THE TRANSITION REGION IN THE SEEDLING OF RICINUS COMMUNIS: A PHYSIOLOGICAL INTERPRETATION

Abstract

The orientation of the xylem and phloem conducting strands differs in the root and stem regions of the axis of vascular plants. The vascular continuity of the entire axis is maintained, however, by an area between root and stem, where rearrangement of the bundle elements occurs. This so-called transition from root to stem structure has been a subject of interest to the plant anatomist since the days when Gerard, and later van Tieghem, followed in detail the changing paths of xylem and phloem throughout this particular zone. The work of van Tieghem and the later morphologists is summarized by Compton (8). Three types of transition are described by van Tieghem, and a fourth is added by Sargant (X7). Some of the more recent investigators retain the descriptive terms of van Tieghem, and talk of the splitting, and fusion of the vascular bundles. Kean (I5) describes the seedling anatomy of Mesenmbryanthe'n'iium in this manner. Eames and MacDaniels ( I I ) summarize the four main methods of division, rotation, and reunion of the conducting strands, and refer to a transition region where root and stem meet, and where the various parts of each organ mnerge into the other. Avery (i) identifies the seedlings of maize, oats, and wheat as representative of van Tieghem's three types of transition, but he considers these three to be probable variations of one main morphological type. A somewhat deeper interpretation of the root-stem transition phenomena is introduced when the subject is approached from the standpoint of phylogeny. If the that ontogeny recapitulates phylogeny is accepted, the first trace of vascular system to appear in the developing seedling is to be regarded as the prototype of the conducting system of vascular plants in general. The seedling has been followed through consecutive stages of growth by a number of different workers, and we find that the results of these investigations have been variously interpreted. The development of the vascular system is fully described by Chauveaud (6) and summarized in the law of basifugal acceleration. In the root there occurs a primitive type of vascular pattern, which is gradually suppressed during the growth of the stem and replaced by a distinctly different arrangement. That is to say, the centrifugal structture of the stem, which results in the suppression or elimination of the original centripetal condition, is secondary and superposed