A PIECE OF COAL. ^3 
top, A, contain a powdery dust a. These spores are 
full of resin, and they are collected on the Continent 
for making artificial lightning in the theatres, because 
they flare when lighted. 
Now this little Selaginella is of all living plants the 
one most like some of the gigantic trees of the coal- 
forests. If you look at this picture of a coal-for- 
est (Fig. 53), you will find it difficult perhaps to 
believe that those great trees, with diamond mark- 
ings all up the trunk, hanging over from the right 
to the left of the picture, and covering all the top 
with their boughs, could be in any way relations of 
the little Selaginella ; yet we find branches of them 
in the beds above the coal, bearing cones larger 
but just like Selaginella cones ; and what is most 
curious, the spores in these cones are exactly the 
same kind and not any larger than those of the club- 
moss. 
These trees are called by botanists Lepidodendrons, 
or scaly trees; there are numbers of them in all coal- 
mines, and one trunk has been found 49 feet long. 
Their branches were divided in a curious forked man- 
ner and bore cones at the ends. The spores which 
fell from these cones are found flattened in the coal, 
and they may be seen scattered about in the coal-ball 
(Fig. 51). 
Another famous tree which grew in the coal-forests 
was the one whose roots we found in the floor or 
under clay of the coal. It has been called Sigillaria, 
because it has marks like seals (sigillum, a seal) all 
up the trunk, due to the scars left by the leaves when 
they fell from the tree. You will see the Sigillarias 
