Shoot development in Betula papyrifera. III. Long-shoot organogenesis

Buds and developing branches of Betula papyrifera were collected weekly from mature trees during three successive growing seasons. Material was prepared to show stages of bud inception, development, flushing, shoot expansion, and male inflorescence inception and early development. Stages of bud development were correlated with axillant leaf and subjacent internodal extension. Long-shoot buds develop in the axils of distal late leaves of the previous season's long shoot. The bud contains one rudimentary leaf with prominent stipules, two or three embryonic foliage leaves which flush as early leaves, and about five smaller primordia which expand in the spring as late leaves. Preformed primordia are produced in mid-May through to early August. A pause in the inception sequence occurs in June, corresponding to the period of rapid late-leaf and internodal expansion of axillant leaves and subjacent internodes. Long-shoot bud determination occurs in mid-July. True late leaves may form during shoot extension. Male inflorescence induction, which occurs before bud break, restricts late leaf production and results in the last-formed leaf primordia maturing as transitional leaves instead of foliage leaves. Male induction limits crown expansion by reducing sites for axillary bud formation. This reduces the photosynthetic potential of the shoot.

The shoot apex and the ontogeny of axillary buds in Cuminum cyminum L

The structure and plastochronic changes of the shoot apex, and the origin, development, procambialization, and vascular relationships of the axillary bud in Cuminum cyminium were investigated. Pre-leaf initiation, leaf initiation, and post-leaf initiation phases of the shoot apex are identified. The inflorescence is axillary. During flowering the main vegetative shoot apex is semispherical, stratified, and devoid of any distinction between the central and peripheral zones. The vegetative axillary bud is differentiated from the peripheral zone of the shoot apex at the second node. It is delimited by an arcuate shell zone which helps in changing the apical position of the bud to foliar. The emergence of the bud is effected by the meristematic activity of tunica and corpus cells. A single prophyll is formed at right angles to the axillant leaf. Usually the bud trace procambium is differentiated during prophyll initiation. Occasionally it may be seen earlier, but not in connection with the earliest visible bud meristem. There are four to six strands of the bud trace directly interconnecting not only the strands of the prophyll and axillant leaf traces but also those of the second or sometimes even the third bud leaf and the axillant leaf. The bud trace procambial connection is formed by basipetal and acropetal differentiation of procambium in which the bud meristem cells and vacuolated ground meristem cells below the bud are involved. The cells of the peripheral zone of the bud apex below the prophyll primordium procambialize in a basipetal direction. As a continuation from the strand of the axillant leaf trace, the adjacent vacuolated ground meristem cells below the bud acropetally differentiate into procambial cells in the direction of the basipetal procambium and they make connection with it. All the strands of the bud trace are not simultaneously developed. The vegetative and inflorescence buds show varying vascular relationships between the strands of the leaf traces and those of the bud traces. The node differentiated during the vegetative phase of the plant is trilacunar and the one formed at flowering time is tetra- or pentalacunar. The nature and number of bud trace strands, however, suggest fundamental similarities between vegetative and inflorescence buds.