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 “méristème d'attente” and “anneau initial” is discussed in relation to the observations made on E. altissima.     

CINERADIOGRAPHIC STUDIES OF THE NORMAL CERVICAL SPINE

By considering the cervical spine as several segments with relatively different motions, an understanding of the total possible motions of the cervical spine can be more easily attained.Reversal of the cervical lordosis is a normal part of the flexion action and can result from positioning of the patient for radiographic studies.The effect of standing or sitting postures, and methods of initiating flexion of the neck should be considered in the evaluation of routine flexion and extension studies.Evaluation of individual cervical segments may be accomplished by the use of different methods of initiating flexion.