4 STRUCTURE. 
more than a crown, whence the fronds issue. This latter form 
does, however, in age, occasionally become considerably 
elongated, even in some of our native species, and these in- 
stances afford an imperfect idea of the trunks of the tree ferns 
of the tropics, which, in some cases, attain fifty or more feet 
in height. This tendency most decidedly exists in the 
Osmundu regalis, whose erect naked stem may occasionally 
be met with from one to two feet high ; and the same 
tendency may sometimes be observed in Lastrea Filix- 
mas, L. Oreopteris, and L. ditatata, and in the Athyrium 
Filix-fcemina, and Polystichum angulare. When of a 
creeping habit, it usually assumes a tortuous, branching 
form, and extends itself either on the surface, or a few 
inches below the surface of the earth, becoming, in fact, 
a branching prostrate stem, from which the fronds spring 
up individually and distinct, and more or less widely 
separated. They are of variable size, sometimes as thick 
as one's thumb, in other cases as fine as threads, and often 
thickly covered with hair-like scales. Sometimes the 
caudex of the stronger growing species extends to a con- 
siderable distance as well as depth ; that of Polypodium 
vulgare spreads widely in a lateral direction, but the 
common brachen (Pteris aquiluia) creeps the most widely, 
and Newman mentions having met with its under-ground 
stems extending in some cases to a perpendicular depth 
of fifteen feet. The creeping caudex where it exists, affords 
great facilities for propagation ; a portion of moderate 
length, bearing a frond, when separated from the rest, and 
placed under proper conditions, producing roots in due 
time, and forming an independent plant. Whether erect 
and tufted, or lengthened and creeping, their growth ac- 
cording to Hofmeister, takes place only by a continued 
multiplication of one apical cell, through alternate walls, 
inclined to the right and left. 
The FRONDS are the most conspicuous portion of the 
plants. Proceeding from the caudex, which is a true 
stem, they are, in some measure, analogous to the leaves 
of other plants ; and, for this reason, the term frond has 
