Growth and curvature in seedlings of Pisum sativum and an ageotropic mutant

In an attempt to explain the influence of gravity on the behaviour of ageotropic plant organs, a pea mutant (Pisum sativum ageotropum) and normal pea (Pisum sativum cv. Sabel) were examined. The mutant has a significantly lower germination rate (large seeds: 25%, small seeds: 10%) than normal pea seeds (55%). Removal of testa increased germination dramatically, the values obtained were 63 and 89%, respectively. Immediately after imbibition the mutant from which the testa had been removed, developed more slowly than normal pea seeds; after 28 h the difference in elongation rate between the two types was reversed. When continuously stimulated geotropically in the horizontal position the elongation in the mutant is larger than in the normal pea roots kept in the same position. During a 24 h period starting 48 h after imbibition the mutant root elongated 45.0 mm while the value for the normal pea root was 11.5 mm. The course of the geotropic curvature in roots of the two types has been followed during a period of 24 h. Normal pea roots develop an asymmetry in the extreme root tip region after 30 min of horizontal stimulation. After prolonged stimulation (exceeding 2 h) the asymmetry has disappeared and the curvature distributed over the entire growth region. When roots of normal pea are stimulated continuously at various angles, the optimum angle of geotropic response is 90° with decreasing responses in the order 135° (i.e. the root tip is pointing obliquely upward) and 45°. The presumed ageotropic behaviour of the mutant has only to a certain extent been confirmed in the present study. When stimulated at 135° a slight positive curvature developed; stimulation at 90° and 45° gave a slight negative curvature.     

Movement of Cell Organelles and the Geotropic Curvature in Roots of Norway Spruce (Picea abies)

The geotropic development in roots of Norway spruce [(Picea abies (L.)] H. Karst, has been followed by light and electron microscopy and compared with the movement of cell organelles (statoliths) in the root cap cells. The geotropic curvature develops in two phases: (a) an initial curvature in the root cap region, which results in an asymmetry in the extreme root tip and which appears after about 3 h stimulation in the horizontal position; and (b) the geotropic curvature in the basal parts of the root tip, which after 8 h is distributed over the entire elongation zone. A graphic extrapolation, based on measurements of the root curvatures after various stimulation periods, indicates a presentation time in the range of 8 to 10 min. The root anatomy and ultrastructure have been examined in detail in order to obtain information as to which organelles may act as gravity receptors. The root cap consists of a central core (columella) distinct from the peripheral part. The core contains three to four rows of parenchymatic cells each consisting of 15 to 18 storeys of statocyte cells with possibly mobile cell organelles. Amyloplasts and nuclei have been found to be mobile in the root cap cells, and the movement of both types of organelles has been followed after inversion of the seedlings and stimulation in the horizontal position for various periods of time at 4°C and 21°C. Three-dimensional reconstructions of spruce root cap cells based on serial sectioning and electron microscopy have been performed. These demonstrate that the endoplasmic reticulum (ER)-system and the vacuoles occupy a considerable part of the statocyte cell. For this reason the space available for free movement of single statolith particles is highly restricted.     

The Geotropic Curvature in Roots of Norway Spruce (Picea abies) Containing Anthocyanins

