ACROGENS. 277 
and through this the wood passes, the centre being occupied by.a 
mass of scalariform duct, so called from the resemblance which: its 
perpendicular sides and transverse lines bear to the sides and 
rounds of a ladder. This form of tissue is interrupted by large 
spiral vessels ; the wood is also arranged in circles, or bundles, with 
_ & wavy outline, but only near to the bark. These circles seem to 
be sent down from the fronds, and as the fronds surround the stem, 
the bundles sent down from them lie side by side until they form a 
circle. There is a peculiarity in the growth of the Tree Fern, that 
theinterval between the cicatrices enlarges as the tree increases, 
showing that the stem of the tree increases in height not only at 
the apex for the time being, but | 
afterwards in the body of the 
The mode of germination in 
the Ferns seems to be this :— 
The sporule, after extrusion 
from the sporangia, bursts its 
envelope and emits a leafy ex- yy 
pansion from its centre, which ¥ % 
subsequently forms a bud and 
na plant. Plate II. repre- 
sents the Arborescent Fern of 
the Brazilian forests. 
Tn our climate these Acrogens 
are far from presenting the 
dimensions which they attain 
lm the tropics. Our Ferns are 
only perennials, with a short 
thizome or spreading roots, 
whose leaves rarely exceed four 
or five inches. Even in the tro- 
Phere the Hymenophylleum * Oi 
and Trickomanes (Fig. 338), ; | 
which grow only in humid Fig. 338.—Trichomanes brebisetum. _ 
Places, at the foot of old trees, or upon rocks bathed in running 
: brooks, are generally of small size. The delicate leaves are desti- ue 
