摘要:

Growth of spring ephemerals in northern forests is limited to early spring when competition for resources from other species is at a minimum. The abundance of resources during this vernal period suggests that spring ephemerals might grow continuously over a wide range of sites, but distributions tend to be patchy. I hypothesized that co‐occurring plants that grow later in the season compete for resources to a limited extent, but that competition from these other species is sufficient to restrict the spread of spring ephemerals into unoccupied sites. Population dynamics ofErythronium americanumwere compared on sites at the Hubbard Brook Experimental Forest in New Hampshire subjected to varying removals of cooccurring plants. During the 3‐year period after removal of only overstory trees the density and frequency of occurrence ofErythroniumincreased by 225% and 180%, respectively, and removal of all co‐occurring vegetation after 1 year's regrowth had an even more pronounced effect (400% and 195% increases, respectively). The abundance ofErythroniumin the undisturbed forest did not change over the 3‐year period. Elevational distribution ofErythroniumalso was directly related to the extent of removal of summer plants. Recruitment was mostly vegetative, but flower production on the tree removal site increased by a factor of six, and some distant sites apparently were colonized by seedlings. For several years following large‐scale disturbances that eliminate co‐occurring vegetation, the vernal growing season is lengthened and resources are more available, and spring ephemerals such asErythronium americanumexploit these resource‐rich opportunities to expand populations and colonize new sites. It appears that, as forest succession proceeds, the size and survival of newly established colonies are gradually constrained by competition from co‐occurring species that grow later in the season.

ISSN:0002-9122    

DOI:10.1002/j.1537-2197.1992.tb13741.x

出版商:Wiley    

年代:1992

数据来源: WILEY

摘要:

Deschampsia caespitosais a widespread grass common in moist areas of the alpine tundra of the Rocky Mountains. Enzyme electrophoresis was used to examine population genetic structure along two soil moisture gradients in Rocky Mountain National Park, Colorado. Introduced plants used in a revegetation project were also sampled at one of the sites. At both sites, there were significant differences among subpopulations in allele frequencies, but these differences were distributed in a patchy fashion and were not correlated with the apparent soil moisture gradients. The degree of genetic subdivision differed between the two sites. At one site, gene flow appeared to be high and differences in allele frequencies are attributed to selection in a mosaic environment. At the other site, gene flow appeared more restricted and differences in allele frequencies between subpopulations are attributed to selection and limited gene flow acting simultaneously. Overall, 15% of the genetic variability is between subpopulations and gene flow is high, even between subpopulations separated by up to 1.5 km, but local conditions can apparently limit gene flow and increase the degree of genetic subdivision. The mean genetic distance between introduced plants and the native subpopulations was significantly higher than the mean genetic distance between all other subpopulations. Despite the high gene flow apparent in alpine tundra subpopulations ofDeschampsia caespitosa, significant genetic structuring of these subpopulations has developed.

ISSN:0002-9122    

DOI:10.1002/j.1537-2197.1992.tb13742.x

出版商:Wiley    

年代:1992

数据来源: WILEY

摘要:

This study examined the benefits associated with resource sharing among interconnected ramets spanning a soil salinity gradient. Clones ofHydrocotyle bonariensis, a rhizomatous dune perennial, expand into salt marsh communities from surrounding upland dune systems in coastal North Carolina. In rhizome‐severing experiments conducted under both field and laboratory conditions,Hydrocotylewas shown to proliferate ramets under saline conditions, provided that these ramets were connected to other ramets growing in nonsaline conditions. Ramets that benefited from resource integration did not appear to be affected by local salt exposure in that these ramets were morphologically similar to those grown under nonsaline conditions. Supporting ramets incurred no net cost in terms of biomass or ramet production, but there was an increased percent allocation to roots and rhizomes. Ramets grown in saline conditions without the benefit of clonal integration showed high mortality and produced little or no net clonal growth. It is likely that the acropetal movement of water allowedHydrocotyleclones to ameliorate the heterogeneous saline conditions associated with coastal environments.

ISSN:0002-9122    

DOI:10.1002/j.1537-2197.1992.tb13743.x

出版商:Wiley    

年代:1992

数据来源: WILEY

摘要:

The tip cell of the protonema of the mossCeratodon purpureus(Hedw.) Brid. is negatively gravitropic when grown in the dark on supplemented agar. Gravitropism, plastid distribution, and plastid movement were studied in living cells using time‐lapse video microscopy and infrared light. A wrong‐way (downward) curvature preceded upward curvature and was detected as early as 2 minutes after reorientation. Upward curvature began 30–45 minutes after reorientation to the horizontal. Cell division temporarily reversed upward curvature, but did not inhibit wrong‐way curvature. Since significant amyloplast sedimentation always occurred before the start of upward curvature, it is possible that these amyloplasts function as statoliths for upward curvature. However, no significant amyloplast sedimentation occurred before wrong‐way curvature. Thus, this early phase of gravitropism cannot require plastid sedimentation for gravity sensing. Most plastids moved within and between zones, and plastid zonation was highly dynamic. Plastids moved toward the apex and toward the base of the cell at rates much slower than cytoplasmic streaming. Despite the dynamic nature of plastid movement and zonation, during upward curvature the distance between sedimented plastids and the apex stayed constant. Time‐lapse analysis has revealed intriguing events not readily seen previously using destructive sampling.