Seedlings of Norway spruce (Picea abies L.) have been found to synthesize anthocyanins in the root tips as well as in the hypocotyls upon irradiation with white light when kept at 4°C for 6–8 days. In addition, it has also been found that the elongation and the geotropic curvature of spruce roots are dependent on the light conditions. The course of the geotropic curvature in spruce roots containing anthocyanins has been followed during a period of 5 h, in which the seedlings were geotropically stimulated continuously in the horizontal position. When the stimulation was performed in white light and in darkness at 21°C, significantly larger curvatures were observed in the roots pretreated at 4°C in darkness than in the roots containing anthocyanins. The specific curvature (curvature in degrees per mm elongation), however, was approximately the same in both types of roots stimulated in white light. This was due to a retarded elongation of the roots pretreated with light at 4°C and containing anthocyanins. A smaller difference in elongation rate between roots with and without anthocyanins was observed in the dark than in the light, but even in the dark the anthocyanin-containing roots grew more slowly than roots without anthocyanins. In order to find out if it is the anthocyanin content or the illumination which affects the elongation and geotropic curvature in the roots, a series of similar experiments was performed using cress seedlings grown at 4°C in light or darkness. Roots of cress seedlings cultivated under conditions which would induce anthocyanin formation in spruce roots exhibited the highest geotropic responses both in light and darkness as compared to cress seedlings grown at 4°C in darkness. As in the case of spruce roots an increase in elongation was observed in cress roots illuminated during the geotropic stimulation. These similarities in the behaviour made it relevant to compare the development of the geotropic curvature in cress and spruce roots.     

Transport and Degradation of Auxin in Relation to Geotropism in Roots of Phaseolus vulgaris

The movement of auxin in Phaseolus vulgaris roots has been examined after injection of IAA^?3H into the basal root/hypocotyl region of intact, dark-grown seedlings. Only a portion of the applied IAA^?3H was transported unchanged to the root tip. The major part of the chromatographed, labelled compounds translocated to the roots was indole-3-acetylaspartic acid (IAAsp) and an unidentified compound running near the front in isopropanol, ammonia, water. The velocity of the auxin transport (7.2 mm per hour) was calculated from scintillation countings of methanol extracts from serial sections of the root. An accumulation of radioactive compounds in the extreme root tip, was observed 5 h after the injection of IAA. The influence of exogenous IAA on the geotropical behaviour of the bean seedling roots was examined. Pretreated roots were stimulated for 5 min in the horizontal position and then rotated parallel to the horizontal axis of the klinostat for 60 or 90 min. The resulting geotropic curvature of IAA-injected and control roots showed significantly different patterns of development. When the stimulation was started 5 h after application of the auxin, the geotropic curvature became larger in roots of the injected plants than in the controls. If, however, the translocation period was extended to 20 h the geotropic curvature was significantly smaller in the roots of the injected plants. The auxin injection did not significally affect the rate of root elongation. The change in geotropical behaviour of the roots is interpreted as a result of the influence of the conversion products of the applied IAA on the geotropical responsiveness.     

Transport of [3H]IAA label in gravistimulated primary roots of maize

The curvature of roots in response to gravity is attributed to the development of a differential concentration gradient of IAA in the top and bottom of the elongation region of roots. The development of the IAA gradient has been attributed to the redistribution of IAA from the stele to cortical tissues in the elongation region. The gravistimulated redistribution of IAA was investigated by applying [³H]IAA to the cut surface of 5 mm apical primary root segments. The movement of label from the stele-associated [³H]IAA into the root, tip, root cap, and cortical tissues on the top and bottom of the elongation region was determined in vertically growing roots and gravistimulated roots. Label from the stele moved into the region of cell differentiation (root tip) prior to accumulating in the elongation region. Little label was observed in the root cap. Gravistimulation did not increase the amount of label moving from the stele; but gravistimulation did increase the amount of label accumulating in cortical tissues on the lower side of the elongation region, and decreased the amount of label accumulating in cortical tissues on the upper side of the elongation region. Removal of the cap prior to or immediately following gravity stimulation rendered the roots partially insensitive to gravity and also prevented gravity-induced asymmetric redistribution of label. However, removal of the root cap following 30 min of gravistimulation did not alter root curvature or the establishment of an IAA asymmetry across the region of root elongation. These results suggest that a signal originating in the root cap directs auxin redistribution in tissues behind the root cap, leading to the development of an asymmetry of IAA concentration in the elongation region that in turn causes the differential growth rate in the elongation region of a graviresponding root.     

