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31.
Snow (1931) showed that the fate of a branch of a plant that had suffered local herbivory could be determined by correlative effects of other branches. This neglected work was continued in the current study, herbivory being simulated by the removal of leaves of different ages from pea plants with two branches. A damaged branch was suppressed when an undamaged alternative branch was present; otherwise the damage never prevented continued development. The removal of mature leaves had a smaller effect than the removal of immature, expanding leaves. When leaves were removed from both branches it was the branches that suffered less damage to their immature leaves that continued shoot development. Branches from which all photosynthetic leaves were repeatedly removed developed only when they retained their immature leaves and remained dominant, inhibiting the development — but not the photosynthesis — of the other branch on the same plant. Accounting for these results requires mechanisms that compare the different branches of the plant and select for development the ones that have the greatest potential for future, rather than present, photosynthesis. It is concluded that a compartmentalized or sectorial distribution of essential substrates can be modified by correlative relations that are probably mediated by hormones. 相似文献
32.
33.
The basic organization of the Plathelminthes 总被引:2,自引:2,他引:0
Ulrich Ehlers 《Hydrobiologia》1995,305(1-3):21-26
The basic organization of the Plathelminthes is summarized. Special attention is given to epidermal structures, musculature, extracellular matrices, nervous system and sensory structures, digestive system, totipotent stem cells, protonephridia, reproductive system and life cycle. The latest common ancestor of the Plathelminthes lacked any parenchymal cells and tissues; Plathelminthes do not represent Parenchymia. Discussions concerning the relationships of the Plathelminthes with other Metazoa must be based on the characteristics of the plathelminth stem species. 相似文献
34.
Shneior Lifson 《Journal of molecular evolution》1997,44(1):1-8
Theories of the origin of life have proposed hypotheses to link inanimate to animate matter. The theory proposed here derived
the crucial stages in the origin of animate matter directly from the basic properties of inanimate matter. It asked what were
the general characteristics of the link, rather than what might have been its chemical details. Life and its origin are shown to be one continuous physicochemical process of replication, random variation, and natural
selection. Since life exists here and now, animate properties must have been initiated in the past somewhere. According to
the theory, life originated from an as yet unknown elementary autocatalyst which occurred spontaneously, then replicated autocatalytically. As it multiplied to macroscopic abundance, its replicas
gradually exhausted their reactants. Random chemical drift initiated diversity among autocatalysts. Diversity led to competition. Competition and depletion of reactants slowed down the rates of net replication
of the autocatalysts. Some reached negative rates and became extinct, while those which stayed positive ``survived.' Thus
chemical natural selection appeared, the first step in the transition from inanimate to animate matter. It initiated the first animate property, fitness, i.e., the capacity to adapt to the environment and to survive. As the environment was depleted of reactants, it was enriched with sequels—namely, with decomposition products and all other products which accompany autocatalysis. The changing environment exerted
a selective pressure on autocatalysts to replace dwindling reactants by accumulating sequels. Sequels that were incorporated
into the autocatalytic process became internal components of complex autocatalytic systems. Primitive forms of metabolism and organization were thus initiated. They evolved further by the same mechanism to ever higher levels of complexity, such
as homochirality (handedness) and membranal enclosure. Subsequent evolution by the same mechanism generated cellular metabolism,
cell division, information carriers, and a genetic code. Theories of self-organization without natural selection are refuted.
Received: 29 March 1996 / Accepted: 30 May 1996 相似文献
35.
Changes in DNA supertwist as a response of Bacillus subtilis towards different kinds of stress 总被引:1,自引:0,他引:1
Abstract The silent parD ( kis/kid ) stability operon of plasmid R1 is normally repressed by the co-ordinated action of the Kis and Kid proteins. In this report it is shown that a mutation in repA , the gene of the plasmid replication protein, that reduces two-fold the copy number of the plasmid, leads to the derepression of the parD system. This derepression can be prevented by a suppressor mutation in copB, a copy number control gene of plasmid R1, that increases the efficiency of replication of the repA mutant. Derepression of the wild-type parD system leads to high plasmid stability. These data show the activation of a plasmid stability operon by a mutation that reduces the efficiency of wild-type plasmid replication. 相似文献
36.
