FILICINEAE. HETEROSPOROUS FILIC1NEAE. 



237 



stem ; while the apical cell of the stem in these plants forms its segments in two rows, 

 that of the root is a three-sided pyramid as in the Ferns and Equisetaceae. A root- 

 sheath, which grows with and covers the root, is formed according to Strasburger 

 from the cells of the stem which lie above the endogenous mother-cell of the roots of 

 Azolla; it is especially remarkable that the root in Azolla has only a single root-cap- 

 cell ; from this cell proceed two layers of tissue, which grow with the root and 



FIG. 192. Development of the leaf of Marsilia Drsmimondi. A, C, D seen from within. B longitudinal section 

 perpendicular to A ; bs apex of the leaf, q s segments of the apical cell, stb the lateral lobes of the lamina in a very 

 young state. After Hanstein. 



envelope it completely on every side; in Azolla caroliniana the root-cap is eventually 

 thrown off, and the tip of the root is then naked. 



The sporangia of the heterosporous Ferns are enclosed in peculiar structures 

 which are termed sporocarps and have the appearance of capsules. In respect of 

 these structures the Salviniaceae are very near the Ferns, for their sporocarps are 

 only a special development of the indusium. The sporangia are enclosed in 

 unilocular capsules, which are formed two or more together on the segments of 

 leaves (Fig. 194 A, JB]; in Salvinia the basal segments of the submerged leaves bear 

 these capsules ; in Azolla it is the outer descending segments of the deeply bipartite 

 leaf, and always of the first leaf in each shoot, which bears the sporocarps. 

 The leaf-segment first developes into a placenta or columella, on which the 

 sporangia arise, while a circular wall, the rudiment of the indusium, rises from 

 beneath out of the base of the placenta, and growing up above its apex ultimately 

 closes over it and forms the wall of the capsule in which the sorus is entirely 

 enclosed. Thus the sporocarp in the Salviniaceae resembles the sorus in the 

 Hymenophyllaceae, with the difference only that there the envelope remains open 

 like a cup, but in the Salviniaceae it closes completely over the sorus as in Cyathea. 

 The sporocarp of the Salviniaceae is therefore a sorus, the closed indusium of which 

 is of much more solid construction than in the homosporous Ferns, and consists of 

 two layers of cells, and these in Azolla have their walls lignified in the upper part. 

 Each sorus, that is, each sporocarp, produces microsporangia only or macrosporangia 

 only, but both kinds of sporocarps are found on the same plant and indeed on the 



