FERNS. 221 



When the fructification is in scattered patches, on the 

 back of the leaf, as in Polypodium, Fig. 24, the Fern is 

 said to be dorsiferous, in allusion to the situation of the 

 floral organs. Examined with a microscope, the fruit 

 is found to consist of a capsule usually surrounded by an 

 elastic transverse ring, which opens when ripe for the 

 discharge of the seeds. These capsules are usually 

 accompanied by an additional integument, named Indu- 

 sium. It is a thin membrane, Fig. 113, enclosing the 

 unripe fruit, and originates sometimes from the veins, 

 and sometimes from the margin of the leaf. In distin- 

 guishing the genera of Ferns, Linnaeus resorted to the 

 situation and shape of the capsules ; but Dr. Smith has 

 found the presence of the indusium, and the mode in 

 which it bursts, the best criterion for determining the 

 genera of Ferns ; and in connexion with the other 

 characters, it is "the only sure ground on which the 

 botanist can rely." 



But the floral organs are not always cenfined to the 

 back of the frond, nor are the leaves and stems always 

 thus intimately incorporated. In Equisetum or the 

 Horsetail, Fig. 114, the fruit is attached to peltate re- 

 ceptacles, Fig. 115, which are collected into a spike, 

 and terminate the stem ; and its numerous seeds are 

 enfolded by four pollen-bearing filaments, Fig. 116, 

 somewhat analagous to those which occur in the flowers 

 of the more perfect plants. 



In LycopodiumoT Ground Pine the capsules are some- 

 times found in the axils of the leaves and sometimes at 

 the summit of the stem. This family of plants is en- 

 tirely unlike those ferns whose herbage constitutes a 

 frond, and it ranks with them, only because no one has 

 been able to detect those organs which serve to char- 

 acterize the more perfect plants. In most of the 

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