Accumulation of amyloplasts on the bottom of normal and inverted rhizome tips of Caulerpa prolifera (Forsskål) Lamouroux

Caulerpa prolifera (Chlorophyta) exhibits a gravimorphogenetic response to inversion by switching the site of new rhizoid initiations to correspond with the new direction of the gravity stimulus. When plants were fixed at 6 and 24 h after being held in either a normal or an inverted position the switch in the site of organ differentiation, upon inversion, was found to be preceded by the accumulation of starch-containing amyloplasts on the bottom of the rhizome. Approximately 1.5–2.0 times as many amyloplasts were found at the bottom of normal or inverted rhizomes as compared with the top in a region extending from 200 to 1,000 μm behind the rhizome tip. All new rhizoid initials were located in the region of amyloplast accumulation and each rhizoid initial contained numerous amyloplasts.     

DEVELOPMENTAL CHANGES IN THE ALGAL COENOCYTE CAULERPA PROLIFERA (SIPHONALES) AFTER INVERSION WITH RESPECT TO GRAVITY

The algal coenocyte Caulerpa prolifera produces three types of organs, each with a different orientation. The effect of gravity in controlling the development of these organs was investigated. We inverted plants by rotating them 180° around the horizontal rhizome axis, then compared development of the inverted plants with plants matched by size and differentiation. Quantitative data were obtained from photographic records. After inversion the site of organ differentiation was changed with no change in the timing: the next rhizoid (and all subsequent ones) differentiated on the “now-under side” of the inverted rhizome, the next leaf formed on the “now-upper side.” Despite the fast change in the site of organ differentiation, other parameters were not changed by inversion (e.g., rate of elongation of rhizomes or leaves, rate of formation of leaves or rhizoids). Because such changes also occurred in plants with balanced lighting from two sides, it is clear that gravity alone can control these changes without reinforcement from top illumination. After leaves were initiated, they did not change their orientation, showing neither positive phototropism (at our light levels) nor negative geotropism, even when elongating substantially. Torsion of the rhizome tip did not precede the change in site of rhizoid initiation.     

QUANTITATIVE STUDY OF DEVELOPMENT OF THE GIANT COENOCYTE,CAULERPA PROLIFERA

The lengths of rhizomes, rhizoid clusters, and leaves of many plants of Caulerpa prolifera cultured under controlled conditions were measured daily. Their rates and patterns of growth were analyzed mathematically with the aid of an electronic computer. All organs elongated at the rate of ca. 4.4 mm per day. Rhizomes elongated linearly for more than 2 weeks, and curvilinear regression usually did not improve the fit significantly. Rhizome elongation is apical. Every 1.18 days, on the average, a rhizoid cluster developed on the lower surface of each rhizome. The length of the younger rhizoid clusters shows a linear relationship to their distance from the rhizome tip. Little or no significant improvement of fit resulted from fitting polynomials of the 2nd through 4th degree. Accordingly, by extrapolating the straight line to zero rhizoid length, we calculated the presumptive site of rhizoid initiation to be 0.84 mm from the rhizome tip. The rate and distribution of growth were compared to those of other coenocytes and of multicellular plants.     

