The Anatomy of Isoetes

Abstract

1. The vascular axis is a non-medullated monostele, composed of tracheids and parenchyma. There is no differentiation into protoxylem and metaxylem. 2. There is no primary phloem in the stem. It is found in the leaf traces and root bundles only. 3. The cambium gives rise to cortex on the outside and secondary xylem on the inside. The so-called "prismatic layer" is secondary xylem. The cambium does not form phloem. 4. The secondary xylem consists of various combinations of the following types of cells: (a) Spiraland annular tracheids. (b) Immature tracheids, slightly lignified, with irregular rings or spiral thickenings. (c) Immature tracheids, unlignified, nucleate or enucleate, with irregular rings or spiral thickenings. (d) Immature tracheids, nucleate or enucleate, with slightly thickened, pitted walls. (e) Parenchyma cells, which may have little protoplasm and small nuclei, or abundant protoplasm and large nuclei. 5. The secondary xylem of I. Nuttallii shows zonation. I. echinospora var. Flettii and I. Tuckermani var. Harveyi do not. I. melanopoda shows it occasionally in old stems. Starch does not occur in the secondary xylem except in the parenchyma zones of I. Nuttallii. 6. The root bundles are collateral and monarch. The protoxylem is found on the side away from the phloem and toward the center of the stem, i. e., it is endarch. 7. The leaf traces are collateral, but tend to become concentric in the middle and upper part of the leaf. Thy xylem portion undergoes great reduction above the sporangium, but the phloem is not reduced correspondingly. The sieve plates are transverse, 8. Near the vascular axis the leaf trace does not show differential tion into protoxylem and metaxylem. In the outer part of the cortex and in the region of the sporangium it is usually exarch. In I. Nuttallii it is occasionally mesarch above the sporangium and in the region of the sporangium.