THE PTERIDOPHYTA : FILICALES, THE FERNS 



489 



and sporophylls are alike, but the sporophylls only occur on older plants. 

 The sporangia are arranged in small kidney-shaped groups called sori, 

 which are produced in two rows on the under surface of each pinnule ot the 

 leaf (Fig. 477). The sporangia are attached to a cushion-like swelling, the 



Fig. 477. — Dryopteris filix-mas. Lower 

 surface of a portion of a fertile 

 pinna showing sporangial sori, still 

 partially covered by the indusia, 

 arranged in two rows on the 

 pinnules. 



placenta, from which also arises a delicate membrane, the indusium, which 

 covers the sorus like a hood (Figs. 478 and 479). The sorus is at first pale 

 green, but becomes chestnut brown as the sporangia ripen. A single vein 

 of the leaf ends in each placenta and supplies it with nutriment. Sporangia 

 develop successively but without any regular order. 



The sporangium originates from a single superficial cell of the placenta 

 (Fig. 480). This cell divides into two, and the upper cell divides again by a 

 wall at an oblique angle to the first wall. By a further oblique division, a 

 tetrahedral cell is formed, the upper part of which is cut off again by a 

 transverse wall. The inner cell thus formed is the archesporial cell (Fig. 481). 

 Meanwhile, the lower cell of the pair formed at the first division has divided 

 up to form a multicellular tissue, the stalk. The cells surrounding the 

 archesporial cell divide further, and give rise to the wall of the sporangium. 



