204 - Multicellular Plants 



Fig. 12-1. The familiar fern plant is the sporophyte 

 generation, consisting of exposed leaves (fronds), an 

 underground stem (the rhizome), and the roots. (From 

 The Plant World, by Fuller and Carothers. Holt, Rine- 

 hart and Winston, Inc.) 



cluster of spore capsules, or sporangia. At 

 higher magnification one can see that each 

 sporangium is a hollow structure, containing 

 numerous spores (Fig. 12-3). While a spo- 

 rangium is ripening, the spores are produced 

 from spore mother cells, or sporocytes. The 

 spore mother cell undergoes meiosis, giving 

 rise to four haploid spores, each covered by 

 a thick cell wall. Finally when it is ripe, the 

 sporangium bursts open, liberating the spores 

 in large numbers (Fig. 12-4). 



Usually the fern discharges its spores dur- 

 ing dry weather, and the wind ma} 1 carry the 

 spores for considerable distances. The mois- 

 ture-proof covering enables the spore to with- 

 stand exposure to dry air, which otherwise 

 would be lethal. Eventually a spore must 

 fall upon damp ground if it is to germinate 

 successfully. 



The Gametophyte. When a spore germi- 

 nates (Fig. 12-4), it does not give rise to the 

 familiar sporophyte from which it came, but 

 grows into the gametophyte of the fern. The 

 fern gametophyte is a small, flat, heart-shaped 

 green plant, called a prothallium (Fig. 12-4). 



Fig. 12-2. The regularly placed brown spots on the under surface of the fern 

 leaf are sori. Each sorus consists of a covered cluster of spore capsules, or 

 sporangia (not visible at this magnification). A, Dryopteris marginalis. B, Poly- 

 podium scouleri. (From The Plant World, by Fuller and Carothers. Holt, Rine- 

 hart and Winston, Inc.) 