ISSN:0002-9122    

DOI:10.1002/j.1537-2197.1992.tb13744.x

出版商:Wiley    

年代:1992

数据来源: WILEY

摘要:

A fossil flora from the Late Paleocene‐Early Eocene Thyra Ø Formation of eastern North Greenland (paleolatitude 77° N) has yielded monocotyledon leaf impressions with characters seen only in the closely related modem species in the families of Heliconiaceae, Musaceae, and Strelitziaceae. The combination of large costae widths and parallel, nonanastomosing, lateral veins that depart at right angles from the costae in the fossil material are features present only in leaves of extant species from these families. Three basic venation patterns also are recognized in the modem species of these families, but except for the generaStrelitziaandPhenakospermum, none of these patterns are present exclusively in any one family.Musopsisn. gen. is created for the fossil material from Greenland, but it is considered a form genus due to the lack of gross morphological features that can be used for separating leaves of the modem genera in Heliconiaceae, Musaceae, and Strelitiziaceae. It is the first known Arctic occurrence of fossil leaf material resembling this modem group of taxa.

ISSN:0002-9122    

DOI:10.1002/j.1537-2197.1992.tb13745.x

出版商:Wiley    

年代:1992

数据来源: WILEY

摘要:

One day after pollination, embryo sacs ofZea mays, containing zygotes and a few endosperm nuclei, were partially isolated and grown in culture. The zygotes underwent normal embryo development, and 40% germinated on a modified Murashige and Skoog medium with 0.1 mg/liter 6‐benzylaminopurine. After the appearance of a root and a shoot, individual plantlets were transferred to soil and grown to sexual maturity. The techniques for partial mechanical isolation and successful growth to maturity of such young embryos provide the tools necessary for the recovery of plants from in vitro‐fertilìzed embryo sacs.

ISSN:0002-9122    

DOI:10.1002/j.1537-2197.1992.tb13746.x

出版商:Wiley    

年代:1992

数据来源: WILEY

摘要:

Dalechampia brownsbergensisandD. fragransco‐occur in Suriname, and both are pollinated by fragrance‐collecting male euglossine bees.Dalechampia brownsbergensisappears to bloom year‐round and is pollinated by relatively few species of bees, includingEuglossa tridentataandE. gaianii.In contrast,D. fragransappears to bloom from late October through early December and is visited and pollinated by at least 13 species of euglossines. Field observations of pollination indicated that the two species did not share pollinators. However, when the flowers ofD. fragranswere “transplanted” into a population ofD. brownsbergensis, the main pollinator ofD. brownsbergensisalso visited the flowers ofD. fragrans.The pollinators ofD. fragrans, however, did not visit the flowers ofD. brownsbergensis. Partial sharing of pollinators may have only a small negative impact on the two sympatric plant species at this site because they flower simultaneously only part of the year, and they are often spatially separated from one another.

ISSN:0002-9122    

DOI:10.1002/j.1537-2197.1992.tb13747.x

出版商:Wiley    

年代:1992

数据来源: WILEY

摘要:

The breeding system of the hedgehog cactus,Echinocereus coccineusEngelm. (Cactaceae), was studied at two sites in the northern Chihuahuan Desert, New Mexico. Two distinct flower types, restricted to separate plants, were observed. One was hermaphroditic and the other male‐sterile. Hand‐pollinations and field observations showed them to be functionally male and functionally female, respectively. There were significant quantitative intersexual differences in floral morphology. The main pollinators of the populations studied are probably two species of halictid bee. Sex ratios based on the number of plants and flowers at the two sites did not differ significantly from 1:1. Significant differences in fruit set existed between functional female plants from the two sites. The floral dimorphism described here appears confined to eastern and northwestern populations, at the periphery of the range ofE. coccineus.Floral dimorphism was not observed in three closely related species.

ISSN:0002-9122    

DOI:10.1002/j.1537-2197.1992.tb13748.x

出版商:Wiley    

年代:1992

数据来源: WILEY

摘要:

According to sex allocation theory, to maintain a mutant male‐sterile plant in a population of hermaphrodites such a plant must compensate its loss of fitness caused by inhibition of pollen production with a higher reproductive success through its female function. In the present study of a gynodioecious population ofSilene vulgaris(Caryophyllaceae) I show that hermaphrodites not only benefit from outcrossing, in that progeny from outcrossed flowers are more vigorous than those from selfed flowers within an individual plant, but they also suffer heavily from self‐pollination between different flowers of the same individuals, which could be demonstrated in experimentally made male‐sterile (emasculated) individuals. Seeds from the emasculation period were heavier and germinated better than when the same individual was an intact hermaphrodite. Naturally male‐sterile (female) individuals produced more fruits due to flowers staying open longer for pollen to arrive via some vector. However, the higher seed number alone could not provide the fitness advantage needed for females to be maintained in the population, but females also produced heavier seeds as compared to the hermaphrodites. Differences in seed survival and seedling establishment in the field are expected to add the advantages necessary for female plants to be selectively plausible.

ISSN:0002-9122    

DOI:10.1002/j.1537-2197.1992.tb13749.x

出版商:Wiley    

年代:1992

数据来源: WILEY

摘要:

Scanning electron microscopic investigations of seed morphology of the five genera of Menyanthaceae illustrate a remarkable diversity of seed characteristics for a small family. Seeds of the monotypic northern hemisphereMenyanthesandFauriaare unomamented and similar, and those of the monotypicLiparophyllumof New Zealand and Tasmania are similar to some Australian species ofVillarsia. Seed characteristics within the larger generaVillarsiaandNymphoidesare variable and mostly species specific, but do not lend support to taxonomic separation of these genera. Interspecific affinities postulated on other grounds are, in many instances, supported by seed morphology. Various seed‐coat features are believed to aid in water or animal dispersal of the seeds of several species. Ant dispersal appears to be important forVillarsiaandNymphoidesin Australia, but does not seem to occur in these genera or in other members of the family outside of Australia.

ISSN:0002-9122    

DOI:10.1002/j.1537-2197.1992.tb13750.x

出版商:Wiley    

年代:1992

数据来源: WILEY