Size-dependence of reproductive allocation of Sargassum thunbergii (Sargassaceae, Phaeophyta) in Bohai Bay, China

Reproductive allocation (the proportion of biomass allocated to reproductive tissue, RA) in Sargassum thunbergii was studied in Yantai, Shandong Peninsula, southern coast of Bohai Bay, China. Annual reproduction initiated in mid-June and peaked in mid-July (90 ± 8% fertile thalli and 75 ± 6% RA mean). Both RA and percentage of fertile thalli exhibited significant temporal variations during reproduction. Sterile thalli were only observed in small length hierarchies at peak reproduction and mean values of RA showed a significantly hierarchical variation, suggesting that the size of thalli played an important role in reproduction and RA were size-dependent. Numerous receptacles were produced along the lateral branches during the reproductive period. A distinct seasonal pattern was observed wherein the presence of lateral branches was followed by the onset of reproduction. RA was positively correlated with the number of lateral branches, as well as the total length of lateral branches. In addition, fertile thalli decayed quickly after peak reproduction. The probability of decay was evidently higher for fertile than for sterile thalli, because all surviving thalli were sterile and short during July–August. So, a trade-off between reproduction and survival may exist at individual levels in S. thunbergii.     

ECOLOGY AND NATURAL HISTORY OF IRIDAEA CORDATA (GIGARTINALES,RHODOPHYTA) GROWTH1

The in situ growth, maturation and senescence, of central California Iridaea cordata (Turner) Bory was studied. This ontogenetic progression was quantified by measuring development and growth of: i) individually tagged blades, and ii) populations of blades within cumulative (1-yr duration) and seasonal experimental plots. Greatest growth and longest life span were exhibited by winter-spring initiated blades, and were correlated with a rapid increase, in irradiance, but not with either seawater temperature or nutrients. Tagging studies showed that reproductive maturation and senescence of blades occurred throughout the year, irrespective of date tagged, growth rate or size. Moreover, the majority of blades continued to elongate following maturation, and some matured within 3 mo of initiation at all seasons but winter. At the population level maturation took place primarily during summer-autumn when 90 ±2% of the population was mature. The majority of the population senesces or “dies back” during autumn-winter. It is concluded that in situ, the blades are derived almost totally via vegetative means involving perennation. This indirectly suggests that sexual reproduction or the success of sporeling development are marginal. Additionally, the species is perennial with annually deciduous blades, characterized by both rapid growth and maturation.     

SEASONAL PATTERNS OF MORPHOLOGICAL VARIABILITY IN SARGASSUM POLYCERATIUM (PHAEOPHYTA)1

Temporal variability in certain morphological and taxonomically important features was quantified for Sargassum polyceratium Mont. from a population in the Content Keys, Florida (U.S.A.). Patterns of blade development, senescence, and loss caused pronounced seasonal changes in blade length-width ratios. Blade length and width were maximal early in the growing season (August-November) and decreased as the annual stems matured. Early in the growing season, plants had broader blades with randomly distributed cryptostomata. Late in the growing season (February-April), plants had more linear blades with cryptostomata approximately arranged in two rows, one on each side of the midrib. The length-width ratio of blades increased acropetally along the stems and were directly correlated to the size of the cryptostomatal opening and inversely correlated with the number of cryptostomata. The branching pattern of the annual stems ranged from short spur branches to well-developed, lateral axillary branches. The frequency of bifurcated blades increased significantly late in the growing season. Vesicle shape and size and pedicel length were temporally stable. Alated pedicels and mucronate vesicles occurred in low frequencies. The variability of the morphological features used to delineate species within the genus Sargassum on the tropical eastern coasts of the Americas is poorly understood.     

