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11.
The initial development of a piosphere created by stocking a new watering trough with ca. 200 sheep at a near-pristine site in arid chenopod shrubland near Whyalla, South Australia, was studied. The following variables were monitored for up to 8 years using fenced plots as controls:shrub mortality; forage biomass and defoliation of the dominant shrubs and short-lived grasses; linear dimensions of Rhagodia ulicina; the density of short-lived species; and the invasion of exotics. Patterns in these variables were looked for which reflected the attenuation of stocking pressure with distance from water; that is, ‘piosphere patterns’. Piosphere patterns were detected for many variables. Those with values increasing towards the trough were mortality of the main forage shrub Atriplex vesicaria. the density of three short-lived taxa, viz. the bassias (Sclerolaena obliquicuspis, S. patenticuspis. Maireana sclerolaenoides), Tetragonia tetragonoides, and annual Zygophyllum spp. (Z. crenatum. Z. ovatum), and growth of short-lived grass species when sheep stocking pressure was low. Variables with values decreasing towards the trough were grass biomass, percentage of A. vesicaria individuals in flower, the percentage foliation of A. vesicaria and of the forage shrub Maireana sedifolia, and shrub forage biomass of A. vesicaria and M. sedifolia. The rate at which these patterns were registered varied. After only 3 months, a piosphere pattern was evident for grass biomass. Marrubium vulgare, an introduced weed, invaded the trough site within the first 6 months. Almost no A. vesicaria shrubs had died after 2 years, but after 8 years there was a marked piosphere pattern in their mortality. The piosphere patterns in forage biomass and defoliation of A. vesicaria and M. sedifolia became more pronounced with time. Some variables did not exhibit a piosphere pattern, such as the dimensions of the shrub R. ulicina and the densities of successive cohorts of the forb Erodium spp., even though there was an overall effect of grazing in reducing their values. There was no mortality of two dominant shrubs, M. sedifolia and R. ulicina, during the first 8 years. 相似文献
12.
Microhabitat selection and life cycle patterns of two Rhyacophila species (Trichoptera: Rhyacophilidae) in southern Ontario streams 总被引:1,自引:0,他引:1
IAN D. MARTIN 《Freshwater Biology》1985,15(1):1-14
SUMMARY.
- 1 Two species of predatory caddisfly larvae—Rhyacophila fuscula (Walker) and R. melita Ross—have overlapping longitudinal distributions in southern Ontario streams.
- 2 The populations were studied at sites in areas of overlap (sympatry) and allopatry to determine what resources were partitioned, and if partitioning varied in the zones of overlap.
- 3 Life cycle and species size variation resulted in substantial physical differences between these univoltine species for most months of the year.
- 4 In May, when larvae of both species were not significantly different in size, a detailed microhabitat field study found no difference in microhabitat use between allopatric populations of R. fuscula and R. melita. Microhabitat selection by R. fuscula remained constant at sympatric sites but R. melita showed a significant change resulting in reduced microhabitat overlap with R. fuscula.
13.
14.
Experiments are reported on the spatial distributions of isotopiccarbon within the mesophyll of detached leaves of the C3 plantVicia faba L. fed 14CO2 at different light intensities. Eachleaf was isolated in a cuvette and ten artificial stomata providedspatial continuity between the ambient atmosphere (0.030.05%v/v CO2) and the mesophyll from the abaxial leaf side. Paradermalleaf layers exhibited spatial profiles of radioactivity whichvaried with the intensity of incident light in 2 min exposures.At low light, when biochemical kinetics should limit CO2 uptake,sections through palisade cells contained most radioactivity.As the light intensity was increased to approximately 20% offull sunlight, peak radioactivity was observed in the spongycells near the geometric mid-plane of the mesophyll. The resultsindicate that diffusion of carbon dioxide within the mesophyllregulated the relative photosynthetic activity of the palisadeand spongy cells at incident photosynthetically active lightintensities as little as 110 µE m2 s1 whenCO2 entered only through the lower leaf surface. Key words: CO2 capture sites, Vicia faba L., Artificial stomata 相似文献
15.
The Hormonal Control of the Amphibian Ovary 总被引:3,自引:0,他引:3
The ultimate control of amphibian gonadal function rests withenvironmen tal factors mediated through the hypothalamus. Itappears that control of ovarian growth resides in the infundibularregion and ovulation in the preoptic area. For normal temporalrelationships between oocyte growth and ovulation to occur,an intact hypothalamo-pituitary complex is necessary. It isuncertain whether the several types of pituitary basophils considered,histologically as gonadotropin producing cells are in fact producingseparate LH and FSH like hormones. Perhaps the concensus indicatesa single hormone has both vitellogenic and ovulatory functions.This hormone stimulates estrogen synthesis and secretion bythe ovarian follicle cells, and this steroid causes oviductgrowth and the hepatic biosynthesis of vitellogenin, the majoryolk platelet precursor. Uptake of this lipoprotein from thecirculation and its conversion to the components of the plateletis mediated by the gonadotropin, the presence of which resultsin the establishment of a rapid micropinocytotic process atthe level of the oocyte surface and of a mechanism for crystallizationof the yolk. A sudden surge of pituitary hormone, when presentedto fully grown oocytes leads to their maturation and ovulation,and to oviducal jelly release in some species. The active hormoneis progestin in nature, again produced by the follicle cells. In this review the known factors involved in the hypothalamohypophysio ovarian axis are discussed together with some considerationof outstanding problemsand the possible relevance ot ovipantvand ovovivipanty in amphibians to the ovarian control foundin viviparous species. 相似文献
16.
