THE ONTOGENY OF THE SHOOT APEX IN DICOTYLEDONS

Abstract

SummaryWork on the early appearance of the shoot apex in the embryo has been carried out along two lines. First, by formal descriptions of early segmentation, and, secondly, by the recognition of zones within the embryo which foreshadow the tissue systems of the seedling. The latter is the more fruitful approach when problems of wide morphological interest are being investigated. A study of this kind has suggested, for example, that the shoot is homologous with the stele of the root, the outer tissues of the root having no counterpart in the shoot.The tunica‐corpus concept can now be accepted only with considerable reservations. The depth of the tunica may fluctuate during development, and the tissues derived from an apical layer may vary quite capriciously. Nevertheless, stratification of the apex is considered a characteristic feature of angiosperm shoot apices. Complementary to this stratification is a zonation within the apical meristem corresponding to that of gymno‐sperms. That is to say, the central cells of the apical meristem are larger with less dense protoplasm than the surrounding peripheral cells. The term initiating cell is proposed for this stage in cellular development.The initiation of leaves and the delimitation of provascular meristem below the apex are briefly described, and the bearing of these facts on the relationship between stem and leaf is discussed. The shoot is regarded as a unit, and it is concluded that the angiosperm leaf may be regarded more properly as an outgrowth of the stem than as a modified branch system.Some recent work on phyllotaxis is discussed, it being shown that an experiment designed to demonstrate that the position of leaf primordia is independent of the pattern of vascular tissue below them is inadequate for this purpose. An interesting theory of phyllotaxis recently propounded by Plantefol fits many facts of apical development.Little is known of developmental changes during vegetative growth. Those which precede flowering are better known, and seem to be explicable in terms of a redistribution of growth. These changes in the position, rate and direction of cell divisions bring about changes in the organization of the apical meristem in reproductive apices. With the cessation of growth in length, the central zone is replaced by cells of the same nature as those of the peripheral zone, which then extends uniformly over the apex. Gregoire's view of the irreducibility of the reproductive and vegetative apices is examined. Most of his evidence has proved ill‐founded, but a difference in organization remains. This change in organization is regarded as one of a series which occur in the development of the shoot; only with the accomplishment of all stages in its development is a shoot complete.