412 



MOSSES AND FERNS chap. 



and A. CaroJiniana numerous root-hairs, which arise from defi- 

 nite cells, evident while the ''epiblema" or superficial layer of the 

 root is still actively dividing — a condition which also occurs 

 in many other Pteridophytes. 'The initials for these root-hairs 

 arise within a belt of actively dividing cells lying immediately 



under the inner root-cap, not far from the apex As 



the root reaches the limit of its development, the hair-forming 

 impulse travels downward until the apical cell itself is split into 

 several parts, each one piliferous." (1. c, p. 416, 417-) 



The Sporangia 



The sporangia in both genera are contained in a so-called 

 sporocarp, which is really a highly-developed indusium. These 

 sporocarps always arise as outgrowths of the leaves, in Salvinia 

 from the submersed leaves, in AzoUa from the ventral lobes. In 

 Salvinia several are formed together (Fig. 233, C), in AzoUa 

 two, except in A. Nilofica, where there are four. Each sporo- 

 carp represents the indusiate sorus of a homosporous Fern. 



In Azolla iilicidoides these sori arise, as Strasburger ( (6), 

 p. 52) showed, from the ventral lobe of the lowest leaf of a 

 branch. My own observations in regard to the origin differ 

 slightly from Strasburger's in one respect. Instead of only a 

 portion of the ventral lobe going to form the sori, the whole 

 lobe is devoted to the formation of these, and the involucre 

 which surrounds them is the reduced dorsal lobe of the leaf, and 

 not part of the ventral one. 



The leaf lobe, as soon as its first median division is complete, 

 at once begins to form the sporocarps, each half becoming trans- 

 ferred directly into its initial cell. In this, walls are formed, 

 cutting off three series of segments (Fig. 240, D). Next a 

 ring-shaped projection arises about it, and this is the beginning 

 of the indusium {id) or sporocarp, which bears exactly the 

 same relation to the young sorus that it does in Trichomanes, 

 and Salvinia shows the same thing. From this point the two 

 sorts of sporocarps in Azolla differ. In the macrosporic ones 

 the apical cell develops directly into the single sporangium; in 

 the microsporangial sorus the apex of the receptacle, which prob- 

 ably represents an abortive macrosporangium (Goebel (22), p. 

 669) forms a columella from whose base the microsporangia 

 develop. (Fig. 241, A.) 



