286 



BOTANY 



antheridium is formed. The latter develops eight sperm-cells, which are in two 

 groups, and are sometimes considered to represent two antheridia (Fig. 250). 

 The sperrnatozoids are multiciliate, like those of the typical Ferns. 



The ripe inacrospore is a large oval cell, with a very thick outer wall (epispore), 

 which in Azolla is curiously sculptured and provided with fine hairlike out- 

 growths, to which the glochidia become attached. In Azolla, also, the pointed 

 apex of the indusium persists, and is only thrown off by the expansion of the 

 growing gametophyte (Fig. 251). The spore is filled with dense granular cyto- 

 plasm, and the nucleus lies in its upper part. The first division of the nucleus 

 is followed by a transverse wall, shutting off a small lenticular cell from the apex 

 of the spore. This becomes the prothalliuin, the lower cell remaining undivided, 

 and serving as a food-supply for the developing gametophyte. In Azolla, how- 



Fr~ 



FIG. 251. Azolla filiculoides. Female gametophyte and archegonium. A, B, 

 longitudinal sections (x 220). C, D, archegonia (X 375). E, two transverse sec- 

 tions of gametophyte with three archegonia ( X 220) . F, section of macrospore 

 and large prothallium (x 68) ; in, indusium. 



ever, the nucleus of this large basal cell subsequently divides, but there is no 

 cell-formation. It js not known whether this nuclear division also occurs in 

 Salvinia. 



The prothallial cell undergoes rapid divisions, and forms a projecting mass of 

 tissue (Fig. 251), which develops chlorophyll, especially in Salvinia, where the 

 gametophyte is much larger than in Azolla, and resembles more nearly that of the 

 homosporous Ferns. The gametophyte is triangular in form, and in Salvinia two 

 of the angles develop into large pendent lobes of green tissue. Several arche- 

 gonia, much like those of the ordinary Ferns, are formed, the number being 

 larger in Salvinia than in Azolla. The structure of the archegonium (Fig. 251, 

 C, D) is much like that of the other Ferns. 



Embryo. The development of the embryo is much like that of the typical 

 Leptosporangiatae. It is still a question whether a primary root is indicated in 



