45^ 
VASCULAR CRYPTOGAMS. 
The development of the sporangium begins with the outgrowth of one of the 
superficial cells of the placenta which bears the sorus. The subsequent divisions are 
the same as those above described with reference to Salvima, so that here also the 
sporangium is soon elevated on a stalk and consists of a wall of a single layer 
of cells and of a tetrahedral central cell or archesporium (Fig. 322, I-III). From 
this a tapetum is cut off by four septa parallel to its sides, which, as in the 
Salviniaceae and in the true Ferns, surrounds the cell from which the spores are 
subsequently derived (Fig. 322, IV, V). The stalk of the sporangium of the 
Marsihacese, in accordance with the foregoing description, consists at first of three 
rows of cells, but these rows are afterwards multiplied by longitudinal divisions. 
As the young capsule of the sporangium gradually increases in size, the cells of the 
wall undergo radial division and the cells of the tapetum both radial and tangential. 
The central cell by repeated bipartition forms the sixteen mother-cells of the spores, 
Fig. 3St. — A very young sporocarp of Marsilia elata (after Russow) ; A a median longitudinal section ; B a transverse 
section; C part of a longitadinal section at right angles to ^. yy fibro-vascular bundles, j sori, canals of the sori, 
Tna macrosporangia, microsporangia (compare Fig. 325). 
each of which developes four tetrahedrally arranged spores in the usual way. 
During this process the cells of the tapetum gradually undergo disorganisation, and 
the cavity of the sporangium becomes filled with a granular plasma in which lie 
the mother-cells and the tetrads of spores and from which the remarkable epispore is 
subsequently formed. Up to this point the course of development in both kinds of 
sporangia is the same, but differences now become apparent. All the spores of the 
sixteen tetrads formed in the microsporangia reach maturity; each of the four spores 
within a mother-cell ^ surrounds itself with a permanent coat, and then the wall of the 
mother-cell becomes absorbed. In the macrosporangia one of the young spores of 
each of the sixteen tetrads grows more vigorously than the other three ; finally all the 
tetrads but one cease to develope, and the largest cell of this one — the future 
^ Russow raises the objection, /. c. p. 62, that I make no mention of special mother-cells in 
describing the development of the spores. His mistake finds its correction in § 3 of Book I. 