Transport of [3H]IAA label in gravistimulated primary roots of maize

The curvature of roots in response to gravity is attributed to the development of a differential concentration gradient of IAA in the top and bottom of the elongation region of roots. The development of the IAA gradient has been attributed to the redistribution of IAA from the stele to cortical tissues in the elongation region. The gravistimulated redistribution of IAA was investigated by applying [³H]IAA to the cut surface of 5 mm apical primary root segments. The movement of label from the stele-associated [³H]IAA into the root, tip, root cap, and cortical tissues on the top and bottom of the elongation region was determined in vertically growing roots and gravistimulated roots. Label from the stele moved into the region of cell differentiation (root tip) prior to accumulating in the elongation region. Little label was observed in the root cap. Gravistimulation did not increase the amount of label moving from the stele; but gravistimulation did increase the amount of label accumulating in cortical tissues on the lower side of the elongation region, and decreased the amount of label accumulating in cortical tissues on the upper side of the elongation region. Removal of the cap prior to or immediately following gravity stimulation rendered the roots partially insensitive to gravity and also prevented gravity-induced asymmetric redistribution of label. However, removal of the root cap following 30 min of gravistimulation did not alter root curvature or the establishment of an IAA asymmetry across the region of root elongation. These results suggest that a signal originating in the root cap directs auxin redistribution in tissues behind the root cap, leading to the development of an asymmetry of IAA concentration in the elongation region that in turn causes the differential growth rate in the elongation region of a graviresponding root.     

Gravitropic curvature of maize roots is not preceded by rootcap asymmetry

We tested whether the first response to gravistimulation is an asymmetry in the root tip that results from differential growth of the rootcap itself. The displacement of markers on the rootcap surface of maize (Zea mays L. cv. Merit) roots was quantified from videotaped images using customized software. The method was sensitive enough to detect marker displacements down to 15 microns and root curvature as early as 8 min after gravistimulation. No differential growth of the upper and lower sides of the cap occurred before or during root curvature. Fewer than a third of all gravistimulated roots developed an asymmetrical outline of the root tip after curvature had started, and this asymmetry did not occur in the rootcap itself. Our data support the view that the regions of gravitropic sensing and curvature are spatially separate during all phases of gravitropism in maize roots.     

Computer-based video digitizer analysis of surface extension in maize roots

We used a video digitizer system to measure surface extension and curvature in gravistimulated primary roots of maize (Zea mays L.). Downward curvature began about 25 +/- 7 min after gravistimulation and resulted from a combination of enhanced growth along the upper surface and reduced growth along the lower surface relative to growth in vertically oriented controls. The roots curved at a rate of 1.4 +/- 0.5 degrees min-1 but the pattern of curvature varied somewhat. In about 35% of the samples the roots curved steadily downward and the rate of curvature slowed as the root neared 90 degrees. A final angle of about 90 degrees was reached 110 +/- 35 min after the start of gravistimulation. In about 65% of the samples there was a period of backward curvature (partial reversal of curvature) during the response. In some cases (about 15% of those showing a period of reverse bending) this period of backward curvature occurred before the root reached 90 degrees. Following transient backward curvature, downward curvature resumed and the root approached a final angle of about 90 degrees. In about 65% of the roots showing a period of reverse curvature, the roots curved steadily past the vertical, reaching maximum curvature about 205 +/- 65 min after gravistimulation. The direction of curvature then reversed back toward the vertical. After one or two oscillations about the vertical the roots obtained a vertical orientation and the distribution of growth within the root tip became the same as that prior to gravistimulation. The period of transient backward curvature coincided with and was evidently caused by enhancement of growth along the concave and inhibition of growth along the convex side of the curve, a pattern opposite to that prevailing in the earlier stages of downward curvature. There were periods during the gravitropic response when the normally unimodal growth-rate distribution within the elongation zone became bimodal with two peaks of rapid elongation separated by a region of reduced elongation rate. This occurred at different times on the convex and concave sides of the graviresponding root. During the period of steady downward curvature the elongation zone along the convex side extended farther toward the tip than in the vertical control. During the period of reduced rate of curvature, the zone of elongation extended farther toward the tip along the concave side of the root. The data show that the gravitropic response pattern varies with time and involves changes in localized elongation rates as well as changes in the length and position of the elongation zone. Models of root gravitropic curvature based on simple unimodal inhibition of growth along the lower side cannot account for these complex growth patterns.     

