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The Botanical Review

, Volume 43, Issue 2, pp 177–216 | Cite as

Secondary sex characters in plants

  • David G. Lloyd
  • C. J. Webb
Article

Summary

Sexually dimorphic seed plants have partial or complete separation of ovule and pollen functions into two separate sexes, here uniformly called male and female. Secondary sex characters (differences between the sexes in structures other than the androecia and gynoecia) of such populations are reviewed.

In a number of perennial species, males exceed females in vigor, growth rate or vegetative reproduction. In several monocarpic or short-lived polycarpic species, including hemp, spinach and species ofSilene, females are larger than males. In asparagus, males exceed females in total growth, but individual shoots of females are larger. In some long-lived species, the greater survival rate of males than of females contributes to a predominance of males, but in species ofSilene andRumex acetosa males have a higher mortality rate than females. Males and females sometimes have different microdistributions, and inMercurialis perennis andRumex acetosella this appears to be associated with different environmental optima of the sexes. Differences between males and females in habit, leaf characters and minor morphological features are described. In a considerable range of species, male inflorescences emerge or bolt earlier and males begin flowering sooner than females. Several authors have suggested that females spend proportionately more of their resources on sexual reproduction (have a higher reproductive effort) than do males. The only available data, those forRumex acetosella, support this. There are more reports of males flowering earlier in life and more frequently, having more numerous inflorescences and more flowers per inflorescence, than of females exceeding males in these features. Differences between individual male and female flowers are reviewed.

Secondary sex characters may be incidental consequences of the developmental or genetic basis of sex determination and not adaptive themselves, or they may be selected as optimal strategies for the sexes, or both kinds of explanation may simultaneously be applied. We propose that many of the observed differences between the sexes are of direct selective value in relation to the distinct roles of males and females in sexual reproduction. In particular, the benefits to males of increasing pollen production and to females of increasing ovule production differ in such a way that the optimal reproductive effort will often be less for males than for females; this appears to explain a number of secondary sex characters.

Keywords

Botanical Review Sexual Reproduction Reproductive Effort Female Flower Male Flower 
These keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.

Résumé

Chez les spermatophytes sexuellement dimorphes, les fonctions de l’ovule et du pollen sont plus ou moins complétement séparées en deux sexes distincts, uniformément appelés ici mâle et femelle. Les caractères sexuels secondaires de populations de ce type (différences entre les sexes autres que celles relatives à l’androcée et au gynécée) sont passés en revue.

Chez beaucoup d’espèces perennes, les mâles sont supérieurs aux femelles en vigueur et en vitesse de croissance ou de reproduction végétative. Chez plusieurs espèces monocarpiques comme chez plusieurs espèces annuelles polycarpiques, comme le chanvre, l’épinard ou certaines espèces deSilene, les femelles sont plus grandes que les mâles. Chez l’asperge, les mâles sont supérieurs aux femelles en biomasse totale mais les rameaux des femelles pris individuellement sont plus volumineux. Chez certaines espèces perennes, la longévité des mâles est supérieure à celle des femelles, de sorte que les mâles prédominent; cependant, chez les espèces deSilene et chezRumex acetosa, le taux de mortalité des mâles est plus élevé que celui des femelles.

Les mâles et les femelles ont parfois des micro-distributions différentes, ce qui correspond, chezMercurialis perennis et chezRumex acetosella, à des conditions de milieu optimum différentes suivant les sexes. Sont décrites des différences entre mâles et femelles en port, en caractéristiques foliaires et en détails morphologiques divers. Pour une gamme très étendue d’espèces, l’apparition de l’inflorescence et la floraison ont lieu plus tôt chez les mâles que chez les femelles. Divers auteurs pensent que les femelles dépensent, en proportion, davantage de leurs ressources pour la reproduction sexuée (elles consentent un plus grand effort reproductif) que ne le font les mâles. Les seules données disponibles, qui concernentRumex acetosella, confirment ce point de vue. La littérature fournit davantage de cas où les mâles fleurissent plus souvent, ont des inflorescences plus nombreuses et davantage de fleurs par inflorescence que les femelles que de cas où l’inverse se produit.

Des différences entre les fleurs des individus mâles et femelles ont été décrites concernant des caractères tels que la dimension des pétales, la production de nectar et les structures qui interviennent dans la reproduction des graines.

Il se peut qu’un caractère sexuel secondaire soit la conséquence fortuite du mécanisme génétique ou ontogénique de la détermination du sexe et ne soit pas adaptatif par lui-même; il se peut aussi qu’il résulte de la sélection pour une stratégie optimum pour les sexes; il se peut enfin que les deux types d’explications s’appliquent à la fois. Nous avons tendance à penser que beaucoup des différences observées entre les sexes ont une valeur sélective directement rattachée aux rôles différents des mâles et des femelles dans la reproduction sexuée. En particulier, les bénéfices qui résultent de l’augmentation de la production de pollen par le mâle et de l’augmentation de la production d’ovules par les femelles diffèrent de telle façon que l’effort reproductif optimum est souvent moindre pour les mâles que pour les femelles. Ceci parait pouvoir expliquer l’existence de nombreux caractères sexuels secondaires.

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Copyright information

© The New York Botanical Garden 1977

Authors and Affiliations

  • David G. Lloyd
    • 1
  • C. J. Webb
    • 2
  1. 1.Botany DepartmentUniversity of CanterburyChristchurchNew Zealand
  2. 2.Botany DivisionDepartment of Scientific and Industrial ResearchChristchurchNew Zealand

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