Ediacaran macroalgal holdfasts and their evolution: a case study from China

A macroalgal holdfast (root-like structure) anchored or grown into sediments is a key trait of metaphytes and eukaryotic algae. Various patterns and taphonomic variants of congeneric holdfasts are preserved on the bedding planes of black shales of the Ediacaran Wenghui biota in South China. The macroalgal holdfast, which commonly consists of a rhizome, rhizoid and pith (perhaps mechanical tissue), can be morphologically classified into ten types within four groups of rhizomes: bare rhizome holdfasts (Grypania, Tongrenphyton and Sectoralga-type holdfasts), canopy rhizome holdfasts (Gemmaphyton, Gesinella, Discusphyton and Baculiphyca-type holdfasts), pithy rhizome holdfasts (Zhongbaodaophyton and pithy-cone-type holdfasts) and differentiated rhizome holdfasts (Wenghuiphyton-type holdfasts). Analysis of the Precambrian macroalgal record indicates that rhizomes played a more important role in the evolution of macroalgal holdfast than rhizoids. The following evolutionary stages of development of macroalgal holdfasts are proposed: (1) growth of the basal thallus into sediments (Grypania-type holdfast); (2) development of a primitive (indistinct) rhizome from the base of a stipe or thallus (Tongrenphyton-type and Sectoralga-type holdfasts); (3) growth of a distinct rhizome with rhizoid (Gemmaphyton-type, Gesinella-type and Discusphyton-type holdfasts); (4) development of a pith in the stipe (Baculiphyca-type holdfast); (5) pith extension into the rhizome (Zhongbaodaophyton-type and pithy-cone-type holdfasts); and (6) rhizome differentiation and development of a complex holdfast system (Wenghuiphyton-type holdfast).     

Membrane recycling occurs during asymmetric tip growth and cell plate formation inFucus distichus zygotes

Summary Zygotes of the brown algaFucus distichus undergo a series of intracellular changes resulting in the establishment of a polar growth axis prior to the first embryonic cell division. In order to examine the dynamics of membrane recycling which occur in the zygote during polar growth of the rhizoid, we probed living Fucus zygotes with the vital stain FM4-64, N-(3-triethylammoniumpropyl)-4-(6-(4-(diethylammo)phenyl)hexatrienyl)pyridinium dibromide. In newly fertilized, spherical zygotes, FM4-64 staining is symmetric and predominantly in the perinuclear region which is rich in endoplasmic reticulum, Golgi, and vacuolar membranes. As rhizoid or tip growth is initiated, this population of stained membranes becomes asymmetrically redistributed, concentrating at the rhizoid tip and extending centrally to the perinuclear region. This asymmetric localization is maintained in the zygote throughout polar growth of the rhizoid and during karyokinesis. Subsequently, FM4-64 staining also begins to accumulate in a central location between the daughter nuclei. As cytokinesis proceeds, this region of stain expands laterally from this central location, perpendicular to the plane of polar rhizoid outgrowth. The staining pattern thus delineates the formation of a cell plate, similar spatially to the accumulation of nascent plate membranes of higher plants. Treatment of Fucus zygotes with brefeldin-A inhibits both asymmetric growth of the rhizoid and formation of a new cell plate. These data suggest that inF. distichus FM4-64 is labeling a Golgi-derived membrane fraction that appears to be recycling between the site of tip growth, perinuclear region, and new cell plate.Abbreviations AF after fertilization - ASW artificial seawater - BFA brefeldin A - ER endoplasmic reticulum - FM4-64 N-(3-triethylam-moniumpropyl)-4-(6-(4-(diethylamino)phenyl)hexatrienyl)pyridinium dibromide     

THE INITIATION AND ELONGATION OF RHIZOID CLUSTERS IN CAULERPA PROLIFERA

Under improved environmental conditions the average rate of elongation of the rhizoid cluster was 5.2 mm/day in the first 3 days. The rate decreases thereafter and is close to zero around 5–6 days after initiation. The elongation, like that of the rhizome, is chiefly by means of “tip-growth.” The presumptive site of the initiation is 1.3–1.7 mm from the rhizome tip. Under present experimental environment, the presumptive time of the initiation was mainly at 1 to 3 hr after the beginning of the light period. Initiation in the dark period was less frequent, and little or no initiation in the middle and later parts of the light period was observed. The average plastochron of the cluster was 0.91 day. The rate of elongation of the rhizoid cluster was less variable than that of the rhizome. Hence, the slope of calculated regression line from the plot, the cluster length vs. its distance from the rhizome tip, is steeper when the rhizome grows slower (4.9–5.8 mm/day) and less steep when the rhizome grows faster (7.3–8.9 mm/day). In spite of some variation in the slope of the regression line, the intersection between the line and X-axis remains about the same. The application of information on the presumptive site and presumptive time of the rhizoid cluster initiation to a developmental study of micro-events taking place prior to the morphological differentiation is proposed.     