Approximately 52% of the nuclear genome of great millet(Sorghum vulgare) consists of repetitive DNA which can be grouped into very fast, fast and slow components. The reiteration frequencies of
the fast and slow reassociating components are {dy7000} and 92 respectively. Approximately 90% of the genome consists of repeated
sequences interspersed amongst themselves and with single copy sequences. The interspersed repeat sequences are of three sizesviz. > 1·5 kilobase pairs, 0·5–1·0 kilobase pairs and 0·15–0·30 kilobase pairs while the size of the single copy sequences is 3·0
kilobase pairs. Hence the genome organization of great millet is essentially of a mixed type
NCL communication No. 3527. 相似文献
37.
Christian Smoliner 《Plant Systematics and Evolution》1984,146(3-4):249-264
The new parasitic algaChlorosphaeropsis epidemica is a member of theChlorosarcinales and lives endophytically in the leaf-tissue ofPotamogeton species. It forms stacks of cells and appears in great numbers, especially in dying leaf-blades. Sexual reproduction is described for the first time in this genus. Apart from its endophytic way of life,Chl. epidemica is characterized by mononuclear cells with a parietal chloroplast of irregular thickness and with a few perforations, sometimes of complicated reticulate shape, and with 1–5 pyrenoids. The usually sphaerical gametes (respectively zoospores) are provided with two equal flagella, a parietal plastid, one pyrenoid and one stigma. In connection with the asexual reproduction the terms vegetative division and desmoschisis are critically discussed. 相似文献
38.
Summary The pattern of fatty acid release from rat synaptic membranes in the presence of phospholipase A2 (Vipera russelli) was compared to that from liposomes comprised of phospholipids. Phospholipase A2 more readily attacked myelin and synaptic membranes than liposomes prepared from total phospholipids derived from myelin. Although hydrolysis of liposomal phospholipids occurred in the absence of added calcium, the presence of 2mm CaCl2 or 2% bovine serum albumin significantly enhanced the phospholipase attack of liposomes, but not synaptic membranes or myelin. Phospholipase exhibited a marked preference for phospholipids containing docosahexaenoic acid (226) in the synaptic membranes, while with liposomes the pattern of released fatty acid reflected the fatty acid composition in the two-position of the phospholipids. Although either calcium or albumin markedly increased the phospholipase hydrolysis of liposomes, neither affected the hydrolysis of synaptic membranes or the pattern of fatty acid release from liposomes. It was concluded that the nonlipid constituents, particularly the proteins, of biomembranes were responsible for the organization of the phospholipids and accounted for the observed differences between liposomes and synaptic membranes with respect to enzymic accessibility. 相似文献
39.
Summary Parts of the Wulst region of the chick embryo brain were maintained for 39 days in vitro. Processes of adjacent glial cell form zonulae occludentes and desmosomal junctions in the uppermost stratum of the cultures. Subjacent to this layer, in the neuropil, axodendritic synapses are abundant. 10–20 m below the surface the perikarya of glial cells and neurons are found. The latter form small clusters, plasma membranes of contiguous cells being directly apposed to each other. Axosomatic synapses terminate on the perikarya. Occasionally one terminal synapses on two nerve cells simultaneously. Two types of cilia arise from basal bodies in the cytoplasm of nerve cells. Type I is a slender protrusion of about 0.5 m in diameter, extending into the neuropil. On transverse sections it displays a 9 + 0 pattern of organization of paired micro tubules proximally, and an 8 + 1 configuration more distally. The length of the cilium is approximately 7 m. Type II cilia also originate in the neuronal cytoplasm. The structure of the proximal portion is identical with that of type I cilia. Toward the tip, however, the type II cilium is characterized by a bulbous enlargement which is filled with loosely folded membranes. The fine structural details of this cilium correlate closely to the outer segments of retinal photoreceptor cells during differentiation. The possible role of a light receptor in the Wulst region of birds, controlling biological rhythms, is discussed.
Acknowledgements. The authors wish to thank Mrs. H. Frenk for her expert assistance in tissue culture and electron microscopic techniques 相似文献
40.
Approximately 43–60% of the total genome in bovine, goat and sheep consisted of interspersed repeated and single copy DNA
sequences. Most of the interspersed repeated DNA sequences were 1500–2400 nucleotide pair long while a minor portion was more
than 4000 nucleotide pair long in goat and sheep and 3200 nucleotide pair long in bovine. About 1/3rd of single copy sequence
were interspersed and their length was in the range of 1000–1500 nucleotide pairs. 相似文献