meris terns of Ferns as a Phylogenetic Study . 363 
divisions may take place (Fig. 75), so that a considerable bulk 
of tissue is formed, in the projecting apex of which a single 
large cell occupies a central position. This ultimately divides 
by a periclinal wall (Fig. 76), and the inner of the two 
resulting cells is the archesporium 1 , which is completely sur- 
rounded by a single layer of other cells, and these, as in other 
Ferns, develop into the wall of the sporangium. The form 
of the archesporium is not definite and uniform as in other 
Ferns ; and in most cases, instead of being pointed, its base is 
irregularly oblique ; but the subsequent divisions, by which 
the layer of tapetal cells is cut off from it, follow with some- 
what greater regularity (Figs. 77, 78), and the most common 
result is that the sporogenous cell (the true archesporium) has 
a tetrahedral form : nevertheless exceptions even to this are 
by no means uncommon (Figs. 79, 80). The whole spor- 
angium, of which Fig. 77 rnay be taken as a good typical 
example, thus comes to consist of a short and massive stalk, 
a median longitudinal section of which shows four rows of 
cells ; this bears the head, which is composed of a superficial 
layer which forms the wall of the sporangium, a tapetal layer, 
which is shaded obliquely in Fig. 77, and consists as yet of a 
single layer of cells. Towards the base of the sporangium, cells 
which have been cut off by extra oblique walls intervene 
between the tapetum and the superficial layer of the wall, 
giving the sporangium a more massive character than in the 
leptosporangiate Ferns (Figs. 77, 78, 80) : there is thus a 
considerable bulk of internal tissue which takes no part in the 
formation of the spores (Figs. 81, 82). Subsequently anti- 
clinal divisions appear in the cells forming the wall of the 
sporangium, but as no periclinals are formed the wall remains 
a single layer of cells in thickness even to the period of 
maturity; towards the base, however, it is supported internally 
by the cells above mentioned (Fig. 82). The tapetum divides 
1 I use the term archesporium for this cell before the tapetum is separated from 
it, because this is customary, and notwithstanding the inconsistency of so using the 
term, while in the higher plants it is applied to the cell from which the spore- 
mother-cells are directly derived. 