Effects of continuous illumination on the rhizoid cluster of Caulerpa prolifera

Under a photoperiodic regime of 12-hr light and 12-hr dark (12L-12D)and continuous illumination (24L-0D), at 24.0±1°C,the rate of elongation of the rhizoid cluster was 8.1 mm/day,and the presumptive site of the cluster initiation was 1.1–1.9mm from the rhizome tip. However, the plastochron interval under24L-0D regime was shorter (0.87 day) than under 12L-l2D regime(1.18 days). Under both regimes, the plants tended to adjusttheir presumptive time of cluster initiation to be in the twoperiods of the day, 0400–0900 hr and 1800–2100 hr.None of the following seemed to correlate with the initiationof a new cluster: a certain number of hours after the initiationof the youngest cluster, a certain distance on the rhizome distalto the youngest cluster, the youngest cluster reaching to acertain length, or a certain value of the rhizoid plastochronindex. This adjustment by the plants thus suggested the plantshaving an ability to perceive the two preferred periods forcluster initiation. ¹ Report of work supported by research grant GB-7885 from theNational Science Foundation, and in part by Research Councilof Rutgers University. (Received June 2, 1972; )     

DEVELOPMENTAL STUDIES ON THE COENOCYTIC ALGA, CAULERPA SERTULARIOIDES

Thallus growth and development in the coenocyti alga Caulerpa sertularioides (Gmelin) Howe have been studied quantitatively. In unsupplemented seawater at 26 C and in a 12:12 hr light/dark cycle, new rhizomes formed near the old growing points on thalli transplanted from the ocean. Adjacent to the apices of new rhizomes, rhizoids were produced downward, regularly spaced and at a rate of about 1.5/day. Upright. shoots developed irregularly in acropetal succession behind the tips of either the parent or the new rhizomes, which arose as branches of the old. Rates of rhizome, rhizoid, and upright shoot elongation were 0.4, 0.81, and 0.54 cm/day, respectively. Thalli survived up to 2 months in unsupplemented seawater. Long-term growth was obtained by varying culture conditions. A substratum of sand, apparently rich in microorganisms, produced long-term thallus growth in seawater, and the form of development changed so that upright shoot formation was promoted and rhizome elongation halved. Similar effects were elicited by indole-3-acetic acid, 5 × 10^-5, M in seawater and by sap expressed from C. racemosa or C. sertularioides and added to seawater at 2.5–10% by volume. The regulation of development in an algal coenocyte is discussed and analogies with regulation in multicellular plants are drawn.     

In-vivo observations of a spherical aggregate of endoplasmic reticulum and of Golgi vesicles in the tip of fast-growing Chara rhizoids

In-vivo differential interference contrast microscopy was used to detect individual Golgi vesicles and a new structure in the tip of fast-growing rhizoids of Chara fragilis Desvaux. This structure is a spherical clear zone which is free of Golgi vesicles, has a diameter of 5 m and is positioned in the center of the apical Golgi-vesicle accumulation (Spitzenkörper). After glutaraldehyde fixation and osmium tetroxide-potassium ferricyanide staining of the rhizoid, followed by serial sectioning and three-dimensional reconstruction, the spherical zone shows a tight accumulation of anastomosing endoplasmic reticulum (ER) membranes. The ER membranes radiate from this aggregate towards the apical plasmalemma and to the membranes of the statolith compartments. Upon gravistimulation the ER aggregate changes its position according to the new growth direction, indicating its participation in growth determination. After treatment of the rhizoid with cytochalasin B or phalloidin the ER aggregate disappears and the statoliths sediment. It is concluded that the integrity of the ER aggregate is actin-dependent and that it is related to the polar organisation of the gravitropically growing cell tip.Abbreviations CB cytochalasin B - DIC differential interference contrast microscopy - DMSO dimethyl sulfoxide - ER endoplasmic reticulum     