Allocation of blade surface to reproduction in Laminaria longicruris of Long Island Sound (USA)

Sorus formation in the kelp Laminaria longicruris de la Pylaie (Phaeophyta) was quantified for a biennial population in Long Island Sound (Connecticut, USA), at the southern limit of its biogeographical range in the Western Atlantic Ocean. The allocation of blade surface to reproduction was measured monthly during two years, with additional samples taken at times of peak growth and reproduction. Sporophytes produced sori year-round, with the highest percentages of fertile plants (75 to 95 percent) occurring from October to December. The mean percentage of sorus area to blade ranged from 1 to 37 percent, remaining low and fairly constant during the spring and summer, but reaching much higher levels in the fall (when growth is minimal) and early winter. The comparatively low mean annual allocation of blade surface to reproduction (5 percent), was postulated to be due to temperature stress on these plants at the southern limit of distribution, but may instead be typical of the species. Such a small allocation of resources to reproduction nevertheless results in billions of spores per m² of substrate, making a major contribution of material potentially available to grazing food webs.     

A SEASONAL COMPARISON OF CARBON,NITROGEN, AND PIGMENT CONTENT IN LAMINARIA SOLIDUNGULA AND L. SACCHARINA (PHAEOPHYTA) IN THE ALASKAN ARCTIC1

Laminaria solidungula and L. saccharina inhabit the Beaufort Sea in the Alaskan High Arctic. Laminaria solidungula is an Arctic endemic, whereas L. saccharina extends from north temperate Pacific and Atlantic waters to the Arctic. Previous studies have shown that the two species have different seasonal timing of growth, but little comparative physiological information exists. As a first step in characterizing these two species from a mixed Arctic population, we measured variations in carbon, nitrogen, and photosynthetic pigment content in blade tissue from plants collected under the fast ice in April and during the open water Period in late July, Both species exhibited seasonal differences in many measured variables; seasonal differences in L. solidungula were most pronounced in growing basal blades. For example, the molar CIN ratio of basal blades averaged 11 in April and 21 in July for L. solidungula and 11.5 in April and 28 in July for L. saccharina. Basal and mature second blades differed in pigment content in April but not in July: chlorophyll a + c in L. solidungula basal and mature second blades averaged 19 and 27 nmol.cm^?2 in April and 30 and 29 nmol. cm^?2 in July, respectively. The corresponding values for L. saccharina were 17 and 29 nmol.cm^?2 in April and 16 and 16 nmol.cm^?2 in July (95% confidence intervals approximately 1–3 nmol. cm ^?2). Carotenoids exhibited similar patterns. Species differences in pigments, carbon, and nitrogen were minor and were probably effects rather than causes of the different seasonal patterns of growth and development. The primary difference between the two species may be the ability of L. solidungula to retain multiple metabolically active blades and to fuel areal growth with stored carbohydrates during winter near-darkness, whereas L. saccharina growth is more closely tied to active photosynthesis in the growing basal blade. The cause of old blade retention in L. solidungula and the possibility of other physiological differences between the two species, including gametophytes, remain to be determined.     

LATITUDINAL TRENDS IN THE GROWTH AND REPRODUCTIVE SEASONALITY OF DELESSERIA SANGUINEA,MEMBRANOPTERA ALATA,AND PHYCODRYS RUBENS (RHODOPHYTA)1

The seasonality of Delesseria sanguinea, Membranoptera alata, and Phycodrys rubens (Rhodophyta) was studied at Helgoland (North Sea, Germany) and Roscoff (Brittany, France). Plants were collected at bimonthly intervals, and growth and reproduction were monitored. Growth of blades was observed mainly in spring, although small blades were found on plants of M. alata and P. rubens all year round. In summer, plants started to degenerate and in autumn they became fertile. The reproductive season of D. sanguinea lasted from October to February/April at both locations, whereas reproductive plants of M. alata and P. rubens were found until April at Helgoland and until June and August, respectively, in Roscoff_: Lower winter temperatures at Helgoland than at Rascoff may have caused these differences in the duration of the reproductive season. Using published data, the seasonal patterns at Helgoland and Roscoff were compared to those found at other locations (e.g. Barents Sea; Maine, USA; Isle of Man, UK) and local temperature/daylength conditions. Blade growth was synchronized across all populations and occurred in spring, when temperatures were usually still suboptimal for growth. Maximum reproduction was generally found in the colder half of the year but started earlier in autumn in the Barents Sea. Adaptive strategies in the seasonal control of growth and reproduction are discussed. Adequate timing of the history events (e.g. appearance of juveniles in spring) appears more important than maximal growth and reproduction of adults during the season with the most favorable temperatures.     

