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51.
Summary We looked at the relationship between available area, as defined by Thiessen polygons around individual plants, and plant size and mortality in even-aged green-house populations of Lapsana communis L. Polygon area was a good predictor of plant weight in these populations. After nine weeks growth, just prior to the onset of self-thinning, the dry weight of plants was directly proportional to the square root of polygon area. After the onset of selfthinning, plant weight appeared to be directly related to polygon area to the 3/2 power. Plants in small polygons were much more likely to die than those in larger areas. Thinning changed the frequency distribution of polygon sizes from highly skewed and unequal to normal and more equal, while inequality in surviving plant sizes did not appear to be affected by thinning. 相似文献
52.
The meaning and measurement of size hierarchies in plant populations 总被引:16,自引:0,他引:16
Summary The term size hierarchy has been used frequently by plant population biologists but it has not been defined. Positive skewness of the size distribution, which has been used to evaluate size hierarchies, is inappropriate. We suggest that size hierarchy is equivalent to size inequality. Methods developed by economists to evaluate inequalities in wealth and income, the Lorenz curve and Gini Coefficient, provide a useful quantification of inequality and allow us to compare populations. A measure of inequality such as the Gini Coefficient will usually be more appropriate than a measure of skewness for addressing questions concerning plant population structure. 相似文献
53.
Young queens of V. orientalis collected from nests in the field at the end of the season, just before the hornets naturally enter hibernation, were evaluated for longevity under varying laboratory conditions. Queens kept under full illumination and heating had a briefer life span than did queens kept under full illumination alone or under complete heating alone. All, however, were shorter lived than control queens kept under the thermal and photoperiodic conditions prevailing at that time in nature. Feeding of theophylline to the queens caused them to emerge from hibernation and succumb to an early death. Feeding of allopurinol to the queens diminished their activities relative to control queens but did not abbreviate their life span compared to the control queens. 相似文献
54.
The regional distribution of L-homocysteine (Hcy) was determined in brains from mouse, rat, guinea pig, and rabbit, using a sensitive radioenzymatic assay. Large interspecies variations in the Hcy content in various parts of the brain were observed, but cerebellum contained the highest amount in all species investigated. In the rat the amount of Hcy in cerebellum (6.4 nmol/g) was about sixfold higher than in most other parts of the brain, whereas in the mouse and guinea pig the amount in cerebellum (about 1 nmol/g) was only twofold higher than in the other brain regions. There was a remarkably high level of Hcy in all regions of the rabbit brain (4-10 nmol/g); the highest concentration was found in the cerebellar white matter. In this species the amount of Hcy in all brain regions examined exceeded that in the liver. 相似文献
55.
Theoretical effects of transmembrane electroneutral exchange on membrane potential 总被引:2,自引:2,他引:0 下载免费PDF全文
Transmembrane electroneutral transport mechanisms [e.g., Na/H exchange, Cl/HCO3 exchange, (K + Cl) cotransport] have recently been identified in a wide variety of cell types. If these exchanges sum to give a net electroneutral Na/K exchange, they may hyperpolarize the membrane potential beyond the value calculated from the Mullins-Noda equation, provided the cell maintains steady state intracellular ionic concentrations. In extreme circumstances, the membrane potential could hyperpolarize beyond the potassium reversal potential. This effect is mediated by the electrogenic Na/K pump. If either Na or K exchanges electroneutrally against a third ion (e.g., Na/Ca exchange), then the exchange may depolarize the membrane potential. 相似文献
56.
Antibodies were elicited in rabbits to the dinucleotide, dpTpA, conjugated to a carrier protein. Among a few deoxyribomono- and oligonucleotides tested for binding to the antibodies by nitrocellulose tilter binding assay, only dpTpA and dpTpA containing oligonucleotides showed binding. The inhibition of the binding of 3H-dpTpA to the antibodies by various nonradioactive mono- and oligonucleotides showed that the antibodies were sequence specific and recognized the whole molecule of dpTpA. dpTpA specific antibodies were purified by a two step affinity chromatography. By competition studies, it was found that the purified antibodies bound denatured DNA at dpTpA sequences. The antibodies did not bind RNA. 相似文献
57.