The Rac1 inhibitor,NSC23766, depolarizes adhesive secretion,endomembrane cycling,and tip growth in the fucoid alga, <Emphasis Type="Italic">Silvetia compressa</Emphasis>

Proper cell morphogenesis is dependent on the establishment and expression of cellular polarity. In the fucoid zygote, cell shape is critical for establishing the developmental pattern of the adult, and is achieved by guiding insertion of new membrane and wall to the rhizoid tip. Selection and growth of the appropriate tip site are accompanied by formation of dynamic actin arrays associated with the actin-nucleating Arp2/3 complex. In eukaryotes, a major pathway for activation of the Arp2/3 complex is via the Rho family GTPase, Rac1, which stimulates the Scar/WAVE complex. To determine whether Rac1 controls actin nucleation in Silvetia compressa (J. Agardh) E. Serrao, T. O. Cho, S. M. Boo et Brawley, we tested the effects of the Rac1-specific inhibitory compound, NSC23766, on actin dependent processes and on actin arrays. We found that NSC23766 disrupted polar secretion of adhesive, polarization of endomembranes, and tip-focused growth in the rhizoid. Similarly, NSC23766 altered actin and Arp2 localization in the growing rhizoid. In contrast, NSC23766 had no effect on selection of the growth site or on cytokinesis. These data suggest that Rac1 participates in nucleation of specific actin arrays in the developing zygote.     

Localization of membrane-associated calcium during development of fucoid algae using chlorotetracycline

During the first day of development, fertilized eggs of fucoid algae generate an embryonic axis and commence rhizoid growth at one pole. Using Fucus distichus (L.) Powell, F. vesiculosus L. and Pelvetia fastigiata (J.Ag.) DeTony we have investigated the role of calcium in axis formation and fixation as well as in tip growth. The intracellular distribution of membrane-associated calcium was visualized with the fluorescent calcium probe chlorotetracycline (CTC). Punctate fluorescence associated with organelle-like structures was found in conjunction with diffuse staining at all developmental stages. This membrane-associated calcium remained uniformly distributed throughout the cortical cytoplasm while the axis was established, but increased in the rhizoid protuberance at germination. In subsequent development, fluorescence was restricted to the cortical cytoplasm at the elongating tip and at sites of crosswall biosynthesis.The requirement for Ca²⁺ uptake during development was investigated through inhibition studies; influx was impaired with transport antagonists or by removal of extracellular calcium. Both treatments curtailed germination and tip elongation but had little effect on axis polarization. Reductions in external calcium that interfered with elongation also markedly reduced the apical CTC fluorescencence, indicating that calcium uptake and localization are prerequisites for tip growth. This apical Ca²⁺ is probably involved in the secretory process that sustains tip elongation. By contrast, calcium was not implicated in the generation of an embryonic axis.Abbreviations ASW artificial seawater - CTC chlorotetracycline - DU developmental unit - EGTA erhylene glycol bis(amino-ethyl ether) N,N,N¹,N¹–tetraacetic acid - NPN N-phenyl-1-napthylamine     

Effects of resource availability on integration and clonal growth inMaianthemum bifolium

The aim of this study was to investigate whether resource availability affects the degree of physiological integration and the growth pattern of interconnected ramets in the clonal plantMaianthemum bifolium (L.)F.W. Schmidt (Liliaceae), a rhizomatous herb of European forests, by studying it at two contrasting South Swedish beech forest sites termed “poor” and “rich”. The degree of physiological integration was studied by tracing the pattern of¹⁴C translocation and in a cutting experiment involving rhizome severing and defoliation treatments. The size of the plants, growth of new rhizomes, branching frequency, distance between shoots and the internode length were compared. The plants were larger, rhizomes had greater specific mass (mg mm^?1), internodes were shorter and branching frequency higher at the rich site. The cutting treatments reduced the growth of new rhizomes at both sites, and new rhizome segments had lower specific mass in treated plants than in controls, showing the importance of physiological integration for new growth. Translocation of¹⁴C in May showed that the young rhizome tip was a strong sink for carbon. Basipetal translocation to older portions of the rhizome system was greater at the rich site than at the poor site. In September, four months after labelling, the rhizome tips were still strongly labelled with¹⁴C and basipetal translocation had increased at both sites. Plants at the rich site appeared to translocate larger amounts of¹⁴C basipetally than plants at the poor site. It is concluded thatM. bifolium shows a plastic response to resource availability by varying rhizome growth and branching frequency, but the degree of physiological integration is probably only indirectly affected through an increased number of sinks (new rhizome branches) along the ramet system at the rich site.     

