xi LEPTOSPORANGIATJE HETEROSPOREJE 405 



ent existence, and even when unfertilised develops but little 

 chlorophyll. No rhizoids occur (this seems to be true of Sal- 

 vinia 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 Lep- 

 tosporangiatse, the cotyledon and stem apex; from the hypo- 

 basal, 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 Leptosporangiates. 



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 stern, 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 re- 

 spectively triangular and four-sided. The former have larger 

 nuclei and divide for a time after the manner of two-sided 

 apical cells, and perhaps the first division of the leaf quadrant 

 may be of the nature of a true dichotomy, and these cells are 

 the apical cells of the two lobes. In the four-sided cell, the 

 radial and tangential divisions succeed each other with much 

 regularity. By the growth of the two initials (Fig. 236, E, 

 x, x f ) the young cotyledon rapidly grows at its lateral margins 