Photoperiodic regulation of receptacle induction in Sargassum horneri (Phaeophyceae) using clonal thalli

We developed a clonal culture of Sargassum horneri to investigate the effect of photoperiod on reproduction in this species. Regenerated vegetative thalli were obtained using lateral branches excised from a thallus grown from a single embryo under short‐day conditions (SD = 10:14 h light : dark cycle). Lateral branches excised from the SD‐regenerated thallus became vegetative thalli that remained in that phase as long as they were cultured under SD. When an excised lateral branch was cultured under long‐day conditions (LD = 14:10 h light : dark cycle), it began to enter the reproductive phase while still less than 50 mm long. Induction of the reproductive phase was accompanied by a distinctive morphological change – suppression of blade formation at the apical region of the branch; elongation of branches without blades was then followed by differentiation of receptacles bearing conceptacles on their surface. Apices of receptacles were able to interconvert between reproductive and vegetative phases, as blades resprouted upon transfer from LD to SD. The critical day length for induction of receptacle formation was between 13 and 14 h; receptacle formation was also induced under SD conditions with night breaks (NBs). These results strongly suggest that reproductive regulation of S. horneri is a photoperiodic long‐day response. NBs with blue and green light were effective for reproductive induction but not with red light. This suggests that blue‐ and/or green‐light photoreceptors are involved in the photoperiodic reproductive response of S. horneri.     

FIELD AND CULTURE STUDIES OF A POPULATION OF ENDARACHNE BINGHAMIAE (PHAEOPHYTA) FROM SOUTHERN CALIFORNIA1

Abundances of the erect, blade phase of Endarachne binghamiae J. Ag. (Scytosiphonales, Phaeophyta) varied seasonally at a southern California rocky intertidal site. Blade cover and density were much greater in the fall through early spring; blades were mostly absent from quadrats during the summer. Blade abundances were negatively correlated with both seasonal variations in seawater temperature and photoperiod. Laboratory culture studies failed to provide evidence for sexual reproduction. The life history appears to be of the “direct” type with plurangia-produced zooids germinating into crustose disks. Most disks developed erect blade clusters under spring/fall (17° C) and winter (13° C) temperatures over the range of natural photoperiods employed (14:10, 12:12, 10:14 h LD). In contrast, cultures held under the summer temperature (21° C) produced almost entirely crustose growths regardless of photoperiod. Similar results were obtained for cultures grown at 100 and 200 μE · m^?2· s^?1. E. binghamiae blades were fertile throughout the year and produced viable zooids indicating that reproductive seasonality did not influence the seasonal pattern of blade abundance. Culture and field studies suggest that the initiation of new erect blade clusters from crustose disks is confined to the cooler months of the year (winter and spring). The summer reduction or absence of E. binghamiae blades appears to be due to increased mortality rates and temperature constraints on the development of new erect bladed thalli. Hypothetical causes of mortality are desiccation stress, sand burial, increased grazing activity and a genetically-based short life span.     

CONTRASTING PATTERNS OF ALLOMETRY AND REALIZED PLASTICITY IN THE SISTER SPECIES MAZZAELLA SPLENDENS AND MAZZAELLA LINEARIS (RHODOPOHYTA)1

