrose. Purified nuclei were obtained in 3 min and yields of 78-95% were obtained from a variety of human hemopoietic cells. Electron microscope analysis of nuclei obtained from HL60 cells showed that 89% of the nuclei were intact and have an appropriate morphology. A low level of contamination with other organelles was revealed by electron microscopy and by using specific assays for plasma membrane, mitochondria, lysosomes, Golgi membrane, and endoplasmic reticulum (0.5-5.5%). The value of the technique is that nuclear proteins and small metabolites which might be lost by rapid leakage from isolated nuclei and the possibility of biochemical modification of cellular constituents are minimized by using a rapid isolation procedure.     

Initiation of DNA synthesis in mouse 3T3 fibroblasts in response to a specific factor.

Antibodies specific for both the F 1 and F 2a-1 calf thymus histone fractions were prepared by use of highly purified histone fractions. With these antibodies, immunofluorescent studies were performed in cultured cells from a Syrian hamster, from human cancer and from rat embryonal cells. Specific staining of nuclei by both of the antibodies was seen in all the cell lines used. In the staining pattern of the cell nucleus, there was a distinct difference between the results obtained from anti-F 1 antibody and from anti-F2a-1 antibody. In the case of the anti-F 1, the nuclei were stained to be coarse-grained or clumped in appearance. However, the result from anti-F 2a-1 showed strong fluorescence in the peripheral part of the nucleus and a faint shaggy appearance in the central part of the nucleus. These differences in the nuclear fluorescent pattern between the results obtained from anti-F 1 antibody and from anti-F 2a-1 antibody were seen in all the cell lines used.     

Nuclear division cycle in Neurospora crassa hyphae under different growth conditions.

Treatment with picolinic acid blocked Neurospora crassa nuclei in G1, and recovery from the treatment allowed a synchronous wave of deoxyribonucleic acid synthesis to occur. Nuclei, which appeared as compact globular bodies during the period of blockage, assumed a ring shape during the following S phase, which was also maintained in the G2 phase. The proportion of compact globular nuclei was much higher in hyphae growing at lower rates, whereas that of ring nuclei increased when the hyphae were growing at higher rates. Horseshoe nuclei (probably mitotic nuclei) and double ring nuclei were also observed in growing hyphae, but their frequencies were low and fairly independent of the rate of growth. The length of the S phase of the Neurospora nuclear division cycle was determined to be about 30 min. From the frequencies of the phase-specific nuclear shapes, the durations of the G1 phase and the combined S plus G2 phases were calculated. The results showed that variations in the growth rates of the mycelia were mainly coupled with variations in the G1 phase of the nuclear division cycle. For mycelia growing in minimal sucrose, the lengths of all of the phases of the nuclear division cycle were estimated.     

Meiotic nondisjunction of chromosomes 1, 17, 18, X, and Y in men more than 80 years of Age

In order to evaluate a possible paternal age effect, testicular sperm cells from three men aged 81, 82, and 83 yr were analyzed by two-color- and three-color-fluorescence in situ hybridization for disomy rates of chromosomes 1, 17, 18, X, and Y as well as for diploidy frequencies. A minimum of 1500 sperm cells per donor and probe was evaluated due to the low number of spermatozoa in the preparations. Diploidy and disomy frequencies were in the same range as found in men aged <30 yr, a slight increase only being noticed for XY nuclei.     

Multifrequency electron spin-echo envelope modulation studies of nitrogen ligation to the manganese cluster of photosystem II

The CalEPR Center at UC-Davis (http://brittepr.ucdavis.edu) is equipped with five research grade electron paramagnetic resonance (EPR) instruments operating at various excitation frequencies between 8 and 130GHz. Of particular note for this RSC meeting are two pulsed EPR spectrometers working at the intermediate microwave frequencies of 31 and 35GHz. Previous lower frequency electron spin-echo envelope modulation (ESEEM) studies indicated that histidine nitrogen is electronically coupled to the Mn cluster in the S2 state of photosystem II (PSII). However, the amplitude and resolution of the spectra were relatively poor at these low frequencies, precluding any in-depth analysis of the electronic structure properties of this closely associated nitrogen nucleus. With the intermediate frequency instruments, we are much closer to the 'exact cancellation' limit, which optimizes ESEEM spectra for hyperfine-coupled nuclei such as 14N and 15N. Herein, we report the results from ESEEM studies of both 14N- and 15N-labelled PSII at these two frequencies. Spectral simulations were constrained by both isotope datasets at both frequencies, with a focus on high-resolution spectral examination of the histidine ligation to the Mn cluster in the S2 state.     

