Les feuilles contiennent de la chlorophylle. Elles sont le siège principal de la photosynthèse (assimilation chlorophyllienne) ; leurs particularités morphologiques et anatomiques reflètent leur adaptation à cette fonction. Leur large surface par rapport à leur volume facilite la captation des rayons lumineux et les échanges gazeux de la photosynthèse (absorption de gaz carbonique et rejet d'oxygène). Ces échanges se font principalement au niveau des stomates, surtout abondants à la face inférieure de la feuille, en connexion avec les espaces intercellulaires du parenchyme lacuneux (réseau assez lâche de cellules situé sous l'épiderme, du côté inférieur de la feuille). Du côté supérieur se trouve le parenchyme palissadique, formé de cellules jointives très riches en chloroplastes (organites cellulaires, siège de la photosynthèse). Le réseau des nervures correspond aux tissus conducteurs, qui apportent aux cellules de la feuille la sève brute (eau et sels minéraux) et emportent la sève élaborée enrichie des glucides fabriqués grâce à la photosynthèse.
Ces caractéristiques de la feuille (large surface pour une faible épaisseur, stomates abondants) entraînent une forte transpiration (ou évapotranspiration), c'est-à-dire la perte d'eau sous forme de vapeur. Celle-ci est nécessaire, notamment pour la circulation de la sève, mais doit être limitée pour éviter le dessèchement. Les adaptations de la feuille réduisant les pertes d'eau comprennent la fermeture des stomates en cas de déficit en eau, la localisation préférentielle de ceux-ci à la face inférieure de la feuille (protégée du soleil) et la présence d'une pellicule cireuse imperméable, la cuticule, sur l'épiderme de la face supérieure. Dans les régions arides, les feuilles de nombreuses plantes présentent une surface réduite et une cuticule cireuse épaisse.
Feuilles modifiées.
Les bractées sont les feuilles plus ou moins modifiées qui se trouvent à l'aisselle des fleurs ou des rameaux d'une inflorescence. Les pièces florales sont aussi de nature foliaire : du point de vue évolutif, elles dérivent de feuilles profondément modifiées. Les feuilles elles-mêmes peuvent subir diverses transformations en relation avec des adaptations particulières : feuilles charnues accumulant de l'eau chez les plantes grasses, feuilles ou folioles transformées en vrille (pois), en pièges à insectes avec des glandes digestives chez des plantes carnivores, en flotteurs chez des plantes aquatiques... Enfin, elles peuvent être réduites à des écailles et dépourvues de chlorophylle dans le cas de certaines plantes parasites (orobanche) et sur les tiges souterraines (rhizomes, bulbes, tubercules).
Henry
feuillet
Troisième réservoir gastrique des ruminants, situé entre le réseau et la caillette. SYN. : omasum, psautier.
Bougler/Gallouin
feuillu
Arbres dont les feuilles à limbe développé sont le plus souvent caduques.
Les feuillus sont en général des angiospermes, c'est-à-dire des végétaux dont les graines sont contenues dans une cavité close. À la différence des résineux, ils ne portent pas de cônes. Leur bois possède des vaisseaux et n'a pas de poches de résine. Leurs fibres courtes leurs donnent des qualités moindres pour la pâte à papier.
Les chênes représentent à eux seuls 31 % des surfaces forestières françaises. Viennent ensuite le hêtre (10 %) puis le châtaignier (3,6 %) et les autres feuillus (merisier, peuplier, érable, frêne, tilleul, sorbier, cytise, bouleau, charme, noisetier).
Les feuillus représentent encore les 2/3 de la forêt française. Ils en représentaient 80 % en 1875. Leur bois, de meilleure qualité que celui des résineux, est généralement destiné à des usages plus nobles (ébénisterie, menuiserie, plaquage, déroulage...) ; il est également payé plus cher. Par contre, ils sont moins productifs que les résineux et leur plantation est plus coûteuse.
Décourt
fève
1. Plante annuelle originaire du Moyen-Orient, cultivée pour ses grosses graines destinées à l'alimentation humaine et animale (espèce Vicia faba, famille des fabacées). 2. Graine de cette plante.
Avec le pois et la lentille, la fève est la plus vieille espèce légumière introduite en agriculture (il y a 10 000 ans). Il existe deux types de fève : la fève fourragère (Vicia faba minor) et la fève maraîchère (V. f. major). Chez cette dernière, les cultivars se distinguent par leur taille et leur degré de précocité.
Culture.
La fève est peu exigeante vis-à-vis du sol et du climat. Sa température optimale de croissance se situe à 18-22 °C, mais elle résiste au froid jusqu'à - 3 °C. La fève se cultive en pleine terre, souvent en intercalaire avec une autre culture dans les pays méditerranéens. Du sud au nord, le semis est effectué de novembre à avril-mai.
Maladies et ravageurs.
Les plus à craindre en culture sont le puceron noir de la fève (Aphis fabae), le mildiou, la rouille, l'ascochytose de la fève et le virus de la jaunisse apicale du pois.
Récolte et production.
Les gousses sont cueillies à partir de mars dans le Sud et de juin dans le Nord. La fève peut également se récolter en graines sèches. La fève est répandue dans les ceintures vertes et, dans le Sud, dans les potagers. L'Espagne est le 1er producteur de l'Union européenne, avec 130 000 t produites de novembre à mai. Le Royaume-Uni a une production estivale de 17 000 t.
Péron
féverole
Légumineuse annuelle très anciennement cultivée dans le bassin Méditerranéen (espèce Vicia faba, famille des papilionacées). SYN. : faverole, fèverole.
En raison de la richesse de sa graine en protéines, la culture de la féverole s'est progressivement étendue en Europe de l'Ouest, sous la pression des besoins pour l'alimentation animale.
La féverole est une légumineuse qui présente une assez grande variabilité morphologique. D'après la grosseur de la graine, on distingue trois sous-espèces : la féverole à petits grains (V. faba minor), la féverole à grains moyens (V. faba equina) et la féverole à gros grains, ou fève (V. faba major).
C'est une plante dont la taille peut dépasser 1 m. Ses feuilles sont glabres et composées de deux ou trois paires de folioles opposées de forme ovale. Son système radiculaire est développé et descend profondément dans le sol. Ses fleurs, de couleur blanche ou violacée, sont disposées par grappes de 2 à 9 fleurs à l'aisselle des feuilles. La pollinisation est allogame, assurée par les insectes pour environ 40 à 50 % des variétés de printemps et pour 30 à 40 % de celles d'hiver. Le fruit est une gousse verte en végétation, noirâtre à maturité et contenant quelques grains brun noirâtre.
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