Abstract
Observations were made of stem sections stained for RNA and protein of Yucca whipplei ranging from germinated seedlings to 6‐month‐old plants. One‐, two‐, and three‐month‐old plants were labeled with tritiated thymidine, fixed in FAA, sectioned, stained with the Feulgen reaction, and prepared for autoradiography. The serial transverse sections were outlined with a drawing tube recording all labeled nuclei on a computer graphics tablet. Computer‐assisted three‐dimensional reconstructions were made to observe the locations of labeled nuclei. The two techniques are in agreement: the thickening meristem is broad near the top of the stem, occupies a narrower band at more basipetal levels, and disappears below the level of recent root initiation. There are no gaps in staining or labeling, and there are no changes in staining or labeling that would distinguish between the activities of the primary thickening meristem and the secondary thickening meristem in those plants which possess both. The meristems are continuous at all stages of development in the young vegetative stem. The STM is interpreted to be a developmental continuation of the PTM.
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