摘要:

Several characteristics of the environment, such as plant litter, may interact with species characteristics, such as seed size, to determine patterns of plant establishment and thereby influence abundance and spatial location of plants long after the factor has itself disappeared. To determine if litter might differentially affect seedling establishment ofBetula alleghaniensisBritton andRhus typhinaL., two woody species with different sized seeds, we conducted two greenhouse experiments. We documented emergence, morphology, and growth patterns of seedlings covered by three amounts each ofFagus grandifoliaEhrh. leaves orTsuga canadensisL. needles. All amounts of litter significantly reduced and delayed finalBetula alleghaniensisseedling emergence, and the resultant seedlings had reduced root: shoot ratios, longer hypocotyls, and were less robust. Litter did not affectRhus typhinaemergence but did alter biomass allocation: stem, leaf, and root biomass ofRhusseedlings was reduced by greater amounts of litter, and by leaf litter relative to needle litter. We attribute the different responses ofBetulaandRhusto differences in seed size (Betulamean seed size = 1.0 mg;Rhusmean seed size = 8.5 mg) and germination cues (Betularequires light,Rhusrequires chemical or heat scarification), and suggest that differential response to litter may contribute to coexistence among these sympatric species.

ISSN:0002-9122    

DOI:10.1002/j.1537-2197.1992.tb13722.x

出版商:Wiley    

年代:1992

数据来源: WILEY

摘要:

A clone ofSpartium junceum(Spanish broom) was used to evaluate the consequence of different soil nitrogen regimes on growth and assimilation of leaves and stems. Nitrogen limitation caused a general reduction in aboveground growth, and NO3‐limitation resulted in a greater reduction in leaf area compared to stem area. Supplemental soil nitrogen, from NH4+, caused increased growth and a greater increase in stem area compared to leaf area. Excess NH4+caused decreased growth of leaves and stems. Under nitrogen‐limiting conditions, a greater amount of nitrogen was in stems than in leaves, particularly on a surface area basis. Both stem and leaf assimilation were reduced under limiting nitrogen, but the net effect was an increase in the stem contribution to daily carbon gain compared to a decrease for leaves. Stem and leaf assimilation had similar linear relationships with tissue nitrogen concentration. Nitrogen use efficiency increased under nitrogen limitation. Thus, as the proportion of whole plant nitrogen in the stems increased compared to that in leaves at low nitrogen availability, the nitrogen use efficiency increased.

ISSN:0002-9122    

DOI:10.1002/j.1537-2197.1992.tb13723.x

出版商:Wiley    

年代:1992

数据来源: WILEY

摘要:

Wind dispersal of seeds ofMetrosideros polymorpha, the dominant tree of rain forests and the main pioneer of volcanic substrates in Hawaii, was measured on a 20‐yr‐old lava flow downwind of aM. polymorphaforest. Seed rain was concentrated in December and January, when 74.5% of the annual total was dispersed. Annual seed rain at the forest edge was 63,893 seeds/m2, of which 5,580 (8.7%) contained embryos and were potentially viable. Beyond the forest edge, the density of embryo‐containing seed decreased to 363, 137, 37, 25, and 20/m2at distances of 25, 50, 100, 150, and 250 m, respectively. In contrast, the density of establishedM. polymorphaseedlings remained relatively constant beyond 25 m, a fact suggesting that the density of safe sites for germination and establishment was also constant over the same distance. It was concluded that colonization of the lava flow was limited by the density of safe sites, i.e., conditions for establishment, rather than by seed rain, i.e., dispersal, which was more than sufficient to saturate the available safe sites.

ISSN:0002-9122    

DOI:10.1002/j.1537-2197.1992.tb13724.x

出版商:Wiley    

年代:1992

数据来源: WILEY

摘要:

The microscale distribution of polyploid variants of the dominant grass big bluestem (Andropogon gerardii) in virgin tallgrass prairie was mapped using flow cytometry. The correlation between DNA content and polyploidy allows the use of flow cytometry for nondestructive determination of polyploidy in intact plants. At Konza Prairie, local plots contained from 0 to 100% hexaploid cytotypes but most showed fine‐scale mixing of the polyploid variants. The relationship of cytotype frequency to moisture availability or burning history was nonsignificant.