Apical Structure of Actively Growing Fern Rhizoids Examined by DIC and Confocal Microscopy

The development and cytology of gametophyte primary rhizoidsof the fern Dryopteris affinis was examined using actively growingmaterial. During development an apical cytoplasmic ‘ accumulation’forms and is associated with active tip growth. This accumulationdeteriorates as terminal differentiation and cessation of growthapproaches. During early development the nucleus moves fromthe rhizoid cell base into the newly extending rhizoid. Later,during the active elongation phase, the nucleus takes up a relativelystable location approx. 100 µm behind the extending apex.Towards terminal differentiation the nucleus lags further behindthe tip. In actively growing rhizoids four distinct zones weredistinguished: a richly cytoplasmic ‘cap’; an apicalregion with tubular vacuolar intrusions; a region distinguishedby a peripheral sheath of cytoplasm and fine irregular cytoplasmicstrands connecting to the nucleus; and the main subapical vacuole.Confocal microscopy of gametophytes stained with fluorescentvital dyes, not previously used to examine fern rhizoid structure,confirmed that the tubular vacuolar system extends well intothe apical cytoplasm, and that the network of fine cytoplasmicstrands leads back from the apical cytoplasm to the nucleus.It also revealed that mitochondria are distributed throughoutthe rhizoid and are not excluded from the extreme apex. Membranestaining by FM 4-64 suggested a high density of membrane vesicleswithin the cytoplasm of the extreme apex. Uptake of this endocytosismarker into endomembranes also suggested rapid plasma membraneturnover in the rhizoid. This study highlights the similarityin the developmental stages and appearance of D. affinis rhizoidsto angiosperm root hairs and their much less distinct apicalzonation compared to pollen tubes. Copyright 2000 Annals ofBotany Company Rhizoid, root hair, confocal imaging, vital stains.     

Phytochrome-dependent modulation and re-induction of growth of the first rhizoid in Dryopteris paleacea Sw

Phytochrome-dependent growth in Dryopteris paleacea Sw. was investigated in young, developing gametophytes with respect to formation and differentiation of rhizoids. Under continuous red light (Rc), the first rhizoids grew synchronously by tip elongation at a constant rate of 240 μm · d⁻¹ until formation and outgrowth of the second rhizoid. Cessation of growth of the first rhizoids and outgrowth of the second rhizoids showed a correlation in time assumed to be mediated by intercellular signaling. The first rhizoids showed two modes of response to actinic irradiations: (i) modulation of rhizoid growth, and (ii) re-induction of growth in non-growing rhizoids. In the former, the promotory effect of actinic irradiations on rhizoids pre-cultured under Rc determined both the time for which rhizoids continued to grow after transfer into darkness and the final rhizoid length. In the latter, re-induced growth was studied using non-growing rhizoids which were obtained after irradiation with a far-red light (FR) pulse at the end of the pre-culture in Rc and transfer into darkness for 3 d to stop growth. Re-induction of growth occurred with a lag phase of 36 to 48 h after formation of the FR-absorbing form of phytochrome (Pfr) by a red light (R) pulse. From the incomplete R/FR reversibility it is evident that, here, coupling of Pfr to signal transduction is possible within minutes. Re-induction of growth possesses the advantage that the effect of actinic irradiations can be studied as an all-or-none response at the level of single gametophytes in future experiments. The present results clearly indicate that the developmental stage of the whole gametophyte, i.e. temporal and spatial patterns undergone during development, affects the regulation of rhizoid growth by the external factor light. Received: 8 June 1998 / Accepted: 22 December 1998     

Further Notes on Polytrichum commune L.

