38 PLANT-LIFE ON LAND [CH. 



It is in the hollow of the curve thus formed that the 

 sporangia are inserted in large numbers, while a second 

 flap, less obvious than the first, forms a further 

 protection, the sporangia lying between the two. A 

 somewhat similar arrangement is seen in the common 

 Shield-Fern (Nephr odium filix-mas), but here the 

 sori, each covered by a kidney-shaped indusium, are 

 borne on the under surface of the leaf (Fig. 5). Each 

 of the sporangia is a stalked body with a capsular head 

 shaped like a biconvex lens (Fig. 6). Its margin is 

 occupied for three quarters of its length by a row 

 of hard cells forming the annulus, or spring by means 

 of which when ripe the sporangium opens and flicks 

 away its contents. It is inside these capsules that 

 the spores are produced, to the number of 48 in 

 each. When ripe they are dry and dusty, and being 

 very minute they may be blown to a distance by any 

 breeze. If a mature leaf be laid to dry face down- 

 wards upon a sheet of paper, a very perfect print of 

 its outline will in a short time be marked off by the 

 spores which it will shed. Each spore is so small as to 

 be invisible to the naked eye, but they are conspicuous 

 collectively owing to their vast numbers. A moment's 

 consideration is enough to realise the immense powers 

 of propagation with which the plant is endowed. 

 Each sporangium bears about half a hundred spores : 

 each minute subdivision of the leaf may produce very 

 many sporangia : the whole leaf is built up of a vast 