Kenneth R. Miller Jules S. Jacob Ursula Smith Stephen Kolaczkowski Michael K. Bowman 《Archives of microbiology》1986,146(2):111-114
The basic cellular organization of Heliobacterium chlorum is described using the freeze-etching technique. Internal cell membranes have not been observed in most cells, leading to the conclusion that the photosynthetic apparatus of these organisms must be localized in the cell membrane of the bacterium. The two fracture faces of the cell membrane are markedly different. The cytoplasmic (PF) face is covered with densely packed particles averaging 8 nm in diameter, while the exoplasmic (EF) face contains far fewer particles, averaging approximately 10 nm in diameter. Although a few differentiated regions were noted within these fracture faces, the overall appearance of the cell membrane was remarkably uniform. The Heliobacterium chlorum cell wall is a strikingly regular structure, composed of repeating subunits arranged in a rectangular pattern at a spacing of 11 nm in either direction. We have isolated cell wall fragments by brief sonication in distilled water, and visualized the cell wall structure by negative staining as well as deep-etching.Abbreviations PF
protoplasmic fracture face
- EF
exoplasmic fracture face 相似文献
58.
59.
Effect of oestradiol and tamoxifen on the testosterone response in male rats to a single injection of hCG 总被引:1,自引:0,他引:1
A single s.c. injection of hCG (100 i.u.) produced a biphasic serum testosterone response in adult male rats, peaks being noted at 2 h (24 ng/ml) and 3 days (16 ng/ml). The levels fell to control during the intervening interval (8 ng/ml), although there were elevated levels of serum hCG. Maintenance of high oestradiol levels by a s.c. injection of 50 micrograms oestradiol benzoate given on Day 2 after the initial hCG injection failed to prolong the refractory period and the secondary peak of testosterone (16 ng/ml) occurred on Day 3. Administration of the antioestrogen, tamoxifen (2 mg or 3 micrograms), 24 h before or simultaneously with hCG did not prevent testicular refractoriness in vivo because serum testosterone levels still declined after 2 h to reach a nadir at 2 days. The basal in-vitro testosterone production by decapsulated testes from animals injected with hCG was enhanced at 2 h. Stimulation by hCG increased the amount of testosterone produced (X 1.5 that in controls). By 12 h basal production decreased and there was no further increment in testosterone in the presence of hCG. This refractoriness to further hCG stimulation prevailed until Day 3, but the total production of testosterone fell so that at 24 h and 2 days testes were producing basal amounts of testosterone. Testes recovered from refractoriness at 4 and 5 days, when basal and stimulated testosterone production were greater than in controls. Injection of 50 micrograms oestradiol benzoate at 2 days did not prolong the in-vitro refractory period and 2 mg or 3 micrograms tamoxifen had no effect on the in-vitro steroidogenic activity, since testes were still refractory to further hCG stimulation from 12 h to 3 days. The results of the present study do not support the hypothesis that oestradiol is involved in the hCG-induced refractoriness of the Leydig cell. The nadir between the peaks of serum testosterone in vivo corresponds to the period during which the testis is refractory to in-vitro stimulation by hCG. 相似文献
60.
Summary The apex of the root has been proposed as the site of maximum nutrient uptake, a view based on the accumulation of some ions
in the cells of apical meristem4. In this study, intact roots were grown across a thin agar sheet containing a radioisotope of iron. Uptake was then measured
quantitatively by autoradiography of the dried agar once the root had been removed. Uptake patterns were not observed to occur
around the roots of plants that had been supplied with sufficient iron. Around the roots of plants showing severe irondeficiency
symptoms, patterns consistently showed depletion of iron in the agar starting at 3.5 cm behind the tip and continuing to the
point of complete suberization of the endodermis. No uptake was observed around the apical 3.5 cm of the root, although the
highest concentrations within the root have centered on the apex.
Agronomy Paper No.869, New York State College of Agriculture. 相似文献