Within each internode and about four plastochrons after its formation, an intercalary growth area appears, leading to a continued lengthening due essentially to anticline mitoses. It temporarily looks like a cambium. The blade of the leaf which is directly above the internode is absolutely necessary for the formation of this growth area. Xylem integrity of the cauline vascular bundle connected with this leaf is also required. Within each leaf, the presence of the blade and the integrity of vascular bundles are also indispensable for the lengthening of the petiole, whose mitotic system resembles the internode's. The hypothesis of a blade influence on the mitotic lengthening system through mechanical pressure due to the vascular development is discussed. Key words: internode, petiole, blade, vascular tissues, Manihot esculenta, intercalary growth.

MEDARD, R. 1994. Determination de 1'Cbauche foliaire, du bourgeon axillaire et de I'entre-noeud chez le Manihot esculenta : Ctude microchirurgicale. Can. J. Bot. 72 : 1329-1342. L'Ctude microchirurgicale permet de dCterminer, chez le Manihot esculenta, la date de la dktermination de la feuille, plus de deux plastochrones avant sa surrection, ainsi que celle de son bourgeon axillaire, contemporaine du dCbut de la surrection. Des lesions effectuCes sur le trajet du futur cordon procambial de l'initium foliaire le plus bgC provoquent un retard de la surrection de cet initium, laquelle s'effectuera aprbs celle de l'initium suivant. Ce retard provoque une inversion de 1'Ctagement vertical sur la tige adulte des deux feuilles qui rCsultent du dCveloppement de ces deux initiums. I1 y a donc formation, lors de chaque surrection foliaire, d'un intervalle de mCristbme ou prC-entre-noeud mis en place au-dessus du premier primordium. Cet intervalle sera ulterieurement t i l'origine de l'entre-noeud. L'espacement vertical des feuilles ne rCsulte donc pas necessairement de dkterminations CchelonnCes verticalement sur l'apex. Par ailleurs, le positionnement de l'initium foliaire en voie de dktermination est influencC par des informations qui circulent dans l'apex, notamment partir du premier primordium en surrection. La nature de ces informations est discutCe. L'ensemble des risultats obtenus est analysC la lumibre des problbmes de phyllotaxie.Mots c l b : manioc, feuille, initiation foliaire, mCristbme apical, phyllotaxie, entre-noeud.MEDARD, R. 1994. DCtermination de 1'Cbauche foliaire, du bourgeon axillaire et de I'entre-noeud chez le Manihot esculenta : Ctude microchirurgicale. Can. J. Bot. 72: 1329-1342. A microsurgical study on Manihot esculenta shows that foliar initiation occurs more than two plastochrons before bulging out. Initiation of the axillary bud of the leaf occurs at the beginning of bulging out. Injuries produced along the path of the future procambial bundle of the older foliar initial create a delay in the bulging out of this initial, which will occur only after the bulging out of the following initial. This delay generates a reversal of the final vertical stem spacing of the two leaves that emerge from those two initials. Therefore, with each leaf bulging out, a preinternode appears above the first primordium. The vertical spacing of leaves does not necessarily result from the vertical spacing of initials. Moreover, the position of the foliar initial is affected by information moving in the apex, especially from the bulging out of the first primordium. The nature of this information is discussed and the results obtained are analysed in the light of the literature on phyllotaxy.

Le dCveloppement du bourgeon axillaire du Manihot esclrlenta. Can. J . Bot. 70 : 2041-2052. L'appareil vkgktatif atrien du manioc est constituC de deux sortes d'axes : sylleptiques et proleptiques. Les axillaires, aprks dktermination, peuvent suivre deux types de dtveloppement, soit un dCveloppement anticipt B l'origine des axes sylleptiques, soit un dtveloppement proleptique. Les differences morphologiques entre les deux categories d'axes s'expliquent facilement par les diffkrences d'tvolution des axillaires. Le dtveloppement proleptique s'effectue en quatre ttapes : ( i ) une phase de dttermination, (ii) une phase de latence intramkristematique, (iii) une phase d'organogenkse et (iv) une phase d'inhibition par dominance apicale. Les deuxibme et quatribrne phases n'existent pas dans le dtveloppement anticipt. La phase de latence intramkristematique est due B I'existence d'une double prtstance de l'kbauche foliaire axillante et du point vkgktatif susjacent. L'hypothkse d'un contrble de la determination et de la latence des axillaires depuis le mtristkrne apical caulinaire est discutee. Les contr8les des deux interruptions de dkveloppement, latence intramCristCmatique prtcoce et arrCt tardif par dominance de l'ensemble de l'apex, ne s'exercent pas a la mCrne Cchelle et ne sont vraisemblablement pas de mime nature. Mots clis : bourgeon axillaire, tige, dtveloppement, manioc, pioleptique, sylleptique. MEDARD, R., SELL, Y., and BARNOLA, P. 1992. Le dkveloppement du bourgeon axillaire du Marlihot esc~rlerzta. Can. J. Bot.70: 2041 -2052. The vegetative aerial apparatus of cassava consists of two kinds of axes: sylleptic and proleptic. After its determination, an axillary meristem may have either an advanced development, which characterizes a sylleptic axis, or a later development, which characterizes a proleptic axis. The morphological differences between the two categories of axes are easily explained as the result of differences in development. Proleptic development undergoes four stages: ( i ) determination, (ii) first latent period in the apical meristem, (iii) organogenesis, and (iv) second latency. The second and fourth stages do not occur in the advanced form of development. The first latent stage is due to a double precedency of both the upper part of the apical meristem and the axillary leaf primordium. The hypothesis of a control of the determination and the first latency of the axils from the whole apical meristem is discussed. The regulation of the two types of interruptions of development do not take place on the same scale and are probably not of the same type.

The purpose of this work was to investigate the basis of early dorsiventrality of the leaf and to determine what effects mature dorsiventral leaf characteristics, such as collateral bundles or lamina with adaxial palisade parenchyma and abaxial spongy parenchyma. Dorsiventrality was expressed initially by the differentiation of the primordium into two areas, one abaxial and the other adaxial, in contact with each other. From this contact and through meristematic activity arise the collateral bundles and the lamina. Any significant injury to the primordia caused total or partial disappearance of the more delicate adaxial area and, consequently, total or partial lack of collateral bundles and (or) lamina. This modification led to the development of various abnormal leaves, such as centric organs, scyphi, and leaves with round-tipped lobes. Minor injuries restricted to the abaxial–adaxial contact could induce lamina teratogenesis in the form of organoid enations. These results indicate that this contact controls the organogenesis of the lamina.

Inside the distal part of a leaf primordium approximately 12 cells thick and differentiated into distinct abaxial and adaxial areas, the following events occur simultaneously, at an early critical stage, in the middle of the second plastochron of leaf development: (i) determination of the differentiation of the two layers of cells bordering the abaxial–adaxial contact area into palisade and spongy parenchyma; (ii) determination of the ability of these two tissues to react to a triggering of mitotic activity, which enables them to initiate the lamina formation; (iii) determination of the formation, between the two tissues, of a system that inhibits dorsiventral mitosis and constitutes the foundation for lamina development. From the fifth plastochron, this threefold control system will lead to the organogenesis of the lamina. Depending on the stage of development, two kinds of anomalies occur on the lamina when the abaxial–adaxial contact area is disturbed. Disturbance during the early critical stage results in organoid enations and later disturbance in light and dark spots. These anomalies are comparable to those resulting from severe virus infections. The above process of development is not compatible with the classical model of lamina development.