Ce groupe se différencie principalement des Jungermaniacées par le mode d’ouverture en deux valves de la capsule, linéaire et pédicellée. Le gamétophyte est à peine plus grand que le sporophyte, qui touche presque le sol à sa base.
Écologie
Répandues sur tout le globe, les Bryophytes se rencontrent dans les biotopes les plus variés (montagnes, plaines, bord de mer, rochers, tourbières, sables). Il leur faut absolument, pour survivre, se développer et se reproduire, une période annuelle de grande humidité, mais leurs cellules supportent sans mourir d’être presque complètement déshydratées pendant d’assez longs moments et passent alors à l’état de vie ralentie. L’implantation des Muscinées est une étape importante dans la colonisation des surfaces nues, en particulier sur les rochers, où ces végétaux maintiennent une humidité relativement constante. Certaines espèces (Polytrichum, Sphagnum), en énormes colonies, forment de grands peuplements qui pompent l’eau de la profondeur (tourbières). Seule la partie supérieure est vivante, et, progressivement, les parties inférieures se décomposent pour former un sol nouveau : la tourbe. Cette dernière peut atteindre de nombreux mètres d’épaisseur et sert, dans les régions d’économie pauvre, de matériel de chauffage (Irlande). C’est un des premiers stades de la transformation des végétaux en matériel carboné minéral.
L’utilisation des Mousses est des plus réduites ; servant autrefois de matériel d’emballage très léger et bien élastique, la Mousse est maintenant concurrencée par les substances chimiques expansées. Parfois on s’en sert pour l’amélioration des sols des jardins. Les Sphaignes sont employées comme support pour des cultures spéciales dans les serres d’Orchidées et de Broméliacées. Hypnum triquetrum, grosse Mousse, sert aussi comme ornement dans les corbeilles de fleurs sèches, parfois artificiellement colorée.
J. M. T. et F. T.
J. Augier, Flore des Bryophytes (Lechevalier, 1966).
Bryozoaires
Animaux marins ou d’eau douce, formant des colonies fixées, dont les individus, très petits, portent une couronne de tentacules autour de la bouche et peuvent se rétracter dans une logette.
Par leur taille et leur aspect, certaines colonies de Bryozoaires (Bugula, Crisia) rappellent de petits végétaux, ce qui explique le nom donné au groupe (gr. bruon, mousse) ; en réalité, il ne contient pas que des colonies arborescentes : Flustra, Retepora forment des lames dressées, plus ou moins ramifiées ; Membranipora recouvre les algues d’une croûte blanche ; Alcyonidium donne des colonies massives et molles. Chez les formes verticales, la hauteur dépasse rarement dix centimètres ; chez les espèces encroûtantes, les dimensions peuvent être plus élevées. Les colonies se fixent sur des supports variés : rochers, coquilles, algues, etc.
Les Bryozoaires les plus nombreux et les plus diversifiés vivent dans les mers, près du littoral ; cependant, on en a trouvé à plus de 5 000 mètres de profondeur. Les formes souples (Flustra) ou à rameaux articulés (Bugula) se trouvent dans les zones soumises aux mouvements de la marée, les formes fragiles (Retepora) ou raides (Porella) dans les zones abritées ou profondes.
Chaque individu (zoïde) de la colonie mesure à peine un millimètre de long ; il vit dans une loge ou cystide, à paroi cornée ou calcaire, et s’y rétracte en cas de danger. Normalement, il laisse émerger une partie de son corps, ou polypide. Malgré sa petitesse, son organisation se révèle complexe et s’éloigne radicalement de celle des polypes de certains Cœlentérés, avec lesquels les zoïdes n’ont qu’une ressemblance toute superficielle ; entourant la bouche, les tentacules ciliés sont assimilés au lophophore des Brachiopodes et captent les micro-organismes alimentaires ; le tube digestif en U se termine par un anus en dehors de la couronne tentaculaire ; on observe une cavité cœlomique. Il n’y a pas d’appareil excréteur, et les déchets s’accumulent dans l’organisme ; on pense que leur excès entraîne la dégénérescence du polypide en un « corps brun » ; la cystide régénère un nouvel individu, qui évacuera par l’anus les restes de son prédécesseur.
Le plus souvent tous les individus d’une même colonie se ressemblent, mais on assiste chez plusieurs espèces, à un polymorphisme des zoïdes ; certains recueillent les œufs fécondés et les incubent (ovicelles), d’autres jouent un rôle défensif (aviculaires) ou nettoyeur (vibraculaires). Les zoïdes communiquent entre eux par un cordon, le funicule, qui pénètre dans chaque loge par un orifice ou par une plaque perforée.
Les individus sont hermaphrodites ou à sexes séparés. La fécondation a souvent lieu dans la cavité générale, ainsi que le début du développement qui, chez les Cyclostomes, s’accompagne de polyembryonie. Une larve ciliée se libère, puis se fixe en donnant un premier individu, point de départ d’une nouvelle colonie, par bourgeonnement successif.
Les Bryozoaires d’eau douce (Phylactolèmes) diffèrent par quelques points des marins : lophophore en fer à cheval ; parois des cystides jamais calcifiées ; développement de l’embryon dans une cavité incubatrice, avec une sorte de placenta ; formation de statoblastes, bourgeons capables de résister à la mauvaise saison, que la colonie ne supporte pas. La Cristatelle peut glisser lentement sur son support.
Les Bryozoaires sont connus depuis l’ère primaire, qui a vu s’épanouir deux groupes maintenant éteints. Les Chilostomes, les plus répandus à l’heure actuelle, n’apparaissent qu’au Jurassique.
Nous avons envisagé ici les Bryozoaires au sens strict, appelés aussi Ectoproctes, à l’exclusion des Endoproctes ou Kamptozoaires, qui leur ont longtemps été réunis. Ceux-ci sont des formes coloniales, minuscules, presque toutes marines, souvent fixées sur des animaux vivants mais sans en être parasites ; l’anus et la bouche sont situés à l’intérieur des tentacules, deux néphridies assurent l’excrétion, et il n’y a pas de cavité cœlomique.
Certains auteurs font des Ectoproctes et des Endoproctes deux embranchements distincts ; d’autres les réunissent aux Brachiopodes et aux Phoronidiens dans l’embranchement des Lophophoriens.
M. D.
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