112 MARATTIACEAE [CH. 



cell has also divided, and gives rise to part of the sporangial wall. As 

 development proceeds the divisions are often sufficiently regular to allow of 

 the genetic grouping of the tissues being clearly followed (C). Meanwhile 

 certain cells at the apex enlarge to form the crest-like ridge above the 

 annulus: from this downwards on the central side of the sporangium the 

 dehiscence will take place. The sporogenous tissue is early defined by the 

 rich protoplasm of its cells, and it is clear that they are referable to a single 

 parent-cell. The cells immediately surrounding it assume the character of 

 a tapetum, which is thus extra-archesporial in its origin (E). This figure 

 represents a sporangium which has arrived at the stage of complete division 

 of the sporogenous mass, and in which the spore-mother-cells are about to 

 separate and round themselves off prior to tetrad-division. The spore-output 

 per sporangium can be computed by estimating the number of spore-mother- 

 cells. The number for each sporangium is about 360, giving 1440 spores if 

 all the mother-cells formed tetrads. 



Outside the sporogenous cells lies the tapetum of one or two layers, 

 covered by the wall composed usually of three layers of which the outermost 

 is the firmest; part of it gives rise to the opening mechanism. The cells 

 shaded in Fig, 404, i^ are large, turgid, and thin-walled: the lateral bands 

 enclosed by brackets consist of prismatic cells with lignified walls, forming 

 a mechanical ring continuous across the apex of the sporangium by a narrow 

 bridge corresponding to the crest of the sporangium, and seen in surface 

 view in D. On the ventral face the cells are smaller and thin-walled 

 (c, Fig. 404, F). This tissue defines the line of dehiscence, and it is traversed 

 by the plane of section of -£". When ripe the shaded cells shrink: the sides 

 of the indurated hoop are drawn together, while the apical bridge like a 

 semi-rigid hinge yields, so that the slit will gape widely, as it is seen to do 

 in Fig. 402, A. In the synangial sori the mechanism is necessarily different, 

 and the indurated hoop is absent. In Danaea and Christensenia there is a 

 slit of dehiscence for each sporangium which as the adjoining cells dry up 

 widens into an almost circular pore (Fig. 402, G-K). In Marattia there is 

 in addition to this a change in form of the whole woody sorus at maturity. 

 Its two sides, originally facing one another closely, move apart as in the 

 opening of a book (Fig. 402, E). 



The estimate of about 1440 spores as the product of a single sporangium 

 o\ Aiigiopteris is relatively low for the Family. It may be compared with 

 the somewhat smaller spore-counts actually made by Halle from sporangia of 

 his recently discovered Rhaetic Fossil, Danaeopsis fecunda. His actual counts 

 were 1 100 to 11 59. Such figures are below the estimates of 1750 for Danaea, 

 2500 for Marattia, and over 7000 for Christensenia, a number which is pro- 

 bably equalled also by the synangial fossil Ptychocarpus. It appears thus 

 that the higher numbers fall consistently to the synangial types, and a like 