The Starch Statolith Hypothesis and the Interaction of Amyloplasts and Endoplasmic Reticulum in Root Geotropism

Roots of cress (Lepidium sativum L, ) seedlings continuouslystimulated at an angle of 135°—root tips pointingobliquely upwards—develop a larger final geotropic curvaturethan roots stimulated at 45° or 90°. This well-knownbehaviour has previously been interpreted as support for thestarch statolith hypothesis. In the present experiments two groups of cress and lettuce (Lactucasativa L.) seedlings were used: (a) the control group in whichthe roots were allowed to curve without adjustment of the stimulationangle, and (b) the test group in which the roots were readjustedat different time intervals to the original stimulation angle.They were stimulated continuously at 45°, 90°, or 135°and the development of root curvatures was followed over a periodof 5–8 h. Initially (1–2 h) the rate of curvature was approximatelythe same for 135° and 90° control and tested cress andlettuce roots. Thereafter the test roots stimulated at 135°followed a linear curvature pattern. Seedlings stimulated at45° and 90° did not show the same linearity in curvaturedevelopment in the test group. The rates of curvature in thetest group were generally higher than in the control group atangles less than 135°. Cress seedlings were examined by light and electron microscopyin order to follow the movement of the cell organelles in thestatocytes. In the statocytes of roots of test seedlings thestarch statoliths were located in the position attained beforethe first readjustment of the stimulation angle. In the statocytesof control roots the starch statoliths followed the curvatureof the root tip sliding along the cell walls and attaining therest position as in normally orientated roots. The behaviour of control and readjusted roots is interpretedas a result of interaction between starch statoliths and endoplasmicreticulum membranes.     

Autonomic Straightening after Gravitropic Curvature of Cress Roots

Few studies have documented the response of gravitropically curved organs to a withdrawal of a constant gravitational stimulus. The effects of stimulus withdrawal on gravitropic curvature were studied by following individual roots of cress (Lepidium sativum L.) through reorientation and clinostat rotation. Roots turned to the horizontal curved down 62° and 88° after 1 and 5 h, respectively. Subsequent rotation on a clinostat for 6 h resulted in root straightening through a loss of gravitropic curvature in older regions and through new growth becoming aligned closer to the prestimulus vertical. However, these roots did not return completely to the prestimulus vertical, indicating the retention of some gravitropic response. Clinostat rotation shifted the mean root angle −36° closer to the prestimulus vertical, regardless of the duration of prior horizontal stimulation. Control roots (no horizontal stimulation) were slanted at various angles after clinostat rotation. These findings indicate that gravitropic curvature is not necessarily permanent, and that the root retains some commitment to its equilibrium orientation prior to gravitropic stimulation.     