Taxus media cv. Hicksii plants were grown one season under a low and high level of nitrogen fertilization. Before growth in the spring the plants were divided into two groups, one of which was defoliated and the other left intact. The growth and spring utilization of the nitrogen and carbohydrate reserves of defoliated plants were compared to the intact plants 0, 2, 4 and 6 weeks after growth started in the spring. The plants were separated into buds (all new growth), roots and stems and analyzed for changes in total nitrogen, basic and non-basic amino acids, hemicelluloses, soluble sugars, organic acids and chlorophyll. The older evergreen needles from plants grown under low nitrogen levels contain 20 % of the carbohydrate and 24% of the nitrogen used in spring growth. The needles from plants grown under high nitrogen levels contained 56% of the carbohydrate and 49% of the nitrogen used in spring growth. Removal of the old needles before spring growth removed this nitrogen and carbohydrate reserve and reduced the total plant chlorophyll content after 6 weeks of growth to 50% of that found in intact plants, with the result that defoliated plants did not show a growth response to nitrogen. Amino acids accumulated in the stems and buds of defoliated plants as carbohydrates became limiting. The defoliated plants removed 25% more available carbohydrates from the roots and stems than intact plants and their buds contained 50% less available carbohydrates. Plants without old needles showed similar growth rates under low and high nitrogen regimes and produced 33% of the dry weight of intact plants grown under high nitrogen levels and 66% of the dry weight of intact plants grown under low nitrogen levels. The old needles of taxus plants contain substantial amounts of reserve nitrogen and carbohydrate and these needles greatly influence the extent and rapidness of growth in the spring. When the needles are removed, the other tissues can supply an adequate amount of nitrogen but the carbohydrate supply becomes limiting for spring growth. 相似文献
17.
RICHARD L. SIMMONS MARTIN L. LIPSCHULTZ ANGELYN RIOS PRASANTA K. RAY 《Nature: New biology》1971,231(21):111-112
DURING outbred pregnancy the mother is exposed to genetically foreign tissue because the offspring inherits transplantation antigens from the father. The survival of the foetus is ensured by the intervention of the trophoblast which does not express transplantation antigens between mother and foetus: mouse trophoblast is not rejected even when transplanted into immune recipients1,3. The mechanism of this failure to express histocompatibility antigens is not understood1–4, but Kirby et al. have suggested that the extracellular fibrinoid surrounding trophoblast cells is involved5,6. Currie has suggested that the thick sialomucinous glycocalyx of the trophoblast cell might “mask” the histocompatibility antigens on the trophoblast7,8 and has demonstrated that neuraminidase unmasked these antigens8. Our experiments, however, show that trophoblast incubated with neuraminidase cannot sensitize allogeneic mice to donor histocompatibility antigens. Furthermore, pretreatment of trophoblastic implants with neuraminidase did not interfere with their proliferation and growth in highly immune allogeneic recipients. 相似文献
18.
TRACE FOSSILS FROM THE PRECAMBRIAN AND BASAL CAMBRIAN 总被引:6,自引:0,他引:6
MARTIN F. GLAESSNER 《Lethaia: An International Journal of Palaeontology and Stratigraphy》1969,2(4):369-393
Certain worm-like configurations on rocks are recognized as shrinkage-crack infillings. Some genuine Precambrian trace fossils are briefly described. The early Cambrian contains a richer assemblage, including some distinctive and widespread form genera. The study of early trace fossils leads to conclusions not only on facies, but also on the evolution of behaviour and functional morphology in soft-bodied organisms. 相似文献
19.
MARTIN C. LEWIS 《The New phytologist》1969,68(2):481-503
20.
MARTIN ORANS 《American anthropologist》1968,70(5):875-897
Insofar as it may be regarded as a purely logicomathematical system, lacking empirical content, economic theory may, by appropriate empirical interpretation, be applied to domains quite apart from the market. Here such use yields a model of caste or jajmani relations. The chief assumptions of the model regarding the stability of a caste system are the following: (1) it requires a high concentration of political power; (2) it requires "prices" that do not change "freely" with supply and demands; (3) it requires consonance of political power, wealth, and ritual rank. These assumptions and various related deductions are tested statistically by data derived from village studies in India, Pakistan, and Ceylon. The evidence indicates that the model is generally plausible and may be applicable to hereditary hierarchies everywhere. 相似文献