桂林甑皮岩洞穴遗址第四纪孢粉分析

本文对桂林甑皮岩洞穴古人类遗址的文化层堆积物和钙华板系统采样进行孢粉分析研究,分析了当时的植被演替和气候变化、古人类的生活环境以及文化层和钙华板形成的地质时代。     

Internode length and anatomical changes in Pisum genotypes crys and cryc in response to extended daylength and applied gibberellin A1

Dwarf pea (Pisum sativum L.) plants with genotypes cry^c and cry^s responded differently when an 8 h photoperiod (8 h daylight, 16 h dark) was extended to 24 h (8 h daylight, 16 h incandescent light). Genotype cry^c showed up to a 4-fold increase in internode length, sustained by increases in both cell length (particularly of epidermal cells) and cell number (particularly of cortical cells) while cry^s plants showed up to a 2-fold increase in internode length sustained mostly by an increase in cell number. Under an 8 h (daylight) photoperiod the two genotypes did not differ in their sensitivity to applied gibberellin A₁ (GA₁) and they showed a similar pattern of response. GA₁ significantly increased internode length, cell length and cell number in both genotypes. Incandescent light did not increase the size of the response to GA₁ except for cry^s plants at high dose rates of GA₁ (29–58 nmol). At saturating doses of GA₁ the two genotypes attained a similar peak internode length; incandescent light increased the peak by about 40%. GA₁ increased the rate of leaf appearance by up to 33% while incandescent light reduced the rate by 4–7%. The elongation response of the more mature internodes of cry^c plants to GA₁ or incandescent light was due primarily to an increase in cell length whereas increased cell number made a significant contribution in the case of internodes which were relatively immature at the time the stimulus was applied. The progressive increase in internode length of both genotypes during ontogeny was due primarily to an increase in cell number. In conclusion, alleles cry^c and cry^s (background le La) do not confer a difference in sensitivity to GA₁ and the increase in internode length in response to incandescent light is probably not the result of a real or perceived increase in GA₁ level. Allele cry^s may partially block a phytochrome mediated response to light and the key difference between genotypes cry^s and cry^c may lie in the greater elongation (extensibility?) of cry^c epidermal cells in incandescent light.     

Divergent pollination systems in the cleistogamous species,Collomia grandiflora (Polemoniaceae)

Summary A structural study of pollination in the dimorphic flowers ofCollomia grandiflora, a cleistogamous species, reveals significant differences in stigma behavior during pollination, stylar structure, the timing of generative cell division, and pollen tube growth rate patterns. The cleistogamous flower shows a loss of protandry and the stigma is receptive only after reflexing and closing of its lobes. In contrast, the chasmogamous stigma is receptive when reflexed and closes when pollen has been deposited on the lobes. Pollen tube penetration of the dry stigma papillae and entry into the style is similar in the two morphs. The chasmogamous style is solid and the cleistogamous style partly hollow. The matrix of secretion produced by the transmitting tract cells is mainly carbohydrate with a trace of lipids. It is fibrillar in nature and appears to be partly comprised of wall material from the transmitting tract cells. In the chasmogamous pollen, the generative cell enters the tube before division, which occurs between 30 and 60 min after pollination. This division correlates with an increased growth rate for the pollen tube. In the cleistogamous pollen, contact with the stigma triggers generative cell division inside the hydrated pollen grain before germination. The two resulting sperm cells exit the grain 15–30 min after pollination when the pollen tube is in the stigma lobes. The cleistogamous pollen tube shows only one phase of growth which occurs at a rate similar to that of the slow, first phase of the chasmogamous pollen.Abbreviations CH chasmogamous - CL cleistogamous - DAPI 4, 6-diamidino-2-phenylindole     

Regulation of the plane of cell division in vacuolated cells

Summary In small leaf explants fromNautilocalyx lynchii (Hook. f.) Sprague (Gesneriaceae) the vacuolated epidermal cells divide after 3–4 days. Most cells divide periclinally, but longitudinal and transverse divisions are also found. Before mitosis the cells form a phragmosome (PS), a cytoplasmic structure which contacts the cell cortex at the future division site. An experimental approach was used to find out at which time the plane of cell division becomes fixed: prior to or during the formation of a PS.When 3 day-old explants were divided into two parts by a longitudinal cut, a high percentage of the cells near the wound divided longitudinally. Cells which already had a PS at the time of wounding most often divided in the plane of the PS. Some of the cells with a non-longitudinal PS, however, formed a longitudinal cell wall after the replacement of the original PS by a longitudinal PS.The observations show that most cells which had not yet formed a PS could be induced to form a cell wall in a new direction. As soon as the formation of the PS had started, however, it became more difficult to induce a change in the plane of cell division. These results suggest that the division site is chosen during the formation of the PS.Abbreviations BMT band of microtubules - DIC differential interference contrast microscopy - l longitudinal - l-o longitudinal-oblique - MT microtubule - p periclinal - PM prometaphase - PPB preprophase band - PS phragmosome - t transverse - t-o transverse-oblique     

