RHIZOCARPEJ£ 



29 



appearance, from those of Flowering Plants. Those of Salvinia are re- 

 markably small, and are inserted about halfway up the epidermal cells, 

 which are eight or nine times their height. Air-pores occur also in the 

 submerged leaves. The very simple roots of Azolla are of endogenous 

 origin. The root-cap originates from a single cell ; in A. Caroliniana 

 (Willd.) the cap is eventually thrown off, leaving the root-tip naked. 



The sporanges are enclosed in unilocular sporocarps, formed two 

 together or in larger numbers ; in Salvinia on the youngest teeth of the 

 submerged leaves, in Azolla on the pendent submerged lobe of the 

 deeply bifid leaves, and only on the lowermost leaf of each shoot. The 

 leaf-segment which is destined to become fertile first of all develops 

 into a columel or placenta^ to which the sporanges are" attached. An 

 annular wall, the rudiment of the in- 

 diisium^ then becomes elevated from 

 the base of the columel, eventually 

 overtops its apex, closes up, and 

 thus forms the wall of the sporocarp. 

 The sporocarp of Salviniaceae is 

 therefore a metamorphosed portion 

 of a leaf, and corresponds to a sorus 

 in the Hymenophyllaceae (Filices), 

 with the difference that in the latter 

 the envelope remains open in the 

 form of a cup, while in the former it 

 closes completely over the sorus, 

 as in Cyathea (Filices). The in- 

 dusium is much more strongly 

 developed than that of ferns, and 

 completely envelops the sorus ; it 

 consists of two layers of cells, the 



walls of which are, in Azolla, strongly lignified in the upper part. Each 

 sporocarp contains one kind of sporange only ; but both kinds always 

 occur on the same individual, and may even spring from the same 

 metamorphosed leaf. In Salvinia the megasporanges are considerably 

 larger than the microsporanges, and the number of the latter in a sporo- 

 carp is greater (see fig. 7). In Azolla the number of microsporanges in 

 a sporocarp is about forty, while the female sporocarps contain only a 

 single megasporange, and consequently only a single megaspore, en- 

 veloped first in the wall of the sporange, and then in the greatly hardened 

 indusium. The microsporanges are nearly globular capsules, with long 

 slender pedicels, the wall consisting, when mature, of a single layer of 

 cells. The megasporanges are pear-shaped, with much shorter and 



Fig. 12. — Fertile shoot oi Azolla filicidoides 

 Lam., with two female sporocarps, « ( x 27). 

 (After Strasburger.) 



