PEDIGREE AND HISTORY OF THE VEGETABLE KINGDOM.
The Natural System of the Vegetable Kingdom.—Division of the
Vegetable Kingdom into Six Branches and Eighteen Classes.—The
Flowerless Plants (Cryptogamia).—Sub-kingdom of the Thallus
Plants.—The Tangles, or Algæ (Primary Algæ, Green Algæ, Brown
Algæ, Red Algæ).—The Thread-plants, or Inophytes (Lichens and
Fungi).—Sub-kingdom of the Prothallus Plants.—The Mosses, or
Muscinæ (Water-mosses, Liverworts, Leaf-mosses,
Bog-mosses).—The Ferns, or Filicinæ (Leaf-ferns, Bamboo-ferns,
Water-ferns, Scale-ferns).—Sub-kingdom of Flowering Plants
(Phanerogamia).—The Gymnosperms, or Plants with Naked Seeds
(Palm-ferns = Cycadeæ; Pines = Coniferæ).—The Angiosperms, or
Plants with Enclosed Seeds.—Monocotylæ.—Dicotylæ.—Cup-blossoms
(Apetalæ).—Star-blossoms (Diapetalæ).—Bell-blossoms
(Gamopetalæ).
Every attempt that we make to gain a knowledge of the pedigree of any
small or large group of organisms related by blood must, in the first
instance, start with the evidence afforded by the existing “_natural
system_” of this group. For although the natural system of animals and
plants will never become finally settled, but will always represent a
merely approximate knowledge of true blood relationship, still it will
always possess great importance as a hypothetical pedigree. It is true,
by a “natural system” most zoologists and botanists only endeavour to
express in a concise way the subjective conceptions which each has
formed of the objective “_form-relationships_” of organisms. These
form-relationships, however, as the reader has seen, are in reality the
necessary result of true _blood relationship_. Consequently, every
morphologist in promoting our knowledge of the natural system, at the
same time promotes our knowledge of the pedigree, whether he wishes it
or not. The more the natural system deserves its name, and the more
firmly it is established upon the concordance of results obtained from
the study of comparative anatomy, ontogeny, and palæontology, the more
surely may we consider it as the approximate expression of the true
pedigree of the organic world.
In entering upon the task contemplated in this chapter, the genealogy of
the vegetable kingdom, we shall have, according to this principle, first
to glance at the _natural system of the vegetable kingdom_ as it is at
present (with more or less important modifications) adopted by most
botanists. According to the system generally in vogue, the whole series
of vegetable forms is divided into two main groups. These main
divisions, or sub-kingdoms, are the same as were distinguished more than
a century ago by Charles Linnæus, the founder of systematic natural
history, and which he called _Cryptogamia_, or secretly-blossoming
plants, and _Phanerogamia_, or openly-flowering plants. The latter,
Linnæus, in his artificial system of plants, divided, according to the
different number, formation, and combination of the anthers, and also
according to the distribution of the sexual organs, into twenty-three
different classes, and then added the Cryptogamia to these as the
twenty-fourth and last class.
The _Cryptogamia_, the secretly-blossoming or flowerless plants, which
were formerly but little observed, have in consequence of the careful
investigations of recent times been proved to present such a great
variety of forms, and such a marked difference in their coarser and
finer structure, that we must distinguish no less than fourteen
different classes of them; whereas the number of classes of flowering
plants, or _Phanerogamia_, may be limited to four. However, these
_eighteen classes of the vegetable kingdom_ can again be naturally
grouped in such a manner that we are able to distinguish in all _six
main divisions_ or _branches_ of the vegetable kingdom. Two of these six
branches belong to the flowering, and four to the flowerless plants. The
table on page 82 shows how the eighteen classes are distributed among
the six branches, and how these again fall under the _sub-kingdoms_ of
the vegetable kingdom.
The one sub-kingdom of the _Cryptogamia_ may now be naturally divided
into _two_ divisions, or sub-kingdoms, differing very essentially in
their internal structure and in their external form, namely, the Thallus
plants and the Prothallus plants. The group of _Thallus plants_
comprises the two large branches of Tangles, or Algæ, which live in
water, and the Thread-plants, or Inophytes (Lichens and Fungi), which
grow on land, upon stones, bark of trees, upon decaying bodies, etc. The
group of _Prothallus plants_, on the other hand, comprises the two
branches of Mosses and Ferns, containing a great variety of forms.
