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Many functional-ecological, morphological, and physiological-factors affect the occurrence of selffertilization. Six modes of self-pollination are distinguished. These differ in whether they utilize specialized flowers, whether they involve the transfer of pollen within or between flowers, whether they are autonomous or mediated by vectors, and their timing relative to opportunities for outcrossing. The various modes of selfing are subject to different structural constraints. Prepotency, the preferential success of cross-pollen in achieving fertilizations when it competes with self-pollen, influences the frequency of selffertilization in some species. The amount of self-fertilization may depend on environmental conditions and the vector species visiting each flower and may vary among the flowers of one plant. To gain information on the prevalence of autonomous self-pollination, 66 species for which the degrees of selfcompatibility and autofertility (seed set in isolation) have been published were surveyed. Partially selfincompatible species (in which the seed set is lower after self-pollination than after separate outcrosses) have on average lower autofertility than self-compatible species (in which self-and cross-pollinations succeed equally well), but some partially self-incompatible species have considerable autofertility and some self-compatible species have none. A number of features of floral morphology and phenology are associated with high Autofertility Indices. Introduction The comparison of self-and cross-fertilization is the central topic of floral biology. Following the discovery by Knight (1799) and Darwin (1868, 1876) that cross-fertilization is advantageous because it produces superior progeny, there was a period of intense activity in pollination biology in the later decades of the nineteenth century.Flower structures that encourage cross-fertilization and reduce self-fertilization were studied widely. After considerable debate during this period, it was also acknowledged that some plants are adapted to regular self-fertilization and have floral syndromes that contrast strongly with those associated with outcrossing (see Darwin [ 1876] and Henslow [1879] for divergent viewpoints and Miuller's [1883] brilliant resolution of the issue).Floral biology became popular again in the second half of the twentieth century. In this period, genetic studies have dominated comparisons of self-and cross-fertilization. The genetic studies have provided much-needed empirical information on several aspects of self-and cross-fertilization, including frequencies of self-fertilization (Barrett and Eckert 1990), the expression and causes of inbreeding depression (Charlesworth and Charlesworth 1987; Barrett and Charlesworth 1991), and the genetic structures of outcrossing and selfing populations (Brown 1990; Hamrick and Godt 1990; Ritland 1990). The theoretical effects of the mating system on the genetic structures of regularly selfing and outcrossing populations have also been analyzed widely (Brown 1990;Ritland 1990).

Dichogamy is the separation of the presentation of pollen and stigmas in time within a plant. It is a common but neglected feature of outcrossing angiosperms. Dichogamy has been almost universally interpreted as an outcrossing mechanism, but many dichogamous species are also self-incompatible (and sometimes also herkogamous and/or with unisexual flowers). In outcrossing species, there is almost invariably a clash between selection to place pollen and stigmas in similar positions for effective pollination and selection to keep the androecia and gynoecia apart to avoid interference between them. We suggest that the separation of pollen and stigmas acts in general to reduce this self-interference and it often also reduces self-fertilisation. Mechanisms preventing self-fertilisation primarily increase maternal fitness, whereas mechanisms avoiding self-interference primarily promote paternal fitness.Five independent ways of subdividing dichogamy are recognised: protandry or protogyny: intrafloral or interfloral dichogamy; complete or incomplete dichogamy; various intervals between the successive presentations of pollen and stigmas; asynchronous, hemisynchronous, or synchronous dichogamy (the latter of several subtypes). In a sample of British species, protandry is almost twice as common as protogyny in biotically pollinated species but protogyny is six times as common as protandry in abiotically pollinated species.To obtain testable hypotheses of the selective forces responsible for dichogamy, four selective forces that influence whether pollen or stigmas are presented first are examined. (1) Effectiveness inWe are pleased to dedicate this paper to our friend and colleague, Eric Godley, on the occasion of his retirement. avoiding self-fertilisation; (2) Selection for prolonged pollen presentation; (3) Optimal positions for dispatching and receiving pollen; (4) Interference between stamens and carpels, involved in seven different contexts: (a) The relative ease of moving androecia and gynoecia; (b) Vertical windpollinated inflorescences; (c) Vertical animal-pollinated inflorescences: (d) Refuge, trap, and brood blossoms; (e) One type of sporophyll facilitates the presentation of the other; (f) Stamen signals or rewards; (g) Post-presentation changes in flowers.The most important factor overall may be the relative ease of moving stamens and carpels after they have functioned. For many species there may be a combination of selective causes of the direction of dichogamy.Besides pollen-stigma interference, other types of interference (in which two activities obstruct each other because they have the same optima) and conflicts (in which two activities have divergent optima) occur in plants.

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Herkogamy is the spatial separation of pollen presentation and pollen receipt within or between blossoms of an individual plant. Several classes of herkogamy are recognised; these are defined by whether all blossoms are identical (homomorphic herkogamy), all blossoms dispatch and receive pollen but reciprocal forms occur (reciprocal herkogamy), or some or all blossoms perform only one function (interfloral herkogamy). Within homomorphic herkogamy, unordered herkogamy, in which pollinator contacts with stigmas and pollen within a blossom are many and occur in no particular sequence, is distinguished from ordered herkogamy in which contacts are few and ordered. For ordered herkogamy further divisions are based on the operation of the pollination mechanism.Herkogamy is usually interpreted as a mechanism which reduces self-fertilisation and promotes outcrossing. However, as many herkogamous plants are also self-incompatible, it is suggested that the various classes of herkogamy also function in part or solely as mechanisms which avoid interference between pollen receipt by stigmas and pollen dispatch from anthers. Although for herkogamous blossoms these two functions are separated in space, many such flowers control pollinator behaviour so that both pollination surfaces are contacted during a single visit. In dichogamous blossoms separation of pollen dispatch and receipt is temporal rather than spatial and this difference has several important consequences for the pollination biology of dichogamous and herkogamous blossoms.

SUMMARYIt is postulated that in one reproductive session the level of maternal expenditure of an angiosperm plant is determined by a temporal series of controls on the number of potential fruit in which an investm.ent is made. The serial adjustment hypothesis has three parts: A. The amount of maternal expenditure is regulated at many developmental units, particularly single flowers and fruit, at three principal sequential stages -the determination of flowers, the development of ovaries and the maturation of fruit. B. At each stage, the initiation or continuation of an investment requires an amount of available resources above a certain threshold. Hence maternal expenditure is continually adjusted to the resources available at each developmental site. C. The pattern of controls that maximizes the maternal fitness of a plant is selected. Many factors affect the relative advantages of regulation at the three stages, therefore species and sexual morphs appear to vary widely in their proportional use of the three stages. Regulation of flower determination has the general advantages of offering bidirectional adjustment of maternal investment, maintaining a constant ratio of maternal to paternal investment, and reducing wasted expenditure. The later stages, ovary and fruit regulation, allow secondary adjustnients of maternal investment in unpredictable circumstances and permit adjustment of the relative numbers of polliniferous and seminiferous flowers. The relative advantages of restricting the numbers of developing ovaries, or of maturing fruit, depend principally on the extent to which differences in the capacity of flowers to mature fruit are evident before anthesis.