The spore mother cells ofEquisetum fluviatile undergo meiotic division, each forming a tetrad of spores. The spore protoplasts are separated from each other by an accumulation of mitochondria (organellar plate) at first and later on by plasma membranes, no cell wall is formed. The first layer of the sporoderm, the exine, originates from the plasmodial tapetum and is deposited at the outer side of the plasmalemma of the young spore. The exine reaches a thickness of about 330 nm. In the phase of spore greening the so-called perine, originating from the tapetum, is placed onto the exine and the inner layer of the sporoderm, the intine, is formed from the spore protoplast. The mature spore, about 40 Μm in diameter, does not enter dormancy and remains viable only for a few days.