86 COMPOUND ORGANS OF PLANTS. 



the parenchyma, the whole of which disappears, leaving no- 

 thing but the naked nerves all covered with sori. A striking 

 instance of this kind is not uncommon in bogs in the form of 

 a plant called the Osmunda speotabilis, or flowering-fern. The 

 lower pinnffi of this fern present the usual leaf-like appear- 

 ance, whilst the upper are entirely changed into sori. 



123. The morphological changes of the frond into sori are 

 well seen in Osmunda Claytonia. Some two to five pairs of 

 the middle pinnas of the frond of this fern are usually meta- 

 morphosed into sori. An occasional specimen may be found 

 bearing pinnse, in which the metamorphosis has been only 

 partially effected, the pinnae being partly parenchymatous and 

 partly soriferous. In such specimens, nature herself teaches 

 us her own process. 



124. When the sori are moderately dispersed over the un- 

 der surface of the frond; or, when developed in great num- 

 bers, if they are confined to its under surface ; or, when only 

 a part of the frond is metamorphosed into sori, then the same 

 frond can still continue to exercise the functions of vegetation 

 as well as of reproduction ; but this ceases to be the case when 

 the whole frond is metamorphosed into sori. Other fronds 

 are then developed from the same rhizome to carry on the 

 vegetative functions, which are barren and without sori, as 

 we see exemplified in the barren and fertile fronds of Osmund 

 cinnamomea, and Onoclea sensibilis. 



^EQUISBTACE^, OR JOINTED FERNS. 



125. This family of plants is remarkably different from all 

 the other Cryptogamia. It differs from the rhizomatous ferns 

 in the structure of its stem and that of its reproductive 