All _Thallus plants, or Thallophytes_, can be directly recognized from
the fact that the two morphological fundamental organs of all other
plants, stem and leaves, cannot be distinguished in their structure. The
complete body of all Algæ and of all Thread-plants is a mass composed of
simple cells, which is called a _lobe_, or _thallus_. This thallus is
as yet not differentiated into axial-organs (stem and root) and
leaf-organs. On this account, as well as through many other
peculiarities, the Thallophytes contrast strongly with all remaining
plants—those comprised under the two sub-kingdoms of Prothallus plants
and Flowering plants—and for this reason the two latter sub-kingdoms are
frequently classed together under the name of _Stemmed plants_, or
_Cormophytes_. The following table will explain the relation of these
three sub-kingdoms to one another according to the two different views:—
{ A. Thallus Plants } I. Thallus Plants
{ (_Thallophyta_) } (_Thallophyta_)
I. Flowerless Plants. {
(_Cryptogamia_) {
{ B. Prothallus Plants }
{ (_Prothallophyta_) }
} II. Stemmed Plants
} (_Cormophyta_)
}
II. Flowering Plants { C. Flowering Plants }
(_Phanerogamia_) { (_Phanerogamia_) }
The stemmed plants, or Cormophytes, in the organization of which the
difference of axial-organs (stem and root) and leaf-organs is already
developed, form at present, and have, indeed, for a very long period
formed, the principal portion of the vegetable world. However, this was
not always the case. In fact, stemmed plants, not only of the flowering
group, but even of the prothallus group, did not exist at all during
that immeasurably long space of time which forms the beginning of the
first great division of the organic history of the earth, under the name
of the archilithic, or primordial period. The reader will recollect that
during this period the Laurentian, Cambrian, and Silurian systems of
strata were deposited, the thickness of which, taken as a whole,
amounts to about 70,000 feet. Now, as the thickness of all the more
recent super-incumbent strata, from the Devonian to the deposits of the
present time, taken together, amounts to only about 60,000 feet, we were
enabled from this fact alone to draw the conclusion—which is probable
also for other reasons—that the archilithic, or primordial, period was
of longer duration than the whole succeeding period down to the present
time. During the whole of this immeasurable space of time, which
probably comprises many millions of centuries, vegetable life on our
earth seems to have been represented exclusively by the sub-kingdom of
Thallus plants, and, moreover, only by the class of marine Thallus
plants, that is to say, the Algæ. At least all the petrified remains
which are positively known to be of the primordial period belong
exclusively to this class. As all the animal remains of this immense
period also belong exclusively to animals that lived in water, we come
to the conclusion that at that time organisms adapted to a life on land
did not exist at all.
For these reasons the first and most imperfect of the great provinces or
branches of the vegetable kingdom, the division of the Algæ, or Tangles,
must be of special interest to us. But, in addition, there is the
interest which this group offers when viewed by itself. In spite of the
exceedingly simple composition of their constituent cells, which are but
little differentiated, the Algæ show an extraordinary variety of
different forms. To them belong the simplest and most imperfect of all
forms, as well as very highly developed and peculiar forms. The
different groups of Algæ are distinguished as much by size of body as by
the perfection and variety of their outer form. At the lowest stage we
find such species as the minute Protococcus, several hundred thousands
of which occupy a space no larger than a pin’s head. At the highest
stage we marvel at the gigantic Macrocysts, which attain a length of
from 300 to 400 feet, the longest of all forms in the vegetable kingdom.
It is possible that a large portion of the coal has been formed out of
Algæ. If not for these reasons, yet the Algæ must excite our special
attention from the fact that they form the beginning of vegetable life,
and contain the original forms of all other groups of plants, supposing
that our monophyletic hypothesis of a common origin for all groups of
plants is correct. (Compare p. 83.)