Detection of aneuploidy and aneuploidy-inducing agents in human lymphocytes using fluorescence in situ hybridization with chromosome-specific DNA probes

The feasibility of utilizing fluorescence in situ hybridization with chromosome-specific DNA probes as the basis of an assay to detect aneuploidy and aneuploidy-inducing agents in interphase human lymphocytes has been investigated. The assay involves counting the number of hybridization regions in interphase cells to determine the number of copies of a specific chromosome of interest, 22,000 interphase nuclei from untreated 72-h lymphocyte cultures were examined following hybridization with probes for chromosomes 1, 7, 9, 17, X or Y. The combined frequencies of nuclei containing 0, 1, 2, 3 and 4 hybridization regions for the various autosomal chromosomes were 0.004, 0.084, 0.909, 0.003 and 0.001, respectively. Based on these frequencies, scoring 1000-2000 cells should allow detection of aneuploid cells with a 0.012 frequency of hyperdiploidy or a 0.11 frequency of hypodiploidy for a specific chromosome of interest (alpha = 0.05, beta = 0.80). This difference in test sensitivity is related to the higher frequency of cells with one apparent spot. A comparison of the ratio of hybridization region to nuclear area in the two-dimensional images used for this analysis indicates that an overlap of the two regions probably accounts for the high frequency of apparent monosomy observed in normal cells. Treatment with the aneuploidy-inducing chemicals, colchicine, vincristine sulfate and diethylstilbestrol resulted in significant dose-related increases in the number of nuclei containing 3 or more hybridization regions. Treatment with the clastogen sodium arsenite produced only a minor increase in apparently hyperdiploid cells whereas treatment with ionizing radiation, another potent clastogen, resulted in a significant increase in nuclei containing multiple hybridization regions. These results suggest that ionizing radiation is an aneuploidy-inducing agent under these conditions although chromosomal breakage within the hybridization region may account for a portion of the increased frequency of nuclei with multiple hybridization regions. These results indicate that the use of fluorescence in situ hybridization with DNA probes is capable of detecting aneuploid cells occurring at relatively low frequencies within a population of cells. Assays based on these techniques should facilitate a more rapid identification of aneuploidy-inducing environmental and therapeutic agents.     

Vegetation structure and pollination in the Venezuelan Central Plain

Classes of pollination agents and pollination system specificities were evaluated according to the vegetation structure in the Venezuelan Central Plain. Two main questions were examined: (i) Do the frequencies of pollination categories estimated at plant species level differ from those estimated considering plant abundance? and (ii) Does plant abundance differ according to categories of pollination in the context of each vegetation attribute? The frequencies of classes of pollination agents and systems of pollination specificity were estimated at plant species and plant abundance level for the community, habitats, life-forms, strata, horizontal disposition, and season. Besides, plant abundance of each category of pollination agent classes and systems of pollination specificity were statistically compared. Most of the comparisons between the frequency of plant species and plant abundance were non-significant (58.3%). The multivariate classifications of life-forms and habitats according to pollination agent classes using plant species and plant abundance separately are very similar. In both analyses, perennial herbs come out as a separate group, which is associated with a comparable frequency distribution of pollination agent classes. The frequency distributions of polyphilous, oligophilous, and monophilous species were statistically higher for plant abundance than for plant species in the disturbed area and in the lowest stratum (D), which is primarily associated with the relation between life-forms and vegetation strata. The average values of abundance for wind-pollinated plants were found to be larger than those for other categories of pollination systems specificity and pollination agent classes, respectively, in the forest–savanna transition and overall community. All anemephilous species have clumped distribution and plant abundance was higher in this case than what was found with the other categories of pollination specificity with clumped distribution. Estimates of pollination agent classes at plant species level proved to be a practical method to estimate pollination frequency, mainly for those studies related to the floristic composition of the community. The large values of abundance for wind-pollinated species are in concordance with the savanna–forest mosaic vegetation growing in the Venezuelan Central Plain, but are in contrast with the comparatively low percentage of wind-pollinated species recorded.     

