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排序方式: 共有220条查询结果,搜索用时 15 毫秒
1.
Summary Fruit of the blackberry, Rubus pennsylvanicus Poir. (Rosaceae), were examined to determine variation in maturation characteristics. Maturation timing and rate varied greatly among individual fruits, resulting in a bi-colored fruiting display comprised largely of two maturation stages, pre-ripe (salmon and scarlet) and ripe (dark brown and black). While ripe fruit were generally larger and heavier than pre-ripe fruit, exhibiting greater fresh and dry fruit weight, diameter, water content, and total seed weight, no significant differences were found in energy content, i.e. numbers of calories per gram pulp, or in pulp:seed ratio. The differences between ripe and pre-ripe fruit appear to be due largely to an increase in water content and seed weight with maturity. The fact that little energetic benefit accrues to the preferential selection of ripe fruit suggests that bi-colored Rubus displays may be considered to be unicolored. 相似文献
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R. Haven Wiley Joe Poston 《Evolution; international journal of organic evolution》1996,50(4):1371-1381
When Darwin first proposed the possibility of sexual selection, he identified two mechanisms, male competition for mates and female choice of mates. Extending this classification, we distinguish two forms of mate choice, direct and indirect. This distinction clarifies the relationship between Darwin's two mechanisms and, furthermore, indicates that the potential scope for sexual selection is much wider than thus far realized. Direct mate choice, the focus of most research on sexual selection in recent decades, requires discrimination between attributes of individuals of the opposite sex. Indirect mate choice includes all other behavior or morphology that restricts an individual's set of potential mates. Possibilities for indirect mate choice include advertisement of fertility or copulation, evasive behavior, aggregation or synchronization with other individuals of the same sex, and preferences for mating in particular locations. In each of these cases, indirect mate choice sets the conditions for competition among individuals of the opposite sex and increases the chances of mating with a successful competitor. Like direct mate choice, indirect mate choice produces assortative mating. As a consequence, the genetic correlation between alleles affecting indirect choice and those affecting success in competition for mates can produce self-accelerating evolution of these complementary features of the sexes. The broad possibilities for indirect mate choice indicate that sexual selection has more pervasive influences on the coevolution of male and female characteristics than previously realized. 相似文献
4.
The pollen of all four genera of Gronovioideae—Cevallia, Fuertesia, Gronovia, and Petalonyx—was examined in light microscopy, and scanning and transmission electron microscopy. The pollen of Cevallia, of Fuertesia, and of Gronovia can be easily distinguished from each other and from all remaining Loasaceae. Only Petalonyx, with a striate tectum, shows a clear relationship to the Mentzelioideae and Loasoideae, the vast majority of which have striate-reticulate or striate tecta. The trichome data are mostly congruent with the pollen data: Cevallia, Fuertesia, and Gronovia each have a distinctive trichome not known to occur elsewhere in the family, while Petalonyx has only the common types. A cladistic analysis of Gronovioideae utilizing Mentzelia as the outgroup proposes that Cevallia, Gronovia, and Fuertesia are a sister group to Petalonyx within the subfamily. The relationships of the four genera to each other and of Gronovioideae to the Loasaceae are discussed. 相似文献
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Litman GW; Rast JP; Shamblott MJ; Haire RN; Hulst M; Roess W; Litman RT; Hinds- Frey KR; Zilch A; Amemiya CT 《Molecular biology and evolution》1993,10(1):60-72
Immunoglobulins are encoded by a large multigene system that undergoes
somatic rearrangement and additional genetic change during the development
of immunoglobulin-producing cells. Inducible antibody and antibody-like
responses are found in all vertebrates. However, immunoglobulin possessing
disulfide-bonded heavy and light chains and domain-type organization has
been described only in representatives of the jawed vertebrates. High
degrees of nucleotide and predicted amino acid sequence identity are
evident when the segmental elements that constitute the immunoglobulin gene
loci in phylogenetically divergent vertebrates are compared. However, the
organization of gene loci and the manner in which the independent elements
recombine (and diversify) vary markedly among different taxa. One striking
pattern of gene organization is the "cluster type" that appears to be
restricted to the chondrichthyes (cartilaginous fishes) and limits
segmental rearrangement to closely linked elements. This type of gene
organization is associated with both heavy- and light-chain gene loci. In
some cases, the clusters are "joined" or "partially joined" in the germ
line, in effect predetermining or partially predetermining, respectively,
the encoded specificities (the assumption being that these are expressed)
of the individual loci. By relating the sequences of transcribed gene
products to their respective germ-line genes, it is evident that, in some
cases, joined-type genes are expressed. This raises a question about the
existence and/or nature of allelic exclusion in these species. The
extensive variation in gene organization found throughout the vertebrate
species may relate directly to the role of intersegmental
(V<==>D<==>J) distances in the commitment of the individual
antibody-producing cell to a particular genetic specificity. Thus, the
evolution of this locus, perhaps more so than that of others, may reflect
the interrelationships between genetic organization and function.
相似文献
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In neonatal and young adult SH and WK rats, maturation of a functional unit consisting of sympathetic nerve terminals and smooth muscle (levator palpebrae) was assessed by measuring the contractile response to tyramine, an agent which releases endogenous norepinephrine from sympathetic nerve terminals. Responses in SH are comparable to WK at 5–6 days postnatally, smaller from the 8th through 19th day and larger in young adults (41–46 days old). Results indicate that functional maturation of the nerve terminal-smooth muscle complex is retarded in SH relative to WK during the 2nd and 3rd postnatal weeks. It is suggested that retardation in the neonatal SH rat is an expression of a genetic defect in the growth of the complex and that the enhanced response in young adult SH is a consequence of the neonatal abnormality. 相似文献
9.
D L Poston 《Social biology》1976,23(3):198-209
Using data from the 1965 U.S. National Fertility Study, white childless wives 30-54 years old were categorized as voluntarily, involuntarily, or temporarily childless. These 3 groups were then compared along many demographic, socioeconomic, marital, and religious lines. It was found that voluntarily and involuntarily childless wives were similar across most characteristics and both groups were different from the temporarily childless women. This last group is considerably younger than either of the other 2 groups; this fact may account for the differences between this and the other 2 groups. Other literature on childlessness is cited and compared. The difficulties with categorization of childless wives are discussed. Before it is generally concluded that differences do not exist between voluntarily and involuntarily childless wives, it will be necessary to study data from younger women who benefited from the "new fertility regime" of recent years. 相似文献
10.
Kate Bramham Carlos E. Poli-de-Figueiredo Paul T. Seed Annette L. Briley Lucilla Poston Andrew H. Shennan Lucy C. Chappell 《PloS one》2013,8(10)