SYSTEMATIC VIEW
_Of the Six Branches and Eighteen Classes of the Vegetable Kingdom._
==========================================================================================
_Primary Groups_ | | |
_or Sub-Kingdoms_ | _Branches or Clades_ | _Classes_ | _Systematic Name_
_of the_ | _of the_ | _of the_ | _of the_
_Vegetable Kingdom._ | _Vegetable Kingdom._ | _Vegetable Kingdom._ | _Classes._
------------------------+-----------------------+----------------------+------------------
{ { 1. Primæval algæ 1. _Archephyceæ_
{ { (Protophyta)
{ {
{ I. { 2. Green algæ 2. _Chlorophyceæ_
A. { Tangles { (Chloroalgæ)
=Thallus Plants= { _Algæ_ {
Thallophyta { { 3. Brown algæ 3. _Phæophyceæ_
{ { (Fucoideæ)
{ {
{ { 4. Red algæ 4. _Rhodophyceæ_
{ { (Florideæ)
{ II. { 5. Lichens 5. _Lichenes_
{ Thread-plants {
{ _Inophyta_ { 6. Fungi 6. _Fungi_
{
{ { 7. Tangle-mosses 7. _Charobrya_
{ { (Characeæ)
{ {
{ III. { 8. Liverworts 8. _Thallobrya_
{ Mosses { (Hepaticæ)
{ _Muscinæ_ {
{ { 9. Frondose-mosses 9. _Phyllobrya_
B. { { (Frondosæ)
=Prothallus= { {
=Plants= { { 10. Turf-mosses 10. _Sphagnobrya_
Prothallophyta { (Sphagnaceæ)
{
{ { 11. Shaft-ferns 11. _Calamariæ_
{ { (Calamophyta)
{ IV. {
{ Ferns { 12. Frondose-ferns 12. _Filices_
{ _Felicinæ_ { (Pterideæ)
{ {
{ { 13. Aquatic ferns 13. _Rhizocarpeæ_
{ { (Hydropterides)
{ {
{ { 14. Scale-ferns 14. _Selagineæ_
{ { (Lepidophyta)
{ V. {
{ Plants with { 15. Palm-ferns 15. _Cycadeæ_
C. { Naked Seeds {
=Flowering Plants= { _Gymnosperma_ { 16. Pines 16. _Coniferæ_
Phanerogamia {
{ VI. { 17. Plants with 17. _Monocotylæ_
{ Plants with { one seed lobe
{ Enclosed Seeds {
{ _Angiosperma_ { 18. Plants with 18. _Dicotylæ_
{ { two seed lobes
----------------------------------------------------------------
_Gamopetalæ_
(Flowers with corolla)
|
_Dialypetalæ_
(Star-shaped flowers)
|
_Monochlamydeæ_ MONOCOTYLEDONÆ
(Flowers with calyx) (One seed-lobed plants)
| |
DICOTYLEDONÆ |
(Two seed-lobed plants) |
| |
\--------------------v-------------/
CONIFERÆ +Angiospermæ+
CYCADEÆ (Pines) (Plants with enclosed seeds)
(Palm-ferns) | GNETACEÆ |
| | | |
\-----------v-----------------------/
+Gymnospermæ+
(Plants with naked seeds) _Phanerogamæ_
| (Flowering plants) _Pterideæ_
_Selagineæ_ | _Rhizocarpeæ_ | (Frondose-ferns)
(Scaled-ferns) |(Water-ferns) | | _Calamariæ_
| | | | | (Shaft-ferns)
| | | | | |
\-------------------------------v------------------------/
+Filicinæ+
_Frondosæ_ _Sphagnaceæ_ (Ferns)
(Leaf-mosses) (Turf-mosses) |
| | | CHARACEÆ
\----------------v------------/ (Tangle-mosses)
| |
_Hepaticæ_ (Liverworts) |
| |
\------------------v--------------------/
+Muscinæ+ (Mosses)
_Fucoideæ_ |
_Florideæ_ (Brown Algæ) | _Lichenes_
(Red Algæ) | _Chlorophyceæ_ (Lichens)
| | (Green Algæ) |
| | | |
\------------------v-----------------/ +Fungi Inophyta+
+Algæ+ (Tangles) (Thread-plants)
| |
\-------v------------------------------/
_Protophyta_
(Primæval Plants)
|
_Vegetable Monera_
Most people living inland can form but a very imperfect idea of this
exceedingly interesting branch of the vegetable kingdom, because they
know only its proportionately small and simple representatives living in
fresh water. The slimy green aquatic filaments and flakes of our pools
and ditches and springs, the light green slimy coverings of all kinds of
wood which have for any length of time been in contact with water, the
yellowish green, frothy, and oozy growths of our village ponds, the
green filaments resembling tufts of hair which occur everywhere in fresh
water, stagnant and flowing, are for the most part composed of different
species of Algæ. Only those who have visited the sea-shore, and wondered
at the immense masses of cast-up seaweed, and who, from the rocky coast
of the Mediterranean, have seen through the clear blue waters the
beautifully-formed and highly-coloured vegetation of Algæ at the bottom,
know how to estimate the importance of the class of Algæ. And yet, even
these marine Algæ-forests of European shores, so rich in forms, give
only a faint idea of the colossal forests of Sargasso in the Atlantic
ocean, those immense banks of Algæ, covering a space of about 40,000
square miles—the same which made Columbus, on his voyage of discovery,
believe that a continent was near. Similar but far more extensive
forests of Algæ grew in the primæval ocean, probably in dense masses,
and what countless generations of these archilithic Algæ have died out
one after another is attested, among other facts, by the vast thickness
of Silurian alum schists in Sweden, the peculiar composition of which
proceeds from those masses of submarine Algæ. According to the recently
expressed opinion of Frederick Mohr, a geologist of Bonn, even the
greater part of our coal seams have arisen out of the accumulated dead
bodies of the Algæ forests of the ocean.