ISSN:0002-9122    

DOI:10.1002/j.1537-2197.1992.tb13725.x

出版商:Wiley    

年代:1992

数据来源: WILEY

摘要:

Several different chytridiomycetes are described from the Lower Devonian (Siegenian) Rhynie chert. Included are both eucarpic and apparently holocarpic forms that occur inPalaeonitella, Aglaophyton, Lyonophyton, Horneophyton, and clusters of algal cells, as well as in the surrounding chert matrix. Holocarpic types consist of endobiotic sporangia, each characterized by one discharge tube. Sporangia can be traced from the thallus stage to the discharge of zoospores. Monocentric and polycentric eucarpic chytrids are associated with the miospores ofAglaophytonand various thick‐walled fungal spores. In these forms the sporangia are variable in size and shape ranging up to 30μm. Most appear to be inoperculate and there is evidence that the sporangium ruptured on the distal surface. Some contain zoospores with flagella. One operculate eucarpic form had parasitized the cellular gametophyte emerging from the proximal surface of anAglaophytonspore. Several of the Rhynie chert chytrids are comparable with a number of extant forms (e.g., Olpidiaceae and Spizellomycetaceae), while others possess features that encompass several groups. These fossil fungi are discussed in the context of their interactions with other organisms in this Lower Devonian freshwater paleoecosystem.

ISSN:0002-9122    

DOI:10.1002/j.1537-2197.1992.tb13726.x

出版商:Wiley    

年代:1992

数据来源: WILEY

摘要:

Extrafloral nectaries are secretory glands, usually found on leaves, that have been shown to promote ant defense against the insect herbivores of many modem day plants. Extrafloral nectaries were found on the 35‐million‐year‐old fossil leaves of the extinctPopulus crassafrom Florissant, Colorado. Extinct ant species (belonging to five still extant genera that have modem ant‐guard species), and other predators and parasitoids (whose modem relatives frequent extrafloral nectaries) also lived at Florissant. The extrafloral nectaries ofP. crassa(and perhaps other plants) probably operated to attract ants and/or other arthropod defenders as early as the Oligocene.

ISSN:0002-9122    

DOI:10.1002/j.1537-2197.1992.tb13727.x

出版商:Wiley    

年代:1992

数据来源: WILEY

摘要:

The effect of microgravity on cellulose synthesis using the model system ofAcetobacter xylinumwas the subject of recent investigations using The National Aeronautic and Space Administration's Reduced Gravity Laboratory, a modified KC‐135 aircraft designed to produce 20 sec of microgravity during the top of a parabolic dive. Approximately 40 parabolas were executed per mission, and a period of 2 g was integral to the pullout phase of each parabola. Cellulose biosynthesis was initiated on agar surfaces, liquid growth medium, and buffered glucose during parabolic flight and terminated with 2.0% sodium azide or 50.0% ethanol. While careful ground and in‐flight controls indicated normal, compact ribbons of microbial cellulose, data from five different flights consistently showed that during progression into the parabola regime, the cellulose ribbons became splayed. This observation suggests that some element of the parabola (the 20 sec microgravity phase, the 20 sec 2gphase, or a combination of both) was responsible for this effect. Presumably the cellulose Iαcrystalline polymorph normally is produced under strain, and the microgravity/hypergravity combination may relieve this stress to produce splayed ribbons. An in‐flight video microscopy analysis of bacterial motions during a parabolic series demonstrated that the bacteria continue to synthesize cellulose during all phases of the parabolic series. Thus, the splaying may be a reflection of a more subtle alteration such as reduction of intermicrofibrillar hydrogen bonding. Long‐term microgravity exposures during spaceflight will be necessary to fully understand the cellulose alterations from the short‐term microgravity experiments.

