A Study of the Plastids in the Cells of the Mature Sporophyte of Isoetes

Abstract

The plastids of Isoetes are unusual in that they exhibit "plastid polarity" at the time of nuclear and cell division in all of the meristematic cells of the mature sporophyte. Each meristematic cell in the metabolic phase contains a single plastid which is appressed to the nucleus. This plastid begins to divide before the nucleus enters prophase. During the prophase the single plastid completes its division to form 2 daughter plastids each of which occupies a polar position in the cell. The spindle fibers of the achromatic figure terminate at the inner surface of each daughter plastid and it would seem that the plastid functions in the formation of the achromatic figure. At the end of cell division each daughter cell contains one of the daughter plastids; thus the individuality of the plastids in these cells is at the same level as the individuality of the nucleus. The structure of the plastids includes a network of grana and connecting fibers. The network is distributed through the stroma of the plastid. Starch accumulates around some of the grana of the plastid and thus these grana become the hila of the starch grains. Those grana which are not involved in starch formation may function in the formation of the spindle elements of the achromatic figure.