PAST HISTORIES OF PLANT FAMILIES
VII. The Lycopods
The present-day members of this family are not at all impressive, and in
their lowliness may well be overlooked by one who is not interested in
unpretending plants. The fresh green mosslike _Selaginella_ grown by
florists as ornamental borders in greenhouses and the creeping “club
moss” twining among the heather on a Highland moor are probably the best
known of the living representatives of the Lycopods. In the past the
group held a very different position, and in the distant era of the Coal
Measures it held a dominant one. Many of the giants of the forest
belonged to the family (see frontispiece), and the number of species it
contained was very great.
Let us turn at once to this halcyon period of the group. The history of
the times intervening between it and the present is but the tale of the
dying out of the large species, and the gradual shrinking of the family
and dwarfing of its representative genera.
It is difficult to give the characters of a scientific family in a few
simple words; but perhaps we may describe the living Lycopods as plants
with creeping stems which divide and subdivide into two with great
regularity, and which bear large numbers of very small pointed leaves
closely arranged round the stem. The fruiting organs come at the tips of
the branches, and sometimes themselves divide into two, and in these
cone-like axes the spore cases are arranged, a single one on the upper
side of each of the scales (see p. 67, fig. 46, A). In the Lycopods the
spores are all alike, in the Selaginellas there are larger spores borne
in a small number (four) in some sporangia (see fig. 53, p. 75), and
others in large numbers and of smaller size on the scales above them. The
stems are all very slender, and have no zones of secondary wood. They
generally creep or climb, and from them are put out long structures
something like roots in appearance, which are specially modified
stem-like organs giving rise to roots.
From the fossils of the Coal Measures _Lepidodendron_ must be chosen as
the example for comparison. The different species of this genus are very
numerous, and the various fossilized remains of it are among the
commonest and best known of palæontological specimens. The huge stems are
objects of public interest, and have been preserved in the Victoria Park
in Glasgow in their original position in the rocks, apparently as they
grew with their spreading rootlike organs running horizontally. A great
stump is also preserved in the Manchester Museum, and is figured in the
frontispiece. While among the casts and impressions the leaf bases of the
plant are among the best preserved and the most beautiful (see fig. 93).
The cone has already been illustrated (see fig. 46 and fig. 9), and is
one of the best known of fossil fructifications.
[Illustration: Fig. 93.—Photo of Leaf Bases of _Lepidodendron_
C, Scar of leaf; S, leaf base. In the scar: _v_, mark of severed
vascular bundle, and _p_, of parichnos. _l_, Ligule scar.]
From the abundant, though scattered material, fossil botanists have
reconstructed the plants in all their detail. The trunks were lofty and
of great thickness, bearing towards the apex a much-branched crown, the
branches, even down to the finest twigs, all dividing into two equal
parts. The leaves, as would be expected from the great size of the
plants, were much bigger than those of the recent species (fig. 93 shows
the actual size of the leaf bases), but they were of the same
_relatively_ small size as compared with the stems, and of the same
simple pointed shape. A transverse section across the apex of a fertile
branch shows these closely packed leaves arranged in series round the
axis, those towards the outside show the central vascular strand which
runs through each.
[Illustration: Fig. 94.—Section across an Axis surrounded by many
Leaves, which shows their simple shape and single central vascular
bundle _v_]
The markings left on the well-preserved leaf-scars indicate the main
features of the internal anatomy of the leaves. They had a single central
vascular strand (_v_, fig. 93), on either side of which ran a strand of
soft tissue _p_ called the parichnos, which is characteristic of the
plants of this group. While another similarly obscure structure
associated with the leaf is the little scale-like ligule _l_ on its upper
surface.
The anatomy of the stems is interesting, for in the different species
different stages of advance are to be found, from the simple solid
protostele with a uniform mass of wood to hollow ring steles with a pith.
An interesting intermediate stage between these two is found in
_Lepidodendron selaginoides_ (see fig. 95), where the central cells of
the wood are not true water-conducting cells, but short irregular
water-storage tracheides (see p. 56), which are mixed with parenchyma.
All the genera of these fossils have a single central stele, round which
it is usual to find a zone of secondary wood of greater or less extent
according to the age of the plant.
[Illustration: Fig. 95.—Transverse Section of _Lepidodendron
selaginoides_, showing the circular mass of primary wood, the central
cells of which are irregular water-storage tracheides
_s_, Zone of secondary wood; _c_, inner cortical tissues; _r_,
intrusive burrowing rootlet; _oc_, outer cortical tissues with corky
external layers _k_. (Microphoto.)]
