Paolillo, D. J., Jr. (U. Illinois, Urbana.) The plastids of Isoetes. howellii. Amer. Jour. Bot. 49(6): 590–598. Illus. 1962.—The plastids of various tissues of the sporophyte of Isoetes howellii have been investigated with the light and electron microscopes. The plastid in all observed stages of development is bounded by an envelope composed of 2 closely placed membranes. The proplastid in meristematic tissues is a flattened, often circular, structure, which is differentiated into a peripheral portion (plastonema) that stains deeply, and an internal portion (plastosome) that stains very lightly with dyes that color protein. Tubules and lamellae, as well as starch, occur in the plastonema before they are found in the plastosome. In the mature photosynthetic tissues of the leaf, chloroplasts with lamellated grana resembling those of the higher plants are found. Amyloplasts are found near the vascular bundle of the leaf. In addition to plastids and other organelles, the cells of the ligule contain highly organized portions of endoplasmic reticulum. Long, tortuous amyloplasts are found in the parenchyma of the stem. The plastids of the root cortex also mature as amyloplasts. The structure of the chloroplast is similar to that reported for Psilotum but different from that reported in some accounts on Selaginella. An examination of various accounts of the chloroplast of Selaginella indicates that environmental effects must be considered in a comparison of the structure of chloroplasts of different species.