STUDIES OF SPORE-PLANTS 



235 



leaves of seed-plants is that fern leaves may have on their 

 lower surface peculiar organs which form spores. In some 

 species of ferns there are on the leaflets rounded spots ; in 

 others there are ridges on the leaflets ; and in still others there 

 are folds at the margin. Examination with a hand-lens shows 

 that these variously shaped spots are similar in essential 

 structure, for they are clusters of spore- 

 cases (sporangia), the organs for pro- 

 ducing spores. When immature, these 

 spore-cases are usually protected by a 

 covering (called indusium) in the form of 

 a cap, or a fold of the edge of the leaf. 



In many ferns the leaves thus combine 

 the regular work of foliage leaves (breath- 

 ing, transpiration, starch-making) and 

 that of spore forming ; but some species 

 have certain specialized leaves which pro- 

 duce spores (spore-leaves or sporophylls) , 

 and thus the foliage leaves are free for the 

 regular work. In such cases the foliage 

 leaves are often called sterile leaves, the 

 spore-bearing ones fertile. In some spe- 

 cies these latter become so much modified FIG. 71. 

 that they do not resemble ordinary leaves 

 (Fig. 71). 



224. Germination of Fern Spores. - 

 When the spore-cases (sporangia) are ma- 

 ture and dry, they burst and scatter the spores. 



Specialized 

 spore-bearing fern 

 leaf (6), leaflet mag- 

 nified (c) , and regular 

 leaf (a). (From 

 Gray.) 



The burst- 

 ing is caused by a peculiar ring (annulus) which almost 

 surrounds each spore-case and acts like a spring. If the 

 spores fall upon favorable soil, they absorb water and ger- 

 minate. The coats of the spore burst and a delicate tube 

 protrudes (Fig. 72, A). This soon divides into cells (Fig. 

 72, B), and grows into a flat and usually heart-shaped 

 structure, called a prothallium (C). On its under side are 