Within the branch of the Algæ we distinguish four different classes,
each of which is again divided into several orders and families. These
again contain a large number of different genera and species. We
designate these four classes as Primæval Algæ, or Archephyceæ, Green
Algæ, or Chlorophyceæ, Brown Algæ, or Phæophyceæ, and Red Algæ, or
Rhodophyceæ.
The first class of Algæ, the _Primæval_ Algæ (Archephyceæ), might also
be called _primæval plants_, because they contain the simplest and most
imperfect of all plants, and, among them, those most ancient of all
vegetable organisms out of which all other plants have originated. To
them therefore belong those most ancient of all vegetable Monera which
arose by spontaneous generation in the beginning of the Laurentian
period. Further, we have to reckon among them all those vegetable forms
of the simplest organization which first developed out of the Monera in
the Laurentian period, and which possessed the form of a single
plastid. At first the entire body of one of these small primary plants
consisted only of a most simple cytod (a plastid without kernel), and
afterwards attained the higher form of a simple cell, by the separation
of a kernel in the plasma. (Compare above, vol. i. p. 345.) Even at the
present day there exist various most simple forms of Algæ which have
deviated but little from the original primary plants. Among them are the
Algæ of the families Codiolaceæ, Protococcaceæ, Desmidiaceæ,
Palmellaceæ, Hydrodictyeæ, and several others. The remarkable group of
Phycochromaceæ (Chroococcaceæ and Oscillarineæ) might also be comprised
among them, unless we prefer to consider them as an independent tribe of
the kingdom Protista.
The monoplastic Protophyta—that is, those primary Algæ formed by a
single plastid—are of the greatest interest, because the vegetable
organism in this case completes its whole course of life as a perfectly
simple “individual of the first order,” either as a cytod without
kernel, or as a cell containing a kernel.
Among the primary plants consisting of a single cytod are the
exceedingly remarkable Siphoneæ, which are of considerable size, and
strangely “mimic” the forms of higher plants. Many of the Siphoneæ
attain a size of several feet, and resemble an elegant moss (Bryopsis),
or in some cases a perfect flowering plant with stalks, roots, and
leaves (Caulerpa) (Fig. 17). Yet the whole of this large body,
externally so variously differentiated, consists internally of an
entirely simple sack, possessing the negative characters of a simple
cytod.
[Illustration: FIG. 17.—Caulerpa denticulata, a monoplastic Siphonean
_of the natural size_. The entire branching primary plant, which appears
to consist of a creeping stalk with fibrous roots and indented leaves,
is in reality only _a single plastid_, and moreover a cytod (without a
kernel), not even attaining the grade of a cell with nucleus.]
These curious Siphoneæ, Vaucheriæ, and Caulerpæ show us to how great a
degree of elaboration a single cytod, although a most simple individual
of the first order, can develop by continuous adaptation to the
relations of the outer world. Even the _single-celled primary
plants_—which are distinguished from the monocytods by possessing a
kernel—develop into a great variety of exquisite forms by adaptation;
this is the case especially with the beautiful _Desmidiaceæ_, of which
a species of Euastrum is represented in Fig. 18 as a specimen.
[Illustration: FIG. 18.—Euastrum rota, a single-celled Desmid, much
enlarged. The whole of the star-shaped body of this primæval plant has
the formal value of a simple cell. In its centre lies the kernel, and
within this the kernel corpuscle, or speck.]