Phenotypic differences between the low wave exposure Mazzaella splendens (Setchell et Gardner) Hommersand and the high exposure Mazzaella linearis (Setchell et Gardner) Fredericq could be due to plasticity or genetic differentiation. Common gardens were used to assess their levels of plasticity after describing allometric relationships. As thalli lengthened, stipe development for M. splendens almost ceased even though blades continued to expand, but M. linearis formed a larger stipe before developing a blade that continued to stay narrow at longer thallus lengths. Common gardens demonstrated that M. splendens regrown in the site of M. linearis produced a wider blade than M. splendens regrown in its natural low energy site and that M. linearis regrown in low wave energy either could not form a wider blade or became narrower than thalli from its high energy site. Tetrasporophytes of M. splendens produced a longer and thicker stipe in the high energy site, but the larger M. linearis stipe was not realized because its wider blades made it vulnerable to hydrodynamic removal. Mazzaella splendens therefore had low survivorship in the high wave energy site, and survivors were not long enough to reproduce. Survivorship and reproduction of M. linearis was similar in both environments. Some of the M. splendens and M. linearis characters are plastic, but this plasticity was insufficient for convergence of phenotypes, and blade width plasticity was maladaptive at least for M. splendens. Developmental systems producing the stipe and blade phenotypes of each species have undergone genetic differentiation.     

REPRODUCTION AND MORPHOLOGICAL VARIATION IN SOUTHERN CALIFORNIA POPULATIONS OF THE LOWER INTERTIDAL KELP EGREGIA MENZIESII (LAMINARIALES)1

Intertidal Egregia menziesii (Turner) Aresch. populations were studied at three Southern California sites to determine temporal and spatial patterns of reproduction and morphology. The timing of sporophyll production and sporophyte recruitment was similar at all sites. Sporophyll production was much greater during winter periods of colder seawater temperatures and shorter day lengths. Sporophyte recruitment occurred from spring through midsummer, ～5 months following maximal sporophyll production. Lateral blade morphologies varied in a consistent manner, suggesting a developmental mechanism for form variation in Egregia thalli. Spatulate blades dominated shorter axes and the bases of longer axes, whereas filiform laterals became abundant toward the tips of longer axes. Filiform laterals (9.8 mg O₂·g^?1·h^?1) had higher light‐saturated net photosynthetic rates than spatulate laterals (6.8 mg O₂·g^?1·h^?1), resulting in a 12% increase in the productivity of Egregia per meter of filiform frond.     

ORGANIZATION OF THE MORPHOPHYSIOLOGIC UNIT IN ANAGALLIS ARVENSIS AND ITS RELATION WITH THE PERPETUATION MECHANISM AND SENESCENCE

The organization of the morphophysiological unit in Anagallis arvensis L. is conditioned by vegetative growth, which is expressed as indeterminate under short-day conditions and as determinate under long-day ones. The perpetuation mechanism is a function of the environment. Under SD conditions ontogenetic morphogenesis is incomplete, and the unit does not form specific reproductive structures; indeterminate growth and permanent rooting capacity provide an efficient mechanism for perpetuation. Under LD complete ontogenetic morphogenesis assures perpetuation by seeds. The appearance of senescence is independent of the nature of the morphophysiological unit and is directly related to growth cessation. It affects organs only when growth is indeterminate (SD) and the whole organism when growth is determinate (LD). Organ senescence seems related to growth itself, and organism senescence would appear closely related to photoperiodic effects. Photoperiod controls organization of the unit, ontogenetic morphogenesis, and organism senescence simultaneously. The number of leaves per node, flowering, organization, and senescence of the organism are all directed toward a more or less evolved state by the effects of the photoperiodic condition.     

Nutrient and productivity relations of the dune grasses Ammophila arenaria and Elymus mollis

