4o6 MOSSES AND FERNS chap. 



and bends forward so as to enclose the stem apex. At the 

 same time the upper marginal cells divide rapidly by oblique 

 walls alternately on the inner and outer sides, so that the coty- 

 ledon also increases in length, and by this time it is about four 

 cells thick. 



As soon as the apical cell of the stem is established, it grows 

 very much as in the mature sporophyte. Each segment divides 

 into a ventral and dorsal half, and each of these into an acro- 

 scopic and basiscopic portion. In case the stem octants are 

 equal at first it is not possible to say which is to form the stem 

 apex, but this is determined by the first division in each cell. 

 One of them divides by a vertical wall into equal parts and be- 

 comes the second leaf ; the other forms the stem apex. If the 

 octants are unequal, the smaller one always forms the leaf. At 

 the base of the cotyledon, between it and the stem, is a group of 

 short hairs (Fig. 236, F, h). 



The primary root of Azolla arises in exactly the same way 

 as that of the typical homosporous Leptosporangiatae, except 

 that here the two root octants seem to be always equal in size, 

 and as practically only one of them forms the root, the other 

 dividing irregularly and becoming merged in the foot, the root 

 is more or less decidedly lateral (Fig. 236, E). After one 

 complete set of lateral segments has been formed, the primary 

 cap segment is cut off from the outer face, but, unlike the other 

 Ferns, this is the only one formed. The cap cell divides later 

 by periclinal walls, so that there are two layers of cells covering 

 the apical cell, and these are continuous with the epidermis of 

 the rest of the embryo, and continue to grow at the base, so that 

 a two-layered sheath is formed about the young root. The 

 lateral segments are shallow and arranged very symmetrically, 

 and the divisions correspond to those in the other Ferns. 



The divisions in the foot are more regular than is usually 

 the case, and this is especially noticeable in sections cut parallel 

 to the quadrant wall (Fig. 236, E). The general arrange- 

 ment of the cells is quite like that of the cotyledon, but the 

 divisions are fewer and the cells larger. Corresponding to 

 the upward grow^th of the cotyledon, the foot elongates down- 

 wards beyond the base of the root, which thus appears as a 

 lateral growth from it, and no doubt led to Berggren's mistake 

 concerning its origin. 



Salvinia in its early stages is much like Azolla, but, accord- 



