A MORPHOMETRIC STUDY OF THE ULTRASTRUCTURE OF ECHINOCEREUS ENGELMANNII (CACTACEAE). VI. THE INDIVIDUALIZED ULTRASTRUCTURES OF DIVERSE TYPES OF MERISTEMS

The structures of spine basal meristem cells were examined by stereological morphometric techniques and found to be different not only from shoot apical meristem cells but also from spine primordium cells in the relative volumes occupied by cellular organelles. Nineteen types of meristematic cells have been studied in E. engelmannii, and all are distinct ultrastructurally. This suggests that whereas they all appear to be primarily involved in mitosis and cytokinesis, there must be other important aspects to their physiology; possibly these cells have already begun differentiation toward their mature states even while they are parts of active meristems. MERISTEMATIC CELLS are often regarded as being undifferentiated and unspecialized, their primary role being to produce masses of cells. These meristem-derived cells are likewise usually considered to be unspecialized, but are presumably the targets for morphogenetic stimuli that initiate their differentiation along specialized developmental pathways, culminating in determined, mature cell-types or tissues. If mitosis is indeed the primary function of meristematic cells, then it would be expected that they would all have the same, or at least nearly the same, ultrastructure. Recent studies of Echinocereus (Mauseth, 198 la), Pisum (Lyndon and Robertson, 1976), Sinapis (Havelange and Bernier, 1974; Havelange, Bernier and Jacqmard, 1974), and Trichocereus (Mauseth, 1982b) prove conclusively that there is no universally common ultrastructure for shoot apical meristem cells. This indicated that whereas mitosis may be the most obvious activity of these meristematic cells, there must be other aspects of their physiology that are significant. However, it is also possible that the meristematic cells, rather than being unspecialized producers of "raw material" for future differentiation, are in fact themselves already highly specialized, and the morphogenetic processes that are necessary to produce the final, mature cell type have already begun and are acting 1 Received for publication 10 August 1981; revision accepted 14 September 1981. Support for this work was provided by NSF Grants PCM76-21429 and PCM79-02626. I thank the Cell Research Institute of the University of Texas for the use of its facilities. even as the cells are dividing and being produced. This concept is strongly supported by the finding (Mauseth, 1982a) that the meristematic cells of leaf and spine primordia in Echinocereus engelmannii differ not only from each other, but also from the cells of shoot apical meristems: even identity of genotype does not result in identity of meristem cell structure. To further examine the concept that cellular differentiation and specialization begin in the meristem and not after the cells leave it, a study of the basal meristems of spines of E. engelmannii was undertaken. These meristems, like those of the shoot apex, are not transitory but function continually for weeks, and their only apparent activity is mitosis and cytokinesis. MATERIALS AND METHODS-Spine basal meristems were obtained from the same E. engelmannii plants from which shoot apical meristems (Mauseth, 1981a), mature tissues (Mauseth, 198 lb), and leaf and spine primordia (Mauseth, 1982a) were taken and were fixed simultaneously with those tissues. All ultrastructural and morphometric techniques are as described previously (Mauseth, 198 la, b, 1982a, b). Results have been presented in Table 1 in the format used for all previous papers of this series to facilitate comparisons. The total sample was eight meristems, 55 protoderm cells, and 146 ground meristem cells. RESULTS AND DISCUSSION-Spine basal meristems are simple structures, being composed only of protoderm and ground meristem. During development, the tunica of the short