The Starch Statolith Hypothesis and the Optimum Angle of Geotropic Stimulation

When roots of cress seedlings (Lepidium sativum L.) are stimulated for 10 min at an angle of 135° (i.e. the root tips are pointing obliquely upward), the resulting geotropic curvatures become larger than after 10 min stimulation at 45°. This well-known behavior has been explained by the better conditions for statoliths, initially located in the floor end of the statocytes, to slide along the cell walls when root tips are pointing upward at 135° than when pointing downward at 45°. Accepting this explanation, one would predict the optimum angle of stimulation to be near 45° when roots had first been kept inverted long enough for their statoliths to accumulate in the opposite end of each functional statocyte. This prediction has been verified in experiments with cress seedlings which were first kept inverted for 16 min, then stimulated for 10 min at given angles, and subsequently rotated parallel to the horizontal axis of the klinostat at 2 rpm. Under these conditions, roots stimulated at 45° curve faster during a 20 to 30 min period on the klinostat than roots stimulated at 135°, but thereafter they stop curving. Roots stimulated at 135°, on the other hand, although initially curving slower than those at 45°, continue curving for at least a whole hour, and attain larger curvatures than the others after 40 min. The optimum shifts from near 45° to near 135° during the course of the klinostat rotation. The behavior of normal and pre-invertcd roots is interpreted as the result of at least two effects: (1) a stimulation due to the movement of amyloplasts, which is enhanced if these are allowed to slide along the cell walls, and (2) a modification of the development of the resulting curvatures by tonic effects, which are inhibitory between stimulation angles 0° and 90°, and absent or enhancing between 90° and 180°.     

Sensitivity to gravistimulus of lentil seedling roots grown in space during the IML 1 Mission of Spacelab

The gravitropic curvature of seedlings of lentil ( Lens culinaris L. cv. Verte du Puy) grown in microgravity and stimulated on the 1 g centrifuge for 5 to 60 min was followed by time lapse photography in near weightlessness in the frame of the IML 1 Mission of Spacelab. In microgravity, the root tip could overshoot the direction of the 1 g acceleration after bending, whereas roots stimulated on the ground did not reach the direction of the gravity vector. On earth, there is, therefore, a regulation (inhibition of root curvature), which is gravity dependent. In space, the initial rate of curvature as well as the amplitude of curvature varied as a function of the quantity of stimulation (Q, in g min). For a given quantity of stimulation, the rate of curvature remained constant for 80 min. The bending has thus a certain inertia, which is linked to the mechanism of differential growth. The presentation time (Tp) of the lentil root was calculated by extrapolation to zero curvature of the regression line representing either the initial rate of curvature or the amplitude of curvature at 2 h after the end of the stimulation. Tp was estimated to 27 and 26 s. respectively. These results confirm the values of Tp obtained by clinostats, and they also lead to a reconsideration of the causes of the kinetics of root curvature.     

Graviresponse and the localization of its initiating cells in roots of Phleum pratense L.

Roots of Phleum pratense L. were photographed during both vertical growth and gravitropic bending, and positions of anticlinal rhizodermal cell walls were digitized on the physically upper and lower flanks of the root in the curvature plane. By using B-splines, arc lengths of these positions, i.e. distances along the root surface, values of curvature, and relative elemental rates of elongation were estimated. The whole graviresponse can be divided into phases according to growth-rate values: (i) an increase of rates on the upper side of the root and a decrease on the lower side during the first 1–11/2h after the root has been moved from the vertical to a horizontal position, (ii) a transient equality of the rates on both sides, (iii) 2–3 h after the beginning of graviresponse, the growth gradient is inverted, and (iv) finally, after about 4 h, the growth rates of both flanks are approximately equal again. Curvature begins 15–20 min after horizontal placement of the root. During the first 2 h of graviresponse, plots of curvature versus arc length show one maximum value. After 2–21/2 h, two maximum values can be observed, the apical one near the root tip always keeping the same distance from the tip, the other one drifting basipetally relative to the growing tip. By evaluating photographs of high magnification, a group of six rhizodermal cells on each side of the root was identified which are the first cells showing gravitropic bending. These cells are located at the beginning of the elongation zone, enclosing the region 480–680 m from the root tip. These cells might be target cells for a signal which the statenchyma, the site of graviperception, sends to the reacting zone of gravicurvature.Abbreviations curvature - RELEL relative elemental rate of elongation A preliminary report was presented at the Meeting of the Deutsche Botanische Gesellschaft, Regensburg, 30 Sept–5 Oct 1990This work was supported by Deutsche Forschungsgemeinschaft. We thank Dr. Brigitte Buchen and Professor Zygmunt Hejnowicz (Botanisches Institut, Universität Bonn, Bonn, FRG) for critical reading of the mansucript.     

