INTRODUCTION. 9 
leaving a concave cicatrix or scar. When the stems assume this 
creeping habit, they usually if terrestrial extend either on the surface, 
or just below the surface of the earth, often becoming branched, the 
fronds springing up individually and distinct, and more or less widely - 
separated. If epiphytal they creep along the surface of the bark of 
the trees that support them, or hang dependent from their branches ; 
they are sometimes as thick as one’s finger, as in the Polypodium 
vulgare, but are often much smaller than this. The true rhizomes 
are generally thickly covered with scales, which are variable in size 
and form, and are sometimes so large and numerous as to render 
the surface quite shaggy. Unless scales are common to the whole 
frond, they seldom extend in Ferns of this habit beyond the point at 
which the fronds are articulated. 
The rhizome or creeping stem, where it exists, affords great facili- 
ties for propagation ; for if a portion of moderate length, furnished 
with fronds and roots, is separated from the rest, and placed under 
proper conditions, it will readily form an independent plant. 
Whether the stem is caudiciform or rhizomiform, erect or creeping, 
accretion takes place, according to Hofmeister, by means of a con- 
tinued multiplication of one apical cell. 
As regards their internal structure, the stems of Ferns are 
more highly developed than those of other acrogenous plants. 
The lower groups of these Acrogens consist merely of cellular tissue, 
but in the more highly organised forms, among which the Ferns take 
precedence, both woody and vascular tissues are found. In some 
Ferns, Marattia for instance, the vascular bundles are regularly dis- 
tributed throughout the whole mass forming the stem. In others, 
such as the Lastrea Filie-mas, they are disposed in a single circle, 
or, as in Pleopeltis leiorhiza, with neighbouring smaller bundles. In 
others, again, as in Trichomanes reniforme, they are reduced to one 
central bundle; while instances occur in which they are disposed on 
either side of hard plates, as in Pteris aquilina. In many of the 

