STRUCTURE AND DEVELOPMENT OF SPORANGIA 297 



A mature sporangium (cf. also Fig. 166, B) consists of a stalk 

 of usually three rows of elongated cells, terminated by a biconvex 

 capsule which is more or less oval in side-view, and encloses 

 the spores within a wall of a single layer of cells. The cells of 

 the wall fit firmly together, and most of them are thin-walled 

 and very flat, appearing more or less polygonal in surf ace- view, 

 but tabular in optical section. The edge of the capsule, how- 

 ever, is occupied by a single 

 row of specially differentiated 

 cells. The greater part of this 

 band, from the base of the 

 capsule on one side to half-way 

 down the other edge, is com- 

 posed of cells which have all 

 but their outer walls strongly 

 thickened, and which constitute 

 the annulus (a. in Fig. 165 and 

 an. in Fig. 166, B). The re- 

 mainder, forming the so-called 

 stomium, are much broader and 

 thin-walled throughout (Fig. 

 166, B, s.), and it is here that 

 the mature capsule ruptures. 



Each sporangium arises as 

 a papillate outgrowth from a 

 single surface cell of the placenta, 

 which is cut off by a cross- 

 wall, and undergoes division by 

 three oblique septa to produce 

 a tetrahedral apical cell (Fig. 



166, C, I, II). The three rows of segments, primarily cut off 

 parallel to the three flat faces of the latter (III), elongate and 

 become the stalk. Next, walls are formed parallel to all four 

 faces of the apical cell (IV), and give rise to the one-layered 

 capsule-wall (Fig. 166, D and E, w.), an inner nutritive layer, 

 or tapetum (t.), and a central cell, the archesporium (a.). The 

 last-named divides to form the spore mother -cells , each of which 

 gives rise to four spores {cf. p. 309). Their development takes 

 place at the expense of the food-materials contained in the 



FIG. 164. On the left part of a 

 Filmy Fern (Hymenophyllum 

 tunbridgense) , bearing sori (s.). 

 The two smaller figures on the 

 right show a sorus, entire and 

 in longitudinal section respec- 

 tively. i>, indusium; s., sorus. 



