PTERIDOPHYTA. 



201 



which in this cas"e as in all others is a primordial cell, is fer- 

 tilised, it surrounds itself with a cell-wall and commences to divide 

 into a number of cells, to form the embryo. 



The first dividing wall (basal wall) is nearly horizontal, and in the direction of 

 the longitudinal axis of the archegonium. The next wall is vertical, and the 

 next perpendicular to the other two. The oosphere, therefore, is now divided 

 into eight octants by these three walls. The basal wall divides the embryo into 

 a hypobasal and an epibasal half. From the first one, by continued divisions, 

 the first root is developed ; from the latter, the stem and leaves. After the for- 

 mation of the octants the development proceeds in somewhat different ways in 

 the various classes. In addition to the stem, leaf, and root, a " foot " is deve- 

 loped from the hypobasal half which remains enclosed in the prothallium, and 

 conveys nourishment from the prothallium to the young plant until it is able 

 to sustain itself (Fig. 202). The formation of these members in the embryo 

 depends on the position of the oosphere in the archegonium and prothallium, 

 and is independent of gravity. 



FIG. 202. Adiantum capillus veneris. Vertical section through a prothallium (//), 

 with a young plant attached on its under side (mag. about 10 time>) ; r the first root, and 

 b the first leaf of the young Fern-plant ; m the foot. In the angle between m and b lies tLe 

 apex of the stem : h the rhizoids of the prothallium ; ce ce unfertilised archegonia. 



In the Mosses the asexual generation is the sporogonium, which 

 is limited in its development and in a great measure dependent 

 upon the sexual generation, upon which it is situated ; but in the- 

 Pteridophyta this generation is an independent and highly developed 

 plant, provided with stem, leaf, and true roots, and has in many 

 instances an unlimited development. The Pteridophyta are the 

 lowest Division with true roots. The root which is first formed ia 

 very similar in nature to the primary root of the Monocotyledons ; 

 it very soon dies and is replaced by others which are more per- 

 manent, and developed upon the stem (adventitious roots) ; roots 

 are wanting in Salvinia, Psilotum, and some Hymenophyllacea3. 

 The differentiation is, however, not so complete as in the Flower- 

 ing-plants, and so many leafy forms are not found. The various 

 members of these plants are anatomically much higher than in the- 