Summary A comparative study of blade photosynthesis and nitrogen use efficiency was made on the dune grasses Ammophila arenaria and Elymus mollis. In the laboratory, an open system gas analysis apparatus was used to examine the gas exchange characteristics of blades as influenced by nitrogen supply. Plants were grown under near-ambient coastal conditions in a greenhouse near Bodega Bay, California, and given either high or low supplies of nitrogen in an otherwise complete nutrient solution. In the field, ¹⁴CO₂ uptake techniques were employed to measure the seasonal patterns of blade photosynthesis of plants growing in situ at Point Reyes National Seashore. Blades used in the lab and field studies were analyzed for total nitrogen content, thus allowing for calculations of photosynthetic nitrogen use efficiency (CO₂ fixed/unit of blade N.).Under laboratory conditions, the introduced Ammophila developed higher rates of light-saturated photosynthesis than the native Elymus, especially under the nitrogenlimited growth regime. Higher rates of photosynthesis and lower concentrations of blade N resulted in a significantly greater nitrogen use efficiency for Ammophila regardless of nutrient treatment. Low N availability induced qualitatively similar physiological responses in both species, including reductions in maximum net photosynthesis, mesophyll conductance, leaf conductance, dark respiration, and blade nitrogen content, and an increase in the CO₂ compensation point.Although the photosynthetic rates of Ammophila blades were higher in the lab, those of Elymus blades were consistently higher in the field. This could have resulted from differential effects of drought on the two species (i.e. Ammophila may have been more sensitive) or a higher photosynthetic capacity in Elymus that reflected the greater (1.2–1.5 X) nitrogen content of its blades. However, the nitrogen use efficiency of Ammophila blades was greater than that of Elymus throughout most of the sampling year, despite lower average rates of field photosynthesis.The results indicated that rates of photosynthesis perunit of blade area do not account for the greater aboveground productivity of Ammophila stands along the Pacific coast of North America. Instead, efficient nitrogen use in photosynthesis maycomplement other structural and physiological traits and thereby enhance long-term carbon gain in Ammophila relative to Elymus.     

PHYSIOLOGICAL INVESTIGATIONS ON ALARIA ESCULENTA (LAMINARIALES,PHAEOPHYCEAE). II. ROLE OF TRANSLOCATION IN BLADE GROWTH1,2

Laboratory studies on blade growth in Alaria esculenta (L.) Grev. showed 3 periods of rapid blade elongation during the year: October–November, February–April and late June. The first two periods are characteristic of many Laminariales; the unique June peak may reflect local nutrient conditions. While the distal blade functions as a source, supplying organic matter to the blade meristem, the stipe can be a source during periods of rapid growth or a sink during late summer when blade growth is slow. Maximum enhancement of elongation rate of blade meristems was observed in 40–50 cm blades; longer blades showed no further increase in growth rate. This blade length-growth promotion relationship may be independent of seasonal variations in meristematic activity. ¹⁴C tracer experiments suggested that separate growth promotion effects by distal blade, sporophylls and stipe were not additive in the intact thallus. The preferential source of assimilate for blade meristem growth was the distal blade. Secondary sources: sporophylls, which were activated following excision of the primary source; and stipe, which began to translocate assimilate when both sources were removed. The role of secondary sources in nature is discussed. Profiles of radioactivity in alcohol-soluble organic matter in blades are evaluated in relation to tracer profiles in higher plants and mechanisms of translocation.     

