202 VEGETABLE ORGANOGRAPHY. 



it, and presents itself under the appearance of a subter- 

 ranean nearly horizontal stock, which emits roots from 

 its lower side, and leaves from its superior extremity ; it 

 is gradually destroyed at its older end, whilst it lengthens 

 by the opposite extremity. One can no more refuse to 

 recognise the identity of this subterranean stock with the 

 aerial stem of the Tree Ferns, than in similar cases in 

 the Liliaceae and other well known families. 



The stems of Ferns, whatever be their direction, are 

 always cvliiidrical, and harder towards the circumference 

 than at the centre. They are eminently distinguished 

 by this — that when they are cut transversely, we always 

 observe in them brown spots, round and symmetrical, 

 but of various forms : it is these which, when seen in an 

 oblique section of Pteris aquilina, have been compared 

 to the figure of a German eagle. These spots are 

 formed, according to Mirbel, by the transudation into 

 the cellules of the most spongy part of the stem, of a 

 juice secreted by certain fibres. From an examination 

 of them in the Tree Ferns, we are obliged to consider 

 them as bundles of very compact fibres, which are sepa- 

 rated by spaces essentially full of cellular tissue. The 

 central part is sometimes hollow in aged stems, by the 

 destruction of the cellular tissue. The stem, cut trans- 

 versely, presents an external ring of cellular tissue, 

 which performs the part of bark as to its position, but 

 does not influence the formation of the wood, which is 

 developed by the interior of the central fibrous cylinder : 

 this cylinder presents a great number of striped vessels ; 

 the ramifications of the stem spring entirely from it, and 

 they do not seem to be any thing but the results of the 

 divergence of the fibres. 



Every part of the surface of the stem appears en- 

 dowed witli the faculty of producing roots ; this we see 

 very clearly in the rliizomata or subterraneous stems of 