Abstract

It baa been show that the one-third divergance of the 1eaf-scale in the rhizome and middle regions is bound up with parallel type of segmentation at the apex, while the encroaching type result in higher divergence.

The general arrangement of the tissues in the oil is influenced by the type of apical aegmentation. The delineation of the primary aegments is soon lost, on account of lateral overlapping, but it can be seen that in the rhizome the three rows of radial plates represent the median regions of the aegments and correspond with the vein region in the acale leaves with which they are connected. As the axis grows, rhizoids, rhizoid wicks and sometimes branch buds are developed from epidermal cells exterior to the radial strands. This, coupled with the fact that the cells of these strands contain a larger amount of starch and other organic materials than the other regions of the cortex, indicates that they are the main conducting channels between the branches and the rhizoid system and the central conducting column.

In the middle region, the primary aegmentation is fundamentally the same as that of the rhizome, the parallel type being maintained until green leaves appear. The chief difference lies in the cell division, which is more frequent and accompanied by a strengthening of the cell walls of the cortex and a concentration of hydroids to the centre In the region corresponding with that occupied by the radial strands, groups of smaller cells forming rudimentary leaf traces can sometimes be distinguished. These are comparable with the more definite leaf traces described by Goebel (1906) in the case of Dawsonia superba R.Br., a 1arge form, 30-40 cm. long, found in Australia and New Zealand.

In the green-leaved region the change to the encroaching type of segmentation has taken place, and is accompanied by increased overlapping and further differentiation of tissues of the leaf trace and other tissues of the axis.     

Centrifugation causes adaptation of microfilaments Studies on the transport of statoliths in gravity sensingChara rhizoids

Summary The actin cytoskeleton is involved in the positioning of statoliths in tip growingChara rhizoids. The balance between the acropetally acting gravity force and the basipetally acting net out-come of cytoskeletal force results in the dynamically stable position of the statoliths 10–30 m above the cell tip. A change of the direction and/or the amount of one of these forces in a vertically growing rhizoid results in a dislocation of statoliths. Centrifugation was used as a tool to study the characteristics of the interaction between statoliths and microfilaments (MFs). Acropetal and basipetal accelerations up to 6.5 g were applied with the newly constructed slow-rotating-centrifuge-microscope (NIZEMI). Higher accelerations were applied by means of a conventional centrifuge, namely acropetally 10–200 g and basipetally 10–70 g. During acropetal accelerations (1.4–6 g), statoliths were displaced to a new stable position nearer to the cell vertex (12–6.5 m distance to the apical cell wall, respectively), but they did not sediment on the apical cell wall. The original position of the statoliths was reestablished within 30 s after centrifugation. Sedimentation of statoliths and reduction of the growth rates of the rhizoids were observed during acropetal accelerations higher than 50 g. When not only the amount but also the direction of the acceleration were changed in comparison to the natural condition, i.e., during basipetal accelerations (1.0–6.5 g), statoliths were displaced into the subapical zone (up to 90 m distance to the apical cell wall); after 15–20 min the retransport of statoliths to the apex against the direction of acceleration started. Finally, the natural position in the tip was reestablished against the direction of continuous centrifugation. Retransport was observed during accelerations up to 70 g. Under the 1 g condition that followed the retransported statoliths showed an up to 5-fold increase in sedimentation time onto the lateral cell wall when placed horizontally. During basipetal centrifugations 70 g all statoliths entered the basal vacuolar part of the rhizoid where they were cotransported in the streaming cytoplasm. It is concluded that the MF system is able to adapt to higher mass accelerations and that the MF system of the polarly growing rhizoid is polarly organized.Abbreviations g gravitational acceleration (9.81 m/s²) - MF microfilament - NIZEMI Niedergeschwindigkeits-Zentrifugen-Mikroskop (slow-rotating-centrifuge-microscope)     