Characteristics of the Ramet Population of Epimedium brevicornum and Analysis of Spatial Point Pattern for E. brevicornum and E. sagittatum

Epimedium brevicornum was sampled from Jinchengshan , a mountain of Nanchong , Sichuan province at altitudes of 560m, 653 m and 774m respectively for investigation of its ramet population variation. The results showed that the ramet population density decreased remarkably and its ratio of root to shoot increased with increasing elevation e. g, at 774 m the ratio of root to shoot was significantly higher than that at 560 m . The spatial distribution pattern of E . brevicornum at three different altitudes and E. sagittatum at the altitude 771 m were also investigated respectively with Congregation index (m*􊄯m) 、Clump index (I)、Cassie index (CA)、Dispersal index (C) and spatial point pattern analysis . The results revealed that the populations of E. brevicornum was clumped at all of the spatial scales , whereas the population of E. sagittatum was randomly distributed at the small spatial scales, and was clumped at the most spatial scales which ranged from 0. 1 m to 5 m . Finally , we discussed the protection of E . brevicornum and E. sagittatum and advantage to use point pattern analysis in the investigation of the spatial pattern of herb plant .     

Karyometric image analysis for intraepithelial and invasive cervical lesions

OBJECTIVE: To derive an objective, numeric measure for the progression of intraepithelial and invasive squamous cell cervical lesions. STUDY DESIGN: Thin-layer cervical cytology preparations from colposcopically confirmed normal cervix, low grade squamous intraepithelial lesions, high grade squamous intraepithelial lesions and carcinoma were identified from a cross-sectional study. Fifty-nine cases representing 4 diagnostic categories were selected, and 2,375 nuclei from epithelial cells representative of the diagnostic category were randomly selected for imaging and measurement from these cases. Additionally, 1,378 visually normal appearing intermediate cells from low and high grade squamous intraepithelial lesions, as well as from carcinoma cases, were identified for analysis. The nuclei were quantitatively characterized, and discriminant analyses were performed to derive a progression curve from normal cytology to carcinoma. RESULTS: The lesion signatures show a clear increase in nuclear abnormality with increasing progression. A progression curve was derived based on mean discriminant function scores for each diagnostic category and on the mean nuclear abnormality values for the nuclei in each category, as expressed by their deviation in feature values from normal reference nuclei. CONCLUSION: A numeric assessment of lesion progression for cervical precancerous and cancerous lesions based on karyometric measurements is possible and may provide an objective, precise characterization of each lesion as well as a basis for improved performance in automated cytology-based cervical cancer screening.     

The cytoarchitecture of the torus semicircularis in the golden skink Mabuya multifasciata

The skink, Mabuya multifasciata, torus semicircularis was subdivided into the central (CN), the laminar (LN), and the superficial (SN) nuclei using Golgi and Nissl methods. The central nucleus consisted of small ovoid neurons surrounding a core of fewer large ovoid-triangular and fusiform neurons. The ovoid cells had scant cytoplasm and two to five dendritic trunks. Most of these processes were directed around the periphery of the central nucleus. The large neurons had clumped, darkly staining Nissl substance and a central nucleus. The sparse dendritic spine population on these cells increased distally on the three to five radiate dendrites. The laminar nucleus was present caudal and ventral to the central nucleus. At more rostral levels it was medial and dorsomedial to the central nucleus. The NL had three to five layers of ovoid and fusiform neurons. Scattered within these layers were a few ovoid-triangular neurons. Ovoid neurons had eccentric or central nuclei. The arborization of their dendrites was generally medial and lateral but was frequently oriented caudomedial and rostrolateral. Fusiform neurons had pale Nissl substance, central nuclei, and one to two dendritic processes. The ovoid-triangular neurons had dense, clumped Nissl substance and at least two dendritic trunks with few spines. The superficial nucleus was dorsal, lateral, and caudal to the central nucleus. Extending ventrolaterally around the central nucleus, the superficial nucleus became confluent with the laminar nucleus, ensheathing the central nucleus ventrally, laterally, and dorsally. Rostrally the central nucleus was covered by the layers of the laminar nucleus. Within the superficial nucleus were ovoid, fusiform and sparse ovoid-triangular neurons. The study indicated that the morphology of the torus semicircularis in the golden skink was similar to that in other lizards. This similarity correlates with the degree of development as it relates to the auditory function, but was independent of the type of inner ear restraint mechanism.     

