246 



THE SPORE-PRODUCING ORGANS 



[CH. 



The further segmentation of the sporangial head has already been de- 

 scribed in the case of Dryopteris, which is that usual for Leptosporangiate 

 Ferns (Chapter I, p. 13). The inclination of the three oblique walls to one 

 another at an approximate angle of 120 degrees, followed by a fourth wall 

 at right angles to the axis of the sporangium, results in a tetrahedral cell 

 within a superficial wall of four cells. This cell gives rise to the tapetum 

 and spore-mother-cells. In this scheme of segmentation the usual structure 

 of the stalk is three-rowed, as in Fig. 243, /, o,p, or sometimes as in n, q, r. 

 It remains to place in relation to this another scheme hitherto overlooked, 

 which occurs in certain Ferns with a sporangial stalk composed of four rows 

 of cells, as in Fig. 243, k. It is found in Metaxya, Dipteris, and Cheiropleuria, 



Fig. 241. Illustrations of the two-sided segmentation of Fern sporangia. 

 a, b, young sporangia of Cheiropleuria, showing aspects at right angles 

 to one another. ( x 165.) c, d, similar drawings of Metaxya. ( x 200.) 

 e, a rather older sporangium of Metaxya seen from above; /, seen 

 from the side, h, /^ = hairs. ( x 200.) 



and it is possible that it may be found also elsewhere (Fig. 241 ). In the young 

 sporangium of these Ferns the initial cleavages appear in two rows, instead 

 of three. The relation of this scheme to that usually seen is comparable to 

 that of a two-sided initial cell of stem or leaf to the three-sided, or tetra- 

 hedral ; and the form of the internal cell is approximately that of half of a 

 biconvex lens, as in the case of a two-sided initial cell. C. Miiller {Ber. d. 

 D. Bot. Ges. ii (1893), P- 54) ^^s made a precise analysis of the sporangial 

 structure in Dryopteris, assigning to each part of the mature head its 

 origin in the initial segmentation. But it is found in the sporangia which 

 show the two-rowed segmentation (e.g. Dipteris, Cheiropleuria) that the 

 annulus and the other parts of the sporangial wall maintain essentially the 

 same relative positions as those of allied P'erns which have a three-rowed 



