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101.
Reasons for the absence of trees on flat valley-bottoms in the subalpine regions of south-eastern Australia were studied at Seventeen Flat near Currango and other sites in the Snowy Mountains of New South Wales. Temperature changes with height above terrain were measured at grassland and woodland sites by thermistors attached to a hydrogen-filled balloon. The occurrence of nocturnal temperature inversions above grassland was established. Minimum temperatures at 80 cm above ground were recorded for thirty-one weeks at thirteen stations along a transect across the grassland and extending into the woodland on either side. Minima were lower and frequency of temperatures below—9°C were higher in grassland than in woodland. Seedlings of subalpine woodland species and other eucalypts were planted at both grassland and woodland stations. At grassland stations seedlings of tree-line species showed frost injury within a short time from planting and none survived two winters. The death of planted trees at grassland stations, the high frequency of temperature inversions and the low minima recorded on grassland compared with woodland suggest that temperature is a primary cause of the absence of trees in grasslands below subalpine woodlands. 相似文献
102.
A Morphometric Analysis of the Redistribution of Organelles in Columella Cells of Horizontally-oriented Roots of Zea mays 总被引:1,自引:0,他引:1
In order to determine what structural changes in graviperceptivecells are associated with the onset of root gravicurvature,the redistribution of organelles in columella cells of horizontally-oriented,graviresponding roots of Zea mays has been quantified. Rootgravicurvature began by 15 min after reorientation, and didnot involve significant changes in the (i) volume of individualcolumella cells or amyloplasts, (ii) relative volume of anycellular organelle, (iii) number of amyloplasts per columellacell, or (iv) surface area or cellular location of endoplasmicreticulum. Sedimentation of amyloplasts began within 1 to 2min after reorientation, and was characterized by an intenselystaining area of cytoplasm adjacent to the sedimenting amyloplasts.By 5 min after reorientation, amyloplasts were located in thelower distal corner of columella cells, and, by 15 min afterreorientation, overlaid the entire length of the lower cellwall. No consistent contact between amyloplasts and any cellularstructure was detected at any stage of gravicurvature. Centrally-locatednuclei initially migrated upward in columella cells of horizontally-orientedroots, after which they moved to the proximal ends of the cellsby 15 min after reorientation. No significant pattern of redistributionof vacuoles, mitochondra, dictyosomes, or hyaloplasm was detectedthat correlated with the onset of gravicurvature. These resultsindicate that amyloplasts and nuclei are the only organelieswhose movements correlate positively with the onset of gravicurvatureby primary roots of this cultivar of Zea mays. Zea mays, root gravitropism, ultrastructure, morphometry, graviperception 相似文献
103.
104.
105.
106.
A new genus of freshwater hoplonemertean from New Zealand 总被引:3,自引:0,他引:3
Potamonemertes percivali , a new freshwater hoplonemertean from the Selwyn River, New Zealand, is described. It is more closely related to the antipodean prosorhochmid supralittoral and terrestrial genera Acteonemertes and Geonemertes than to the only other freshwater hoplonemertean genus Prostoma,* but possesses a distinctive combination of characters which establish it as a new genus and species. 相似文献
107.
On the sandy loam at East Mailing, moderate annual dressings of inorganic sources of nitrogen and potash exerted no consistent influence on the incidence of apple scab, even when the dressings were continued over an eleven-year period on established trees of four different varieties on M.IX rootstock. Likewise, no influence of such dressings continued over five seasons was shown on the incidence of apple canker on the rootstock M.VIII, which was grown under conditions favourable for severe infection.
There was evidence from early observations that grassing-down to excess can reduce the incidence of scab, but only at the expense of nitrogen starvation in the trees. Short-term mixed leys had no such effect.
The incidence of branch blister, a functional disease associated with adverse soil conditions and retarded vigour, was greatly reduced in the variety Cox's Orange Pippin by potash manuring, and was further influenced by the rootstock. 相似文献
There was evidence from early observations that grassing-down to excess can reduce the incidence of scab, but only at the expense of nitrogen starvation in the trees. Short-term mixed leys had no such effect.
The incidence of branch blister, a functional disease associated with adverse soil conditions and retarded vigour, was greatly reduced in the variety Cox's Orange Pippin by potash manuring, and was further influenced by the rootstock. 相似文献
108.
109.
Calcium and plant organelles 总被引:2,自引:0,他引:2
Abstract. The role of intracellular organelles in the regulation of cytosolic Ca2+ levels and whether changes in these levels affect organelle metabolism is considered. We have assessed the biochemical properties of the Ca2+ transporting systems in mitochondrial, chloroplast and microsomal fractions. It is proposed that although all of these organelles can transport Ca2+ to varying extents it would appear that in some tissues at least mitochondria do not play a significant role in the maintenance of cytosolic Ca2+ . The most important Ca2+ transporting systems are probably the ATP dependent Ca2+ extrusion across the plasma membrane and Ca2+ uptake by endoplasmic reticulum, as well as light driven Ca2+ uptake by chloroplasts. Changes in cytoplasmic [Ca2+ ] do appear to regulate the activity of NAD kinase in chloroplasts, the mitochondrial external NADH dehydrogenase and intra-mitochondrial glutamate dehydrogenase, all of which play a key role in plant cell metabolism. Since some of these enzymes are affected by primary stimuli such as light or hormones, it is concluded that Ca2+ may act as a second messenger mediating some of the primary responses. 相似文献
110.
D. MOORE 《The Annals of applied biology》1984,104(1):161-166
The importance of the silica content of the plant as a protection against dipterous stem-borers was studied. Levels of silica in the stubble were inversely related to the degree of infestation by stem-borers in 13 varieties of established swards of Italian ryegrass and also in newly-sown swards of four varieties. There were significant reductions in larval colonisation of Italian ryegrass (cv. RvP) grown on soil watered with sodium silicate solution. A small range of silica content in the stubble was related to large differences in the level of attack by stem-borers. 相似文献