388 PART III. THE CLASSIFICATION OF PLANTS. 



The archegonia are exposed, for the purpose of fertilisation, by the splitting 

 of the coats of the macrospore along the three ridges already described : the 

 prothallium does not, however, project from the spore, nor does it become 

 green. After fertilisation, the oospore developes into the embryo as described 

 above : the foot of the embryo grows down into the large cells of the basal por- 

 tion of the prothallium, absorbs the nutritive substances which were stored 

 up in them, and thus supplies the embryo with food until such time as its leaves 

 and roots are sufficiently developed to enable it to nourish itself in the usual 

 way. 



C. HOMOSPOROUS LEPTOSPORANGIAT^E (Filices). 



The orders constituting this group have so much in common 

 that they may be advantageously considered all together. 



SPOKOPHYTE. The body is differentiated into stem, leaf, and root 

 (generally) : the leaves are large in proportion to the stem, and are 

 relatively few in number. 



The stem has either radial or dorsiventral symmetry. In the 

 former case it is commonly short and straight ; it grows into the air 

 erect, or at any degree between the vertical and the horizontal ; 

 its surface is generally completely covered by the insertions of the 

 spirally arranged leaves, and by adventitious roots : it becomes, 

 however, elongated, to a considerable height sometimes, in the 

 Tree-Ferns. 



In the latter case, the stem grows as a rhizome either on or in 

 the soil, or on the surface of some tree upon which the plant lives 

 as an epiphyte : the leaves are borne on its dorsal surface, either 

 in two rows {e.g. species of Aneimia and Polypodium), or in a 

 single row (e.g. Lygodium palmatum, Polypodium Heracleum and 

 P. quercifolium) : from the lower (ventral) surface, spring the 

 adventitious roots. 



The growth in length of the stem, is effected by a growing-point 

 with a single apical cell (with the occasional exception of 

 Osmunda) : the apical cell is, as a rule, a three-sided pyramid 

 with its spherical base at the surface : but in Pteris aquilina it 

 is usually a two-sided lenticular cell, with its longer axis in the 

 dorso- ventral plane. 



The radial stems branch but little, least of all when the stem 

 is elongated, as in the Tree-Ferns ; and such branching as there 

 is appears to be mainly adventitious, the buds springing from the 

 bases of the leaves. In the dorsiventral stems there is normal 

 lateral branching, which takes place ki the transverse plane : the 