Analysis of growth patterns during gravitropic curvature in roots ofZea mays by use of a computer-based video digitizer

A computer-based video digitizer system is described which allows automated tracking of markers placed on a plant surface. The system uses customized software to calculate relative growth rates at selected positions along the plant surface and to determine rates of gravitropic curvature based on the changing pattern of distribution of the surface markers. The system was used to study the time course of gravitropic curvature and changes in relative growth rate along the upper and lower surface of horizontally-oriented roots of maize (Zea mays L.). The growing region of the root was found to extend from about 1 mm behind the tip to approximately 6 mm behind the tip. In vertically-oriented roots the relative growth rate was maximal at about 2.5 mm behind the tip and declined smoothly on either side of the maximum. Curvature was initiated approximately 30 min after horizontal orientation with maximal (50°) curvature being attained in 3 h. Analysis of surface extension patterns during the response indicated that curvature results from a reduction in growth rate along both the upper and lower surfaces with stronger reduction along the lower surface.     

Ethylene Interacts with Auxin in Regulating Developmental Attenuation of Gravitropism in Flax Root

Previous research shows that gravity-sensing in flax (Linum usitatissimum) root is initiated during seed imbibition and precedes root emergence. In this study we investigated the developmental attenuation of flax root gravitropism post-germination and the involvement of ethylene. Gravity response deteriorated significantly from 3 to 11?h after root emergence, which occurred at around 19?h after imbibition (that is, from “age” 22 to 30?h). Although the root elongation rate increased from 22 to 30?h, the gravitropic curving rate declined steadily. Older roots were able to tolerate higher levels of exogenous IAA before inhibition of elongation and gravitropism occurred. The age-dependent effect of IAA on root growth and gravitropism suggests that young roots are more sensitive to auxin and respond to a smaller vertical auxin gradient than older roots upon horizontal gravistimulation. The ethylene synthesis inhibitor AVG (2-aminoethoxyvinyl glycine, 10?μM) or ethylene action inhibitor Ag⁺ (10?μM) stimulated gravitropic curvature of 30?h roots by 24 and 32%, respectively, but had no effect on 22?h roots, suggesting that as roots age, ethylene begins to play a role in root gravitropism. The auxin transport inhibitor NPA (N-naphthylphthalamic acid, 50?μM) reduced gravitropic curvature of 30?h roots by 24% but had no effect on 22?h roots. On the other hand, treating roots simultaneously with the auxin transport inhibitor and ethylene synthesis or action inhibitor stimulated gravitropic curvature of 30?h roots but not 22?h roots. Taken together, these data indicate that as roots develop, their weakened gravity response is due to decreased auxin sensitivity and possibly auxin transport regulated by ethylene.     

Induction of Curvature in Maize Roots by Calcium or by Thigmostimulation : Role of the Postmitotic Isodiametric Growth Zone

We examined the response of primary roots of maize (Zea mays L. cv Merit) to unilateral application of calcium with particular attention to the site of application, the dependence on growth rate, and possible contributions of thigmotropic stimulation during application. Unilateral application of agar to the root cap induced negative curvature whether or not the agar contained calcium. This apparent thigmotropic response was enhanced by including calcium in the agar. Curvature away from objects applied unilaterally to the extreme root tip occurred both in intact and detipped roots. When agar containing calcium chloride was applied to one side of the postmitotic isodiametric growth zone (a region between the apical meristem and the elongation zone), the root curved toward the side of application. This response could not be induced by plain agar. We conclude that curvature away from calcium applied to the root tip results from a thigmotropic response to stimulation during application. In contrast, curvature toward calcium applied to the postmitotic isodiametric growth zone results from direct calcium-induced inhibition of growth.     