Development of the siphonous green alga Penicillus and the Espera state*

The thallus of Penicillus is composed of two filament types: axial filaments of indeterminate growth and laterals of determinate growth. In vegetative reproduction new plants arise from horizontal rhizoids. Four stages can be distinguished in development. In the primordium stage the tip of a rhizoid swells and forms a primordium. In the germling stage ascending and descending axial filaments arise from the primordium, the former grow into a fascicle and give rise to lateral saccate branches, each of which forms an ascending and a descending arm and branches further into rhizoid-like branchlets. Together these structures constitute the foundation of the stipe. At the same time the descending axial filaments elongate and become main rhizoidal filaments with lateral rhizoidal branchlets. In the early juvenile stage the stipe is formed. The elongating ascending axial filaments form a medulla while their laterals produce a cortex. In the late juvenile stage the axial filaments form the capitulum. The Espera state of Penicillus lacks a stipe since the ascending axial filaments do not join in a fascicle. Espera has been grown in laboratory cultures from Penicillus plants collected in the Caribbean region and also been found uncommonly in nature in this area. This state may be a response to environmental stress. The flattening of thalli and their orientation perpendicular to the direction of waves are discussed. A comparison of Penicillus and Codium indicates that at least two types of development exist in multiaxial Eusiphoniidae.     

东北草地异质生境芦苇种群根茎芽年龄结构及输出规律

为明确异质生境条件下芦苇种群根茎芽年龄结构及输出规律,揭示芦苇种群的营养繁殖特性,采用单位土体挖掘取样,分别计数各龄级根茎芽的调查与统计方法,对东北草甸草原草甸土和盐碱土两个生境单优群落芦苇种群根茎芽动态进行比较分析。结果表明,两个生境芦苇种群根茎芽库主要均由6个龄级组成;草甸土生境在6—10月均为增长型年龄结构;盐碱土生境6—7月份为衰退型年龄结构,8月份为稳定型年龄结构,9—10月份为增长型年龄结构。根茎芽数量1—4a普遍以草甸土生境高于盐碱土生境,5—6a普遍以盐碱土生境高于草甸土生境,各龄级根茎芽数量与月份之间均符合y=a+bx直线关系(P0.05)。随着龄级的增加,休眠芽比率呈逐渐下降趋势,而萌发芽比率则呈逐渐上升趋势,5个生育期的休眠芽比率和萌发芽比率与龄级之间均符合y=a+bx直线关系(P0.01)。各龄级根茎的休眠芽具有一个相对稳定的萌发输出过程,草甸土生境根茎休眠芽按每年11%的比率萌发输出,而盐碱土生境根茎休眠芽按每年7%的比率萌发输出。虽然芦苇种群根茎芽年龄结构及年龄谱在异质生境中存在显著差异,但却有着相同的季节变化规律,均以不断形成新根茎的芽来维持着种群的营养繁殖更新。     

REGENERATION IN THE COENOCYTIC MARINE ALGA,CAULERPA, WITH RESPECT TO GRAVITY

Small pieces of the green algal coenocyte Caulerpa are capable of regenerating complete new plants. This study investigated the effect of gravity on the site of differentiation of newly forming organs during regeneration. Pairs of 3.0-cm blade segments from C. prolifera and C. mexicana, as well as 3.0-cm rhizome segments of C. prolifera, were randomly assigned to either upright or inverted positions. This orientation was maintained throughout each experiment. Results revealed that the blade segments maintained a strict polarity of regeneration regardless of inversion and/or centrifugation. Rhizoids and rhizomes formed at the original basal end of the segment, while blades formed at the apical end. This polarity was seen throughout the length of long blades. Rhizome segments, on the other hand, failed to exhibit a strict polarity of regeneration with respect to gravity.