STRUCTURE AND DEVELOPMENT OF SPORANGIA 297 



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

 of usuall}' three rows of elongated cells, terminated by a biconvex 

 capsule which is more or less oval in side-^'iew, and encloses 

 the spores within a wall of a single la\'er of cells. The cells of 

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

 and \'ery flat, appearing more or less polygonal in surface-view, 

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

 e^•cr, is occupied by a single 

 row of speciallv differentiated 

 cells. The greater part of this 

 band, from the base of the 

 capsule on one side to half-waj' 

 down the other edge, is com- 

 posed of cells which have all 

 but their outer walls strongly 

 thickened, and which constitute 

 the annuliis {a. in Fig. 165 and 



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



mainder, forming the so-called' 

 siomium, 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 bj' a cross- 

 wall, and undergoes division by- 

 three oblique septa to produce 

 a tetrahedral apical cell (b^ig. 



166, C, I, II). The three rows of segments, primaril)- 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-la3-ered 

 capsule-wall (Fig. 166, D and E, u\), an inner nutritive laj-er, 

 or tapdiim {(.), and a central cell, the archesporiimi {a.). The 

 last-named divides to form the spoix mother-cells, each of ^^hich 

 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 {Hviuenophvlliiin 

 tiiiibyidgeiise), bearing sori (s.). 

 The two smaller figures on the 

 right show a sorus, entire and 

 in longitudinal section respec- 

 tivcl)'. !., indusium; s., sorus. 



