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Development of Procambium, Xylem, and Phloem in the Shoot Apex of Selaginella

A quantitative study of vascular development in shoot apices of Selaginella rupestris was undertaken, using sample sizes large enough to justify statistical treatment of the data, and with special interest in comparing results from this microphyllous species with results from megaphyllous angiosperms. Selaginella rupestris developed microphyll primordia that initially contained no procambium. When procambium formed in the leaf trace, it was at an isolated locus, as was the later-forming protoxylem and protophloem in the trace. In the stele the first xylem always differentiated closer to the apex than did the phloem; its average distance from the apex of 302 μm in main shoots was highly significant from the 807 μm of the protophloem. The air spaces, which result from the unusual endodermis development in Selaginella, always differentiated even closer to the apex than did the xylem, averaging 101 μm above the protoxylem. A qualitative survey of 15 other species showed a similar pattern in the stelar tissues.