PLASTOCHRONIC CHANGES AND THE CONCEPT OF APICAL INITIALS IN EPHEDRA ALTISSIMA

PAOLILLO, DOMINICK J., JR., and ERNEST M. GIFFORD, JR. (U. California, Davis.) Plastochronic changes and the concept of apical initials in Ephedra altissima. Amer. Jour. Bot. 48(1): 8-16. Illus. 1961.-The occurrence of distal mitoses and the differentiation of derivatives of the distal cells of the apex of E. altissima are correlated with stages of the plastochronic cycle. The relative rates of growth of elements within the various regions of the apex are characterized in terms of elongation gradients. The numbers of mitoses found in median sections of individual apices are recorded. It is concluded that the distal zone is the source of all other cells of the apex and should be regarded as a zone of initials. The concept of "rneisteme d'attente" and "anneau initial" is discussed in relation to the observations made on E. altissima. THE structure of the shoot apex of Ephedra altissirna has been studied by Gifford (1943) and earlier workers (Schmitz, 1874; Groom, 1885; Koch, 1891). Gifford (1943) evaluated the structural complexity of the apex of E. altissima as being intermediate between that of the Taxodiaceae and that of the angiosperms, but the report did not deal with plastochronic changes. Seeliger (1954) discussed the plastochronic changes in the apex of E. fragilis but did not relate the stage of the plastochron to the occurrence of cell divisions in the sub-apical initial region. Recent literature has dealt with the question of the existence of an inactive zone at the most apical portion of the vegetative apical meristem (Buvat, 1952; Gifford, 1954; Cutter, 1959). Little has been reported on the relationship between the stage of the plastochron and the degree of mitotic activity in the various regions of the shoot apical meristem, although such a relationship has been suggested (Cutter, 1959). Usually, a composite picture of apices from all stages (Buvat, 1952) or from an isolated stage (Lance, 1957; Popham, 1958) has been described. Compounding data from apices of several stages of the plastochron might disguise any differences that may exist among the various stages. Reports on a single stage of the plastochron cycle may produce a bias when the results are generalized. Moreover, the frequency of mitotic division does not supply the information needed to decide whether a given region of the apex is to be regarded as an initial zone. It is the opinion of the present authors that the decisive character of an initial zone is simply the contribution of cells to the other regions of the apex, with the maintenance of the strategic position of some of its cells as initials. The relative frequencies of mitoses in the various regions of the apex are a reflection of the growth patterns of the apex. Meas1 Received for publication February 27, 1960. 2 Research completed during the tenure of a Predoctoral Fellowship, National Science Foundation. 3 Investigation supported in part by the National Science 1o,-nAnt;n (CGrant G-6130). ures of the relative frequencies of division are not an adequate criterion for the elimination of one zone or another as an initial zone. Further comments on these topics are reserved for the discussion section of this paper. MATERIALS AND METHODS.-Shoot tips of E. altissimna with trifoliate nodes (Gifford, 1943) have been utilized in this study because the apices of such shoots are massive enough to show differentiation into zones above the youngest leaf primordia. and are not so large that the interrelationships of the zones are obscured. The slides prepared for an earlier study (Gifford, 1943) furnished a large portion of the material utilized in the present report. Additional slides were prepared from similar apices of the same collections by sectioning paraffinembedded material at 8-1/O and staining with hematoxylin, safranin, and fast green. For convenience, the plastochron was divided into 5 stages: minimal, early post-minimal, late postminimal, pre-maximal, and maximal. It is not intended that these divisions represent absolute time periods since no data are available on the absolute duration of a plastochron. The apex is herein taken as that portion of the shoot tip above the youngest whorl of leaf primordia. A given apex is at minimal stage just after the emergence of foliar buttresses. Maximal stage of the apex is attained just prior to the formation of foliar buttresses (cf. fig. 7, fig. 12). Minimal stage probably provides the best point of entry for description of the cycle (fig. 1 represents a minimal apex). Apices of other than minimal stage were placed in their relative positions in the plastochron on the basis of the comparison of the height of the apex above the axil of the P1 primordia and the height of the Pi primordia above the same reference point. Heights were measured parallel to the longitudinal axis of the shoot, and the ratio, height of apex in micra/height of P1 in micra, is considered the plastochron index. Divisions of the plastochron are as follows: early postminimal, ratio 2.00; late post-minimal, 1.99-