In order to study these correlations even more it seemed necessary to work on the cellular level; in 1966, Lang (12) discussed the problem of "intercellular regulation in plants." In defined in vitro conditions, certain epidermal cells of T fournieri stem segments embark on cell division ultimately leading to bud neoformation. The correlations which these particular cells undergo would be different from those of other epidermal cells. In a previous study, it was shown that cell division centers, observed at the 5-day stage, are not formed at random on the stem epidermis but in relation to the position of underlying vascular tissue (9). An interpretation of this phenomenon, based on hormone movement from this vascular tissue, was put forward. This hormone movement would be one of the correlative factors unequally influencing epidermal cells. For the same material, it has also been shown that among the epidermal cells which divide and go on to form cell division centers, not all form bud meristems (7,24). A certain competition among these centers for nutrient or hormonal factors would mean further changes in intercellular correlations in the epidermal layer.It should be possible to define different cell populations among epidermal cells depending on their position and the different correlations to which they are submitted. The present study has been undertaken to characterize these cell populations at a very early stage of organogenesis according to the time of DNA synthesis and that of entry into division. Several authors (3, 10, 11, 14-16) have analyzed cell populations in stem or root meristems but, perhaps for lack of simple techniques, never in single differentiated cell layers capable of organogenesis.
MATERIALS AND METHODSStem segments of T fournieri Lind, 10 mm long, were cultured aseptically on a simple medium previously described (6), to which [3HJthymidine was added at 0.025 mCi ml-'. The wide face of the stem (which was rectangular in section) was applied to the radioactive medium; only the epidermal cells of this side were studied in the present article.After a continuous treatment on this medium for a length of time (Ti) of 3,6,9,12,15,18,20,22,24, 26, 29, 32, 36, 42 or 48 hr at 27 C with a 16-hr daily light period, the segments were either transferred to an identical medium but without [3Hjthymidine or underwent immediate fixation. In the latter case, the epidermis (10 x 2.5 mm) which was in contact with the radioactive medium was peeled from the segment with no adjacent parenchyma tissue and fixed in Brachet fixative (13) for 1 hr. It was then rinsed for 3 hr in tap water and spread and dried on a slide previously coated with a 0.5% (w/v) gelatin solution. The slides were then covered with Kodak ARIO stripping film for autoradiography in a dark room where they were left for 7 days. They were then developed using photographic products before microscopic observation.The explants transferred to a nonradioactive medium were first rinsed (30 min) in an unlabeled thymidine solution, then placed ...