CARBON ALLOCATION IN MACROCYSTIS PYRIFERA (PHAEOPHYTA): INTRINSIC VARIABILITY IN PHOTOSYNTHESIS AND RESPIRATION1

Measurements of net photosynthesis (PS, O₂ evolution), dark respiration (R, O₂ consumption), and light and dark carbon fixation (¹⁴C) were conducted on whole blades, isolated blade discs, sporophylls, apical scimitars and representative portions of stipe and holdfast of the giant kelp Macrocystis pyrifera L.C. Ag. On a dry weight basis, highest net PS rates were observed in apical scimitar segments and whole blades (3.81 and 3.07 mgC · g dry wt^?1· h^?1, respectively), followed by sporophylls (1.42 mgC·g dry wt^?1· h^?1) and stipe segments (0.15 mgC·g dry wt^?1· h^?1). No PS capacity was observed in holdfast material. Respiration rates showed similar ranking ranging from 1.22 mgC·g dry wt^?1·h^?1 for apical scimitar to 0.18–0.22 mgC·g dry wt^?1· h^?1 for holdfast material. Considerable within blade variability in both PS and R was also found. Steepest PS and R gradients on both an areal and weight basis were found within immature blades followed by senescent and mature blade material. Highest net PS rates were associated with the blade tips ranging from 3.08 (mature blades) to 10.3 mgC·dry wt^?1·h^?1 (immature blades). Highest rates of R generally occurred towards the basal portions of blades and ranged from 1.03–1.80 mgC·g dry wt^?1·h^?1 for immature blades. The variability within and between blades was high, with coefficients of variation approaching 50%. The observed patterns can be related to the decreasing proportionment of photosynthetic tissue and increasing proportionment of structural tissue as occurs from the blade tip to the blade base. Rates of light carbon fixation (LCF) revealed longitudinal profiles similar to oxygen measurements for the different blade types, with the absolute rates being slightly lower. Patterns of dark carbon fixation (DCF) were less easily interpreted. Highest rates of DCF (0.04–0.06 mgC·g dry wt^?1·h^?1) occurred at the basal portions of immature and senescent blades. Longitudinal profiles of total chlorophyll (a + c) on both an areal and weight basis were very similar to the profiles of PS. Normalized to chlorophyll a, PS displayed an unusual longitudinal profile in immature tissue; however, such profiles for mature and senescent tissues were similar to those for PS on an areal basis. It was demonstrated that it is difficult, if not impossible, to select single tissue discs that are representative of whole blades. The metabolic longitudinal profiles reveal a characteristic developmental pattern; the previous working definitions of immature, mature, and senescent blades, based on morphology and frond position thus have a physiological basis.     

Distribution and utilization of assimilated carbon in red clover during the first year of vegetation

Summary The pattern of distribution of¹⁴C labelled assimilates and translocation with time was measured in red clover during one reproductive cycle. Measurements were made on whole plants grown outdoors in pots by exposing the aerial parts to¹⁴CO₂ during one photoperiod. Simultaneously, root respiration and N₂ fixation were recorded.At the beginning of the vegetative period, 2/3 of the assimilates remained in the leaves (basal leaves), and 1/3 were directed to the root system. Then the development of branches required as much as 40% of the C and the root allocation decreased. Reproductive structures diverted 17% of the current photosynthates. Nitrogen fixation was optimal during the maximum extension of the basal leaves and decreased during the development of branches. During this period, C allocation to the nodulated roots was high with an estimated amount of 3.2 mg of C per mg of N fixed.With time, translocation occured within the foliage, from basal leaves to the leaves of the branches and to the new basal leaves developed after senescence of the branches. Remobilization to the reproductive structures remained minimal indicating that flower and seed growth was supported by current photosynthesis.     

ALLOCATION,GROWTH RATES AND GAS EXCHANGE IN SEEDLINGS OF STEPHANOMERIA EXIGUA SSP. CORONARIA AND ITS RECENT DERIVATIVE S. MALHEURENSIS

The annual diploid plant species Stephanomeria malheurensis is known only from a single locality where it grows interspersed with its progenitor S. exigua ssp. coronaria. S. malheurensis is thought to have originated recently at its present site and, consequently, serves as a model to study how the phenotype of a species is assembled following its origin. The following were examined in the present study: dry matter allocation, growth parameters, net photosynthesis, and dark respiration of seedlings under three temperatures in order to identify specific features of the new species that might relate to its ecological properties. Under each test condition, S. malheurensis had leaves with a significantly smaller surface area, greater dry weight per unit area, heavier roots, and a higher root/shoot ratio. Both species had similar relative growth rates during the measured period of rosette growth, but in S. malheurensis this resulted from the combination of a lower leaf area ratio and a higher net assimilation rate, whereas in ssp. coronaria it resulted from the opposite combination. The mean rates of apparent net photosynthesis and dark respiration were similar in both species. The differences between the species may reflect the establishment of genes in S. malheurensis which still are present in ssp. coronaria. The implications of this and the possible ecological significance are discussed.     

