358 PART IV. CLASSIFICATION'. 



spore undergoes repeated division ; this is followed by free cell-formation 

 in the apical region (the pointed end where the three ridges meet) of the 

 macrospore, the result being the formation of a small-celled tissue ; sub- 

 sequently cell-formation extends into the basal portion of the spore, a 

 tissue being formed there consisting of relatively large cells with coarsely 

 granular contents. Thus the macrospore becomes completely filled with 

 a mass of cellular tissue which constitutes the female prothallium : the 

 xipper small-celled tissue is the essentially reproductive portion, whilst 

 the lower large-celled tissue simply serves as a depository of nutritive 

 substances. 



The female organ, the archegonium, is developed from one of the super- 

 ficial cells of the small-celled prothallial tissue, after the manner described 

 on page 351. It appears that two or three archegonia are usually formed : 

 but if none of these primary archegonia are fertilised, a small number of 

 additional archegonia may be subsequently developed. 



The ai-chegonia 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 qospore developes into the 

 embryo: the foot of the embryo grows down into the large cells of the 

 basal portion 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. HQMQSPORQUS LEPTOSPORANGIATvE (Filices). 



The orders constituting this group have so much in common 

 that they may be advantageously considered all together. 



SPOROPHYTE. 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 root$ : 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. Lygodtum palmatum, Polypodium Heracleum and 

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

 adventitious roots. 