Some stems instead of this compact central stele have a ring of wood with
an extensive pith. Such a type is illustrated in fig. 96, which shows but
a part of the circle of wood, and the zone of the secondary wood outside
it, which greatly exceeds the primary mass in thickness. This zone of
secondary wood became very extensive in old stems, for, as will be
imagined, the primary wood was not sufficient to supply the large trunks.
The method of its development from a normal cambium in radiating rows of
uniform tracheides is quite similar to that which is found in the pines
to-day. This is the most important difference between the living and the
fossil stems of the family, for no living plants of the family have such
secondary wood. On the other hand, the individual elements of this wood
are different from those of the higher families hitherto considered, and
have narrow slit-like pits separated by bands of thickening on the
longitudinal walls. Such tracheides are found commonly in the
Pteridophytes, both living and fossil. Their type is seen in fig. 96, B,
which should be compared with that in figs. 78, A and 62, B to see the
contrast with the higher groups.
[Illustration: Fig. 96.—A, _Lepidodendron_ Stem with Hollow Ring of
Wood W and Zone of Secondary Wood S. B, Longitudinal View of the Narrow
Pits of the Wood Elements.]
To supply the vascular tissues of the leaf traces, simple strands come
off from the outer part of the primary wood, where groups of small-celled
protoxylem project (see _px_ in fig. 97). The leaf strands _lt_ move out
through the cortex in considerable numbers to supply the many leaves,
into each of which a single one enters.
[Illustration: Fig. 97.—Transverse Section of Outer Part of Primary
Wood of _Lepidodendron_, showing _px_, projecting protoxylem groups;
_lt_, leaf trace coming from the stele and passing (as _lt_^1) through
the cortex]
As regards the fructifications of _Lepidodendron_ much could be said were
there space. The many genera of _Lepidodendron_ bore several distinct
types of cones of different degrees of complexity. In several of the
genera the cones were simple in organization, directly comparable with
those of the living Lycopods, though on a much larger scale (see p. 67).
In some the spores were uniform, all developing equally in numerous
tetrads. The sporophyll was radially extended, and along it the large
sausage-shaped sporangia were attached (see fig. 98). The tips of the
sporophylls overlapped and afforded protection to the sporangia. The axis
of the cone had a central stele with wood elements like those in the
stem. The appearance of a transverse section of an actual cone is shown
in fig. 99. Here the sporangia are irregular in shape, owing to their
contraction after ripeness and during fossilization. Other cones had
sporangia similar in size and shape, but which produced spores of two
kinds, large ones resulting from the ripening of only two or three
tetrads in the lower sporangia, and numerous small ones in the sporangia
above.
[Illustration: Fig. 98.—Longitudinal Diagram, showing the arrangement
of the elongated sporangia on the sporophylls
_a_, Main axis, round which the sporophylls are inserted; S,
sporangium; _s_, leaflike end of sporophyll.]
The similarity between the _Lepidodendron_ and the modern Lycopod cone
has been pointed out already (p. 67), and it is this which forms the
principal guarantee that they belong to the same family, though the size
and wood development of the palæozoic and the modern plants differ so
greatly.
The large group of the Lepidodendra included some members whose
fructifications had advanced so far beyond the simple sporangial cones
described above as to approach very closely to seeds in their
construction. This type was described on p. 75, fig. 54, in a series of
female fructifications, so that its essential structure need not be
recapitulated.
[Illustration: Fig. 99.—Transverse Section through Cone of
_Lepidodendron_
A, Main axis with woody tissue; _st_, stalks of sporophylls cut in
oblique longitudinal direction; _s_, tips of sporophylls cut across; S,
sporangia with a few groups of spores. (Microphoto.)]
The section shown in fig. 100 is that cut at right angles to that in
which the sporangia are shown in fig. 98, viz. tangential to the axis. A
remarkable feature of the plant is that there were also round those
sporangia which bore the numerous small spores (corresponding to pollen
grains) enclosing integument-like flaps similar to those shown in fig.
100, _sp. f_.
[Illustration: Fig. 100.—Section through one Sporangium of
_Lepidocarpon_
_sp_, Sporophyll; _sp.f._, flaps of sporophyll protecting sporangium;
S, large spore within the sporangium wall _w_; _s_, the three aborted
spores of the tetrad to which S belongs.]
