THE STEM OF VASCULAR PLANTS. 203 



the Ferns of our climates. I have in my possession the 

 lower part of a Tree Fern, which M. Perrottet has had 

 the kindness to send to me from Martinique ; it is 

 covered for about three feet above the neck, with a 

 thick and compact plexus, formed of an immense 

 number of little fibrous dry brown roots, which are 

 situated all around the trunk, forming a kind of cover- 

 ing for it ; these roots have, themselves, surrounded in 

 their growth the climbing stems of Caladiiim, which, 

 seen in the adult state, appear to have perforated this 

 radical tissue. 



§ 7. — The Stem of the Lycopodiums. 



We observe in the Lycopodiums two distinct but con- 

 tinuous parts ; on the exterior is a kind of envelope of 

 round cellular tissue ; in the centre is found a small 

 cylindi'ical column, composed of striped and dotted ves- 

 sels, surrounded by elongated cellules ; and the branches 

 are formed of little bundles, which separate from the 

 central cylinder, and where the cellular tissue is deve- 

 loped on the exterior, according as it is found free from 

 the pressure of the neighbouring fibres. This cellular 

 envelope of the Lycopodiums, and of several other 

 Endogens, has only a general resemblance to that of 

 the bark of Exogens, but there is nothing in it like 

 the cortical layers or the liber. 



The Isoetes is a kind of Lycopodiaceous plant of inun- 

 dated places. Its stem, instead of being elongated and 

 filiform, as in the other genera of the family, is thick, 

 oval, slightly triangular, and has the appearance of a 

 tubercule : this stock presents a bundle of primitive roots, 

 which spring from the base ; and three lateral bundles, 

 which are developed as adventitious roots in three longi- 