Trade-off between vegetative regeneration and sexual reproduction of <Emphasis Type="Italic">Sargassum thunbergii</Emphasis>

An ecological demographic study was conducted from January to December 2009 at Changshan Island on the northern side of Shandong Peninsula, China. The study investigated stem regeneration and the resource trade-off between sexual reproduction and vegetative regeneration of the brown alga Sargassum thunbergii at the individual thallus level. Stem regeneration from the holdfast occurred year round but with significant seasonality, peaking in the early summer. Both percentage of fertile stems and reproductive effort were negatively correlated with stem regeneration in the middle and later periods of sexual reproduction. Furthermore, the correlation coefficients increased significantly with time. These observations suggested that an increase in resource allocation to sexual reproduction would lead to a decrease in allocation to vegetative regeneration during the reproductive period. Reproductive effort was lower and stem regeneration was higher in the wave-exposed location compared with the sheltered location, indicating that allocation of resources between sexual reproduction and vegetative regeneration varied with location. When reproductive tissues were removed, the number of new stems produced from holdfasts increased, and when new stems were removed, an increase in reproductive effort occurred. These results suggest that trade-off between sexual reproduction and vegetative regeneration occurs in S. thunbergii.     

Reproductive biomass allocation in three Sargassum species

Allocation of biomass to sexual reproductive (receptacle) tissue and vegetative (holdfast) tissue differed absolutely and relatively in three Sargassum species that form the bulk of the intertidal algal canopy in the northern Gulf of California. Sargassum herporhizum devoted a greater proportion of its thallus mass into its rhizoidal holdfast than did S. sinicola var. camouii or S. johnstonii, whose holdfasts are solid, more compact, and composed of a lower percentage of water. Conversely, more sexual receptacle tissue was produced by these two species with small holdfasts during the spring reproductive period. Sargassum sinicola var. camouii, which is the only species of the three that becomes fertile in the fall, produces a comparable amount of sexual tissue during this second period of reproduction. Removal of Sargassum from single-species patches showed that canopy regrowth by S. herporhizum with its encroaching rhizoidal holdfast was more rapid and complete than that of the other two species, which invest most of their reproductive efforts into sexual propagules that can disperse long distances. Sargassum herporhizum also displayed a more rapid and complete recovery of canopy cover in patches cleared of thalli and in control patches following the annual summer dieback. These two divergent modes of reproductive biomass allocation suggest that ability to encroach upon nearby open sites and ability to colonize distant discrete islands of suitable habitat represent two distinct reproductive strategies requiring different patterns of biomass allocation. Moreover, for energetic reasons, a species may not be able to excel at both modes of reproduction.     

EVIDENCE FOR XYLEM CONSTRICTIONS IN THE PRIMARY VASCULATURE OF PSILOPHYTON DAWSONII,AN EMSIAN TRIMEROPHYTE

Serial peels through the branch junctions of Psilophyton dawsonii were examined in an attempt to determine if “hydraulic constrictions” (i.e., a localized decrease in mean diameter of xylem elements) sensu Zimmermann (1983) had occurred during bifurcation. Based on the mean diameters of primary xylem tracheids measured acropetally and basipetally to branch junctions, evidence for xylem constrictions (= localized decrease in mean tracheid diameter) was found within the basalmost portions of four out of six minor axes examined near their attachment to major axes (anisotomous junctions). Xylem constrictions were not detected in branch junctions between axes of equal girth (isotomous junctions). Xylem constrictions were detected within the base of five out of seven junctions between fertile and main axes. The mean diameters of tracheids of the branch trace near its origin are larger both basipetally and acropetally from the constriction. There is no evidence for a localized decrease in the mean diameter of the xylem strand in the region of a constriction. Therefore, xylem constrictions are not the result of epidogenesis. Based on the production of “hydraulic constrictions” in extant plants, which serve to localize the formation of embolisms in lateral branches during water stress, it is speculated that P. dawsonii could protect the vasculature of major axes by a similar anatomical feature.