This type of fructification is the nearest approach to seed and pollen
grains reached by any of the Pteridophytes, and its appearance at a time
when the Lycopods were one of the dominant families is suggestive of the
effect that such a position has on the families occupying it, however
lowly they may be. The simple Pteridophyte Lycopods had not only the tall
trunks and solid woody structure of a modern tree, but also a semblance
of its seeds. Whether this line of development ever led on to any of the
higher families is still uncertain. The feeling of most specialists is
that it did not; but there are not wanting men who support the view that
the lycopod affinity evolved in time and entered the ranks of the higher
plants, and indeed there are many points of superficial likeness between
the palæozoic Lycopods and the Coniferæ. Judged from their internal
structure, however, the series through the ferns and Pteridosperms leads
much more convincingly to the seed plants.
In their roots, or rather in the underground structures commonly called
roots, the Lepidodendrons were also remarkable. Even more symmetrically
than in their above-ground branching, the base of their trunks divided;
there were four main large divisions, each of which branched into two and
these into two again. These structures were called _Stigmaria_, and were
common to all species of _Lepidodendron_ and also the group of
_Sigillaria_ (see fig. 102). On these horizontally running structures
(well shown in the frontispiece) small appendages were borne all over
their surface in great profusion, which were, both in their function and
microscopic structure, rootlets. They left circular scars of a
characteristic appearance on the big trunks, of which they were the only
appendages. These scars show clearly on the fragments along the ledge to
the left of the photograph. The exact morphological nature of the big
axes is not known; their anatomy is not like that of roots, but is that
of a stem, yet they do not bear what practically every stem, whether
underground or not, has developed, namely leaves, or scales representing
reduced leaves. Their nature has been commented on previously (p. 69),
and we cannot discuss the point further, but must be content to consider
them as a form of root-bearing stem, practically confined to the Lycopods
and principally developed among the palæozoic fossils of that group.
[Illustration: Fig. 101.—Transverse Section through a Rootlet of
_Stigmaria_
_oc_, Outer cortex; _s_, space; _ic_, inner cortex; _w_, wood of
vascular strand (wood only preserved); _px_, protoxylem group.]
In microscopic structure the rootlets are extremely well known, because
in their growth they have penetrated the masses of the tissues of other
plants which were being petrified and have become petrified with them.
The mass of decaying vegetable tissue on which the living plants of the
period flourished were everywhere pierced by these intrusive rootlets,
and they are found petrified inside otherwise perfect seeds, in the
hearts of woody stems, in leaves and sporangia, and sometimes even inside
each other! Fig. 95 shows such a root _r_ lying in the space left by the
decay of the soft tissue of the inner cortex in an otherwise excellently
preserved _Lepidodendron_ stem (see also fig. 101). In fig. 101 their
simple structure is seen. They are often extremely irregular in shape,
owing to the way they seem to have twisted and flattened themselves in
order to fit into the tissues they were penetrating. No root hairs seem
to have been developed in these rootlets, but otherwise their structure
is that of a typical simple root, and very like the swamp-penetrating
rootlets of the living Isoetes.
The Stigmarian axes and their rootlets are very commonly found in the
“underclays” and “gannister” beds which lie below the coal seams (see p.
25), and they may sometimes be seen attached to a bit of the trunk
growing upwards through the layers. They and the aerial stems of
Lepidodendron are perhaps the commonest and most widely known of fossil
plants.
Before leaving the palæozoic Lycopods another genus must be mentioned,
which is also a widely spread and important one, though it is less well
known than its contemporary. The genus _Sigillaria_ is best known by its
impressions and casts of stems covered by leaf scars. The stems were
sometimes deeply ribbed, and the leaf scars were arranged in rows and
were more or less hexagonal in outline, as is seen in fig. 102, which
shows a cast and its reverse of the stem of a typical _Sigillaria_.
[Illustration: Fig. 102.—Cast and Reverse of Leaf Scars of
_Sigillaria_. In A the shape of the leaf bases is clearly shown, the
central markings in each being the scar of the vascular bundle and
parichnos]
In its primary wood _Sigillaria_ differed from _Lepidodendron_ in being
more remote from the type with a primary solid stele. Its woody structure
was that of a ring, in some cases irregularly broken up into
crescent-shaped bundles. The secondary wood was quite similar to that of
_Lepidodendron_.
_Stigmaria_ and its rootlets belong equally to the two plants, and
hitherto it has been impossible to tell whether any given specimen of
_Stigmaria_ had belonged to a _Lepidodendron_ or a _Sigillaria_. Between
the two genera there certainly existed the closest affinity and
similarity in general appearance.
These two genera represent the climax of development of the Lycopod
family. In the Lower Mesozoic some large forms are still found, but all
through the Mesozoic periods the group dwindled, and in the Tertiary
little is known of it, and it seems to have taken the retiring position
it occupies to-day.