386 MOSSES AND FERNS chap. 



wings is the front, and the primary archegonium occupies the 

 highest point, as in Azolla, and the two secondary ones form a 

 line with it parallel to the forward edge, which develops a 

 meristem and other archegonia in rows parallel to the first 

 ones, in case these fail to be fertilised. 



In Azolla the prothallium has but little power of independ- 

 ent existence, and even when unfertilised develops but little 

 chlorophyll. No root-hairs occur (this seems to be true of 

 Salvinia also), and the growth only proceeds until the materials 

 in the spore are exhausted. To judge from Berggren's figures ^ 

 A. Caroliniana has a larger prothallium but fewer archegonia 

 than A. filiculoides. 



The Embryo 



The fertilised ovum, previous to its first division, elongates 

 vertically. The basal wall is usually transverse instead of 

 longitudinal, as in the other Leptosporangiates, although in 

 exceptional cases it may approach this position in Azolla. 

 From the epibasal half in the latter arise, as in the other 

 Leptosporangiatae, the cotyledon and stem apex ; from the 

 hypobasal, foot and root. The quadrant walls do not always 

 arise simultaneously, but as soon as they are formed the 

 primary organs of the embryo are established and are arranged 

 in the same way as in other Ferns. Berggren ^ asserts that 

 the root does not develop until later, and is derived from the 

 foot ; but in sections it is very evident from the first, and 

 corresponds in position exactly with that of other Lepto- 

 sporangiates. 



In all but the stem quadrant the octant walls are exactly 

 median, and this may be true of the latter ; but in the stem 

 quadrant the octant wall may make an acute angle with the 

 quadrant wall, and the larger of the two cells then forms at 

 once the two-sided apical cell of the stem, and from now on 

 divides alternately right and left. Where the octant wall is 

 median, it is probable, although this could not be positively 

 proved, that the stem apex forms for a short time three sets of 

 segments instead of two. 



In the cotyledon the median octant wall is followed by a 

 vertical wall in each octant, forming two cells that appear 

 respectively triangular and four-sided. The former have larger 

 ^ Berggren (2), Figs. 4-16. ~ Berggren, I.e. p. 4. 



