ORIGIN AND DEVELOPMENT OF AXILLARY BUDS: SYRINGA VULGARIS L.

A SURVEY of botanical literature shows that the origin and the early stages in the development of axillary buds have been investigated a number of times, yet the later phases of development have, in contrast, received comparatively little attention. The works of Koch (1893), Schmidt (1924), and Louis (1935) are especially noteworthy for their detailed accounts of the initial stages in bud formation. Recently several other papers (Majumdar on Heracleum, 1942; Majumdar and Datta on Heracleum and Leonurus, 1946; Reeve on Garrya, 1943; Wardlaw on Matteuccia, Onoclea, and Dryopteris, 1943a, 1943b, 1946; Sterling on Sequoia, 1945a, 1945b, and on Pseudotsuga, 1947; Miller and Wetmore on Phlox, 1946) have further added to our knowledge on this subject, though on widely separated groups of plants. With few references to the later stages of development, especially vasculation, it has not been possible to make generalizations or to formulate a clear concept of bud ontogeny. The differentiation of xylem and phloem in axillary buds is considered in only one paper (Miller and Wetmore, 1946). Further, there has existed a marked divergence of opinion on some of the basic concepts of bud formation; the differences concern such fundamental problems as the origin of bud primordia, the origin of bud traces, and the differentiation of the procambium. With incomplete information on certain developmental stages on the one hand, and with contrasting descriptions and interpretations on the other hand, it seemed desirable that a comprehensive study of the main phases of bud development be made. In this first paper of a series which is concerned with axillary buds of plants with different phyllotactic patterns, different nodal connections, and different bud arrangements, attention is centered upon the origin, the development of general form, and the vasculation of axillary buds of Syringa vulgaris. Consideration is given to the timing and the orderly succession of the progressive developmental stages and to the nature of the vascular strands connecting axillary buds with the vascular tissue of the main axis. MATERIAL AND METHODS.-Syringa vulgaris (fig. 1) was selected as a species for this first study because the plant has opposite leaves, a condition favorable for sectioning and examination of nodal structure; and because the leaf traces are unilacunar, the least complex type of vascular connection. Collections were made every 3 or 4 days or weekly during March and April, 1946, at the time when the buds were opening and during the early period of shoot elongation. Additional material was 1Received for publication September 8, 1948. The writer wishes to express her sincere appreciation and gratitude to Professor Ralph H. Wetmore for his kindly interest, his guidance, and constructive criticism throughout the course of this investigation. obtained at intervals throughout the late spring, summer, and fall months of 1945 and during the spring and summer of 1946. Buds and young twigs were fixed in Craf solution, dehydrated in a series of n-butyl alcohol (Zirkle, 1930), and embedded in rubberized paraffin. Serial sections, transverse and longitudinal, were cut 8 and 10 , thick. Two staining procedures were used: Heidenhain's iron-alum haematoxylin with safranin as the counterstain (Jeffrey, 1917), ......~~~~~~.. ........ . . , J......,