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POrULAR SCIENCE REVIEW. 
characters by which the different stems are distinguished are 
derived from variations in the arrangement and forms of the 
scars produced by the bases of the leaves. In recent tree ferns 
these vary in different parts of the same stem, and this may be 
expected to have been the case in the coal plants ; and, indeed, 
Binney and Hooker have noticed the scars on which have been 
founded four species of Sigillaria existing on a single stem. 
The paucity of specific and generic forms reminds one of the 
extensive native forests of conifers, in the northern hemisphere, 
which consist of a very few, sometimes as in the Wellingtonia 
groves of only a single species, but the conditions under which 
they grew were very different. They covered extensive plains, 
growing on a wet, clayey soil. The temperature was not neces- 
sarily very high, nor the composition of the air different from 
what exists at present. The growth and decay of the forests 
were very rapid, and the changes indicated by the succession of 
coal, shale, and sandstone more speedily followed each other 
than one would at first suppose. Binney has observed an erect 
coniferous stem which has deposited around it some thickness of 
sandstone, a layer of under clay, a bed of coal, another of shale, 
and other successive deposits, all which were formed while it 
was a growing tree. Such a series of rapid changes, not pro- 
duced by violent volcanic agency, is unknown in the present 
day. Both physically and botanically the coal measures pre- 
sent many remarkable problems to the naturalist. 
EXPLANATION OF PLATE XVI. 
Fig. 
V 
V 
1 . 
2 . 
Q 
O. 
if 
4 . 
5 . 
6 . 
v 
V 
V 
V 
V 
1 ) 
7 . 
8 . 
9 . 
10 . 
11 . 
12. 
13 . 
Strobilus of Lepidodendron. — Natural size. 
Two scales and sporangia of Flemingites. — Natural size. 
Single sporangium of ditto, showing the triradiate base. — 
Magnified. 
Two scales and sporangia of Lepidodendron. — Natural size. 
Spores of ditto. — Magnified. 
Section of Strobilus of Calamites showing the scales and the 
peltate scales, with sporangia. — Magnified. 
Strobilus of Calamites. — Natural size. 
Scalariform tissue from Lepidodendron. — Magnified. 
Reticulations on the woody fibre of JDadoxylon. — Magnified. 
Restored stem of Dadoxylon , with its large pith. — (Stembergia.) 
Trigonocarpon . — Natural size. 
Restored stem of Lepidodendron. 
,, ,, Calamites. 
