SPORE FORMATION 313 



the epidermis and sometimes forming broad strands in the 

 interior. This rigid tissue (selerenchyma) is composed of elon- 

 gated cells with very heavy, much-thickened walls, which are 

 often yellowish in color. This tissue is developed from the 

 thin-walled cells (parenchyma), called the ground tissue, that 

 compose the greater part of the interior of the stems. 



306. Spore formation. The sporophyte nature of the fern 

 plant Becomes clear at the time of fructification. Certain ones 

 or sometimes all of the fern fronds as they grow older develop 

 spore cases, or sporangia. These are variously situated on the 

 fronds, sometimes appearing as clusters or spots, called sori 

 (singular, sorus, meaning a heap), on the under surface and some- 

 times in lines along the under edge. A sorus may be naked, 

 but it is frequently protected by a membranous outgrowth, or 

 indusium, from the surface of the frond (Fig. 271, C, i). 



The sporangia are stalked and somewhat flattened many- 

 celled cases, each of which develops from a single surface cell 

 of the frond. There are sixteen spore mother cells in the inte- 

 rior of the spore case, each of which gives rise to a group of 

 four spores (tetrad). The method of spore formation, four spores 

 in each mother cell, is thus identical with that of the bryophytes. 



The sporangium of many common ferns is composed of thin- 

 walled cells except along the edge, where there is a line with 

 much-thickened w r alls, which extends from the stalk about two 

 thirds around on the outside (Fig. 275, A). This line of cells is 

 called the ring, and as the sporangium ripens and becomes dry, 

 the ring is forcibly held like a bent spring. Finally, the deli- 

 cate cells at the side of the spore case opposite the ring are 

 unable to stand the strain and are torn apart so that the ring 

 straightens somewhat and a wide rent is made in the side of 

 the sporangium (Fig. 275, B, C}. The spores are thrown out vio- 

 lently through the rent for a considerable distance. This is the 

 structure of the spore case in the family Polypodiacem, for the 

 several families of the Filicales have sporangia which differ from 

 one another in form and in the structure and position of the ring. 