ISSN:0002-9122    

DOI:10.1002/j.1537-2197.1992.tb13728.x

出版商:Wiley    

年代:1992

数据来源: WILEY

摘要:

A study of the effects of three levels of silicon nutrition, representing low, intermediate, and high levels, shows that silicon does not enhance the vegetative growth ofBromus secalinus(cheat), yet it increases the rate of seed development and the efficiency of seed set. Grasses supplied with silicon have a higher percent of viable seeds compared to those grown in very low levels of silicon. Silicon supply, however, is not an absolute requirement for cheat reproduction, since grasses growing in low silicon levels set flowers and seeds, and the average weight of individual seeds or the mean number of inflorescences per plant are not influenced by silicon nutrition. Concentrations of silicon in tissues of cheat increase over time and with higher levels of silicon supply. Considerable differences are found in the silicon content of various vegetative and reproductive parts of cheat. Among vegetative parts, levels of silicon are lowest in stems and highest in green and senescent leaf blades. Among reproductive parts, silicon is most highly concentrated in the husks (glumes, lemmas, and paleas) surrounding filled seeds and is almost nonexistent in the seed itself. Mechanisms influencing silicon deposition and the ecological significance of the present findings are discussed.

ISSN:0002-9122    

DOI:10.1002/j.1537-2197.1992.tb13729.x

出版商:Wiley    

年代:1992

数据来源: WILEY

摘要:

We examined reproductive attrition inFremontodendron decumbensto characterize sexual reproduction in this rare California shrub. Reproductive individuals produced an average of 2,900 flower buds in a season, with no significant difference in bud production between two seasons. Because of intense insect predation,;80% survived after 5.75 years). Most seedlings produced in unburned chaparral by planting heat‐treated seeds in openings between shrubs were destroyed by predators (rodents and insects). All seedlings that escaped predation died during the summer drought. We concluded that sexual reproduction was limited by (in order of importance): 1) lack of fire, 2) predehiscence predation by insects, and 3) postdehiscence predation by rodents. Size distributions from two populations revealed that, despite the apparent absence of sexual reproduction in unbumed chaparral, two unbumed sites contained a large proportion of individuals in small size classes. Excavation of several small individuals demonstrated they were sprouts from the roots of nearby larger shrubs. Because asexual reproduction by rootsprouting circumvents the high attrition of sexual reproductive effort on unbumed sites, rootsprouting may be a significant reproductive strategy of some ‘sprouter’ species in chaparral.

ISSN:0002-9122    

DOI:10.1002/j.1537-2197.1992.tb13730.x

出版商:Wiley    

年代:1992

数据来源: WILEY

摘要:

Self‐compatibility (SC) appears to be a derived trait in many unrelated taxa. A major selective force that could lead to the evolution of SC from self‐incompatibility (SI) is inadequate pollination. Here we show that seed set is strongly pollenlimited in populations of mayapple,Podophyllum peltatum(Berberidaceae). Hand‐pollination led to a 26‐fold increase in seed set as compared to natural levels in 1989, and to a fivefold increase in 1990. The species is SI, but in a survey of 49 colonies in Ohio we found three that had more than 50% fruit set from self pollen (as compared to about 90% fruit set from hand‐outcrossed flowers). However, when hand‐selfing yielded fruits, the number of seeds per fruit was only about 10% of the seed set from hand‐outcrossing. We discuss the relative fecundity of putative SC and SI genotypes in terms of conditions needed for the spread of SC in mayapple populations (little or no morphological change would be needed to ensure within‐flower selfing). Seed set from putative SC genotypes appeared to be low enough to prevent them from replacing SI genotypes, even in the face of infrequent pollinator visits.

ISSN:0002-9122    

DOI:10.1002/j.1537-2197.1992.tb13731.x

出版商:Wiley    

年代:1992

数据来源: WILEY