ANALYSIS OF EXTENSION AND CURVATURE DURING THE GRAVIRESPONSE IN LEPIDIUM ROOTS

Intact roots of Lepidium sativum were photographed at 1.5- to 6-min intervals before and after being tilted from a vertical to a horizontal position. Relative extension rates were calculated for segments of the root by measuring the rate of movement of charcoal particles on the surface of the root. Curvature was measured at points separated by 0.8 mm, from the tip to the root hair zone. For the first hour after the root was tilted to a horizontal position, the relative extension rate for the side of the root producing the outer side of the curve is higher than that during straight growth. The relative rate for the inner side falls to near zero during the first hour in the horizontal position. There is thus a positive gradient in extension rate from the inner to the outer side of the curve. For most of the second hour, the two sides have approximately equal relative rates of extension. In the last 12 min of the second hour, as the root tip is moving the final distance to a vertical position, the rate on the inner side shows a high extension rate. The gradient in extension rate from the inner to the outer side of the curve has thus become negative during the second hour. This reversal in the extension gradient is necessary to prevent the root from continuing to grow in a circle past the point where the root tip is vertical. Curvature reaches maximal values of 0.7 mm^-1 to 1.2 mm^-1 for the inner side and 0.5 mm^-1 to 0.7 mm^-1 for the outer side. (Units express reciprocals of the radius of curvature in mm.) Two locations where curvature is highest are separated by a flatter region. The charcoal particles marking specific locations on the root surface move away from the tip at the same rate as the features of the curvature pattern.     

Movement of Amyloplasts in the Statocytes of Geotropically Stimulated Roots. The Pre-Inversion Effect

Previously inverted Lepidium roots were placed in a horizontal position and the amyloplasts in the statocytes of the root cap allowed to fall through their entire range of movement across the cell. Under these conditions the amyloplasts first follow a mainly downward course for 6 to 8 min at a speed between 0.5 and 0.8 μm per min. For the next 10 min they move slightly more slowly in a direction away from the apical end of the cell, still sinking somewhat, but without reaching the plasmalemma along the lower wall. Previous experiments have shown that conditions assumed to allow the amyloplasts to slide parallel to the longitudinal cell walls are those that give rise to the largest geotropic curvatures. Such conditions are for instance (1) stimulation at 135° (root tips pointing obliquely upward) and (2) inversion of roots for 16 min followed by stimulation at 45°. Treatments assumed not to permit extensive sliding of the amyloplasts produce smaller geotropic curvatures, namely (3) stimulation at 45° without pre-inversion and (4) inversion followed by stimulation at 135°. The location of the amyloplasts after these four kinds of treatment has now been determined on photomicrographs and the assumptions concerning the paths and extent of sliding of the amyloplasts confirmed. Observations on electron micrographs showed that under all conditions the amyloplasts are separated from the plasmalemma by other organelles, such as ER, nucleus or vacuoles. In roots rotated for 15 min parallel to the horizontal axis of the klinostat at 2 rpm, the amyloplasts are not clumped together as densely as in normal, inverted or stimulated roots, but they are not scattered over the entire cell volume. The statolith function of the amyloplasts is discussed in view of these and other observations.     

An empirical function for the description of root growth

Vertical growth of young cress roots (Lepidium sativum L.) which have not yet attained their steady state is analyzed in terms of the relative elemental rate of elongation (RELEL). To this end, an empirical model function is suggested which describes, by means of six parameters, the movement of marker particles on the root surface. These parameters are interpreted as morphological and physiological quantities. Their determination by independent measurements is in good agreement with that obtained by fitting the model function to the experimental data. The RELEL is then evaluated analytically, so that numerical smoothing and differentiating algorithms with all their problems are avoided. The result demonstrates that—apart from the root cap—nearly the whole root is elongating, including the root hair region and part of the basal portion beyond it. This explains the basal curvature component that is observed during the graviresponse of cress roots.