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 Salvinia, 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-IIT). 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, /F, V). The stalk of the sporangium of the 

 Marsiliaceae, 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. 32t,— a very young sporocarp oi Marsilia data (after Russow); A a median longitudinal section ; B a transversa 

 section ; C part of a longitudinal section at right angles to A. ff fibro-vascular bundles, s s sori, sk canals of the sori, 

 fna macrosporangia, w»i 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, I. c. p. 62, that I make no mention of special mother-cells ii 

 describing the development of the spores. His mistake finds its correction in § 3 of Book I. 



