107 
but, as tliey are never found united, it is impossible to iden- 
tify them. Not all these trunks belong to the Tree Ferns, 
indeed, the greater part do not. Many of them are called 
Sigillaria, which it is not our province to describe. Very 
few specimens of the Tree Fern stems are found in the 
British coal beds. Still there is no doubt whatever that 
there were once Tree Ferns in this our island, and that 
traces of them are to be found in our coal mines. 
At first it might seem difficult to distinguish between the 
stems or trunks of other trees and those of the Tree Ferns. 
The scientific botanist has certain data, by which he can 
discern the difference in various plants. If he were taken 
to the tropics where such trees grow, and were shown a 
cylindrical block without a vestige of frond, or leaf, or fruc- 
tification, his science would help him to ascertain to what 
kind of tree it belongs. It is well known that if a cross 
section be made of the trunks of the trees growing among 
us, a number of circles arising from a common centre are 
visible in the wood. This is especially apparent in fir-wood 
or deal. Such trunks have also a covering of bark. These 
trees are said to be exogenous, growing outward. Such are 
all the European trees. If the wood appears disposed irre- 
gularly in spots, and consists of tubes placed parallel to one 
another without the least appearance of these circles, and 
not the least trace of bark, then it is an endogenous plant, 
growing inward. Such are the palm trees and the canes, all 
our fiags, lilies, and grasses. If the cross-cut of wood should 
show a sort of zig-zag formation, and unconnected layers 
with segments of circles having their ends outwards, or with 
something like an oak tree, or such odd appearances as is 
seen in the woodcut g, page 61 , then it may be inferred that 
such is an acrogen, and most probably belongs to some Tree 
Fern. The scars occasioned by the breaking ofi; of the 
fronds, as already alluded to, are another proof of the charac- 
ter of plant to which it belongs. Thus we see how we may 
discover the various classes of wood, not only in the now 
existing vegetable world, but even when the cylindrical 
trunk is compressed into a flat thin layer of coal. In 
some cases the distinctions mentioned may not be sufficient. 
To remedy this defect, other rules for the investigation of 
the character of stems are given, w hich are too minute to 