A two thousand year history of a northern Swedish boreal forest stand

Abstract. Successional processes within northern Swedish boreal forest are investigated for the last 2000 years by analysis of pollen, charcoal fragments and insect remains preserved in a deep mor humus layer on a small island in a large lake. Frequent disturbances by fire, blow-down, insect attack and flood have been the chief influence on vegetation dynamics. The gradual climatically induced replacement of Betula pubescens by Picea abies is superimposed on the disturbance dynamics. Picea first became established during the Little Ice Age, which was the longest period without fire in the record. A thin layer of volcanic ash from Iceland confirms the chronology at the onset of the Little Ice Age. The present importance of Pinus sylvestris and Ericaceae on the island is a product of a recent increase in disturbance frequency. The disturbance regime ensures that the communities stay in equilibrium with prevailing climate.     

European post-glacial forests: compositional changes in response to climatic change

Abstract. Multivariate analysis of an extensive palyno-logical database for Europe has enabled reconstruction of broad-scale vegetation history. Whereas many major features of present vegetation patterns were established early in the Holocene, floristic composition of the forests has changed continuously up to the present day. For example, although ‘mixed deciduous forests’ had reached approximately their present extent in northwest Europe by 8000 B.P., Tilia peaked in abundance in these forests during the middle post glacial, whereas Pinus was most abundant in these forests during the early post-glacial and Fagus increased in abundance only in recent millennia. Pollen-climate response surfaces for major pollen taxa show how their distribution and abundance patterns relate to contemporary climate. Past forest-compositional changes were responses to climatic changes, the nature of which can be inferred from pollen-climate response surfaces. Post-glacial climate changes have been different in magnitude and direction in different regions of Europe. For example, in recent millennia the vegetation changes indicate decreasing summer temperatures in northern Europe but increasing summer temperatures in the Mediterranean region. The way in which vegetation responded to past climatic changes gives insight into the likely response of vegetation to future climate changes induced by the ‘greenhouse effect’.     

CHANGES IN CELL COMPOSITION AND LIPID METABOLISM MEDIATED BY SODIUM AND NITROGEN AVAILABILITY IN THE MARINE DIATOM PHAEODACTYLUM TRICORNUTUM (BACILLARIOPHYCEAE)1

The effects of nitrogen starvation in the presence or absence of sodium in the culture medium were monitored in batch cultures of the marine diatom Phaeodactylum tricornutum Bohlin. During nitrogen starvation in the presence of sodium, cell nitrogen and chlorophyll a decreased, mainly as a consequence of continued cell division. These decreases were accompanied by decreases in the rates of photosynthesis and respiration. There was no change in either cell volume or carbohydrate, but both carbon and lipid increased. During nitrogen starvation in the absence of sodium, cell division ceased. Cell nitrogen and chlorophyll a remained constant, and respiration did not decrease, but the changes in the photosynthetic rate and the lipid content per cell were similar to cultures that were nitrogen-starved in the presence of sodium. The carbon-to-nitrogen ratio increased in both cultures. Nitrogen, in the form of nitrate, and sodium were resupplied to cultures that had been preconditioned in nitrogen- and sodium-deficient medium for 5 d. Control cultures to which neither nitrate or sodium were added remained in a static state with respect to cell number, volume, and carbohydrate but showed slight increases in lipid. Cells in cultures to which 10 mM nitrate alone was added showed a similar response to cultures where no additions were made. Cells in cultures to which 50 mM sodium alone was added divided for 2 d, with concomitant small decreases in all measured constituents. Cell division resumed in cultures to which both sodium and nitrate were added. The lipid content fell dramatically in these cells and was correlated to metabolic oxidation via measured increases in the activity of the glyoxylate cycle enzyme, isocitrate lyase. We conclude that lipids are stored as a function of decreased growth rate and are metabolized to a small extent when cell division resumes. However, much higher rates of metabolism occur if cell division resumes in the presence of a nitrogen source.     

The results of 2-year pollen monitoring of an urban network in Perugia,Central Italy

Continuous pollen monitoring of an urban network consisting of three stations has been undertaken for a period of 2 years in Perugia, central Italy. The aim has been to establish whether the Perugia pollen trap, active since 1983, is still representative of the area following recent urbanisation. Quantitative differences were found between the stations, reflecting different vegetational areas, but only slight differences were detected in relation to the timing of the principal period of pollination. Therefore, although individual pollen traps are necessary to characterize fully the different areas, one trap is sufficient to determine the key allergenic thresholds in the studied area.