Spatial and genetic structure within populations of wild American ginseng (Panax quinquefolius L., Araliaceae)

Spatial structure and fine-scale genetic structure were analyzed for the medicinal plant American ginseng (Panax quinquefolius L.) to more fully understand biological processes within wild populations. P. quinquefolius has been harvested for more than 250 years and is now considered threatened or rare throughout its range. Plants within four protected and four unprotected populations were significantly clumped based on Ripley's univariate analysis. Analysis with Ripley's bivariate test determined that juvenile plants were significantly clumped with adult plants at the shortest distance classes in all populations. Although plants were highly clumped, we found that significant fine-scale genetic structure was restricted to the shortest distance classes based on estimates of coancestry (f(ij)). In most cases, estimates of f(ij) were more significant among juveniles than among adults, especially at the shortest distance classes. The spatial structure of ginseng seems to result from the establishment and persistence of plants in favorable microhabitats coupled with limited seed dispersal around maternal individuals. There were no differences in patterns of fine-scale genetic structure between protected and unprotected populations.     

Dynamics of Spatial Pattern for the Gastrotrich Tetranchyroderma bunti in the Surface Sand of High Energy Beaches

This paper is a quantitative study of the spatial pattern of the intertidal gastrotrich. Tetranchyroderma bunti, and its migration pattern during a tidal cycle on three high energy beaches of Bogue Banks, North Carolina, USA. Included in the study were the macrodistribution and micro-distribution. T. bunti is gregarious and forms a clumped spatial pattern in the sand. The four clump sizes found were 2.2, 3.3, 4.4, and 5.5 cm with a ratio of 9: 6: 4: 1, respectively. The spatial pattern of these clumps suggests that the general spatial pattern of the population in its most dense area is aggregated clumped. Field experiments show that this is a stable pattern during at least part of the low tide period, and laboratory experiments suggest that the low water content of the sand at low tide contributes to this stability by decreasing the movement of the gastrotrichs. A significant difference (p<0.05) was found in the density of the gastrotrichs parallel to the shore line, but part of this difference was due to local shifts in the spatial pattern of the population instead of a change in the longshore abundance. T. bunti showed a tidal migration pattern opposite to that of the ground water movement.     

Root clumping may affect the root water potential and the resistance to soil-root water transport

We have appraised for clumped root systems the widely-accepted view that the resistance to water flux from soil to roots (‘soil resistance’) is low under most field conditions, so that root water potential would closely follow the mean soil water potential. Three root spatial arrangements were studied, simulating either the regular pattern generally assumed in models, or two degrees of root clumping frequently observed in the field. We used a numerical 2-dimensional model of water transfer which assumes a control of evapotranspiration by root signalling. Calculations were carried out at two evaporative demands and for two contrasting soil hydraulic properties. The rate of soil depletion, the timing of the reduction in evapotranspiration and the difference between root water potential and mean soil water potential were all affected by the root spatial arrangement, with a greater effect at high evaporative demand and low soil hydraulic conductivity. Almost all the soil water reserve was available to plants without reduction in evapotranspiration in the regular case, while only a part of it was available in clumped cases. In the regular case, calculated ‘soil resistances’ were similar to those calculated using Newman's (1969) method. Conversely they were higher by up to two orders of magnitude in clumped root spatial arrangements. These results place doubt on the generality of the view that ‘soil resistance’ is low under common field conditions. They are consistent with the results of field experiments, especially with recent data dealing with root-to-shoot communication.