It is very probable that similar primæval plants, the soft body of
which, however, was not capable of being preserved in a fossil state, at
one time peopled the Laurentian primæval sea in great masses and
varieties, and in a great abundance of forms, without, however, going
beyond the stage of individuality of a simple plastid.
The group of _Green Tangles_ (Chlorophyceæ), or _Green Algæ_
(Cloroalgæ), are the second class, and the most closely allied to the
primæval group. Like the majority of the Archephyceæ, all the
Chlorophyceæ are coloured green, and by the same colouring matter—the
substance called leaf-green, or chlorophyll—which colours the leaves of
all the higher plants.
To this class belong, besides a great number of low marine Algæ, most of
the Algæ of fresh water, the common water hair-weeds, or Confervæ, the
green slime-balls, or Glœosphæræ, the bright green water-lettuce, or
Ulva, which resembles a very thin and long lettuce leaf, and also
numerous small microscopic algæ, dense masses of which form a light
green shiny covering to all sorts of objects lying in water—wood,
stones, etc.
These forms, however, rise above the simple primary Algæ in the
composition and differentiation of their body. As the green Algæ, like
the primæval Algæ, mostly possess a very soft body, they are but rarely
capable of being petrified. However, it can scarcely be doubted that
this class of Algæ—which was the first to develop out of the preceding
one—most extensively and variously peopled the fresh and salt waters of
the earth in early times.
In the third class, that of the _Brown Tangles_ (Phæophyceæ), or _Black
Algæ_ (Fucoideæ), the _branch_ of the Algæ attains its highest stage of
development, at least in regard to size and body. The characteristic
colour of the Fucoid is more or less dark brown, sometimes tending more
to an olive green or yellowish green, sometimes more to a brownish red
or black colour.
Among these are the largest of all Algæ, which are at the same time the
longest of all plants, namely, the colossal giant Algæ, amongst which
the Macrocystis pyrifera, on the coast of California, attains a length
of 400 feet. Also, among our indigenous Algæ, the largest forms belong
to this group. Especially I may mention here the stately sugar-tangle
(Laminaria), whose slimy, olive green thallus-body, resembling gigantic
leaves of from 10 to 15 feet in length, and from a half to one foot in
breadth, are thrown up in great masses on the coasts of the North and
Baltic seas.
To this class belongs also the bladder-wrack (Fucus vesiculosus) common
in our seas, whose fork-shaped, deeply-cut leaves are kept floating on
the water by numerous air bladders (as is the case, too, with many other
brown Algæ). The freely floating Sargasso Alga (Sargasso bacciferum),
which forms the meadows or forests of the Sargasso Sea, also belongs to
this class.
Although each individual of these large alga-trees is composed of many
millions of cells, yet at the beginning of its existence it consists,
like all higher plants, of a single cell—a simple egg. This egg—for
example, in the case of our common bladder-wrack—is a naked, uncovered
cell, and as such is so like the naked egg-cells of lower marine
animals—for example, those of the Medusæ—that they might easily be
mistaken one for another (Fig. 19).
[Illustration: FIG. 19.—The egg of the common bladder-wrack (Fucus
vesiculosus), a simple naked cell, much enlarged. In the centre of the
naked globule of protoplasm the bright kernel is visible.]
It was probably the Fucoideæ, or Brown Algæ, which during the primordial
period, to a great extent constituted the characteristic alga-forests of
that immense space of time. Their petrified remains, especially those
of the Silurian period, which have been preserved, can, it is true,
give us but a faint idea of them, because the material of these Algæ,
like that of most others, is ill-suited for preservation in a fossil
state. As has already been remarked, a large portion of coal is perhaps
composed of them.
Less important is the fourth class of Algæ, that of the _Rose-coloured
Algæ_ (Rhodophyceæ), or _Red Sea-weeds_ (Florideæ). This class, it is
true, presents a great number of different forms; but most of them are
of much smaller size than the Brown Algæ. Although they are inferior to
the latter in perfection and differentiation, they far surpass them in
some other respects. To them belong the most beautiful and elegant of
all Algæ, which on account of the fine plumose division of their
leaf-like bodies, and also on account of their pure and delicate red
colour, are among the most charming of plants. The characteristic red
colour sometimes appears as a deep purple, sometimes as a glowing
scarlet, sometimes as a delicate rose tint, and may verge into violet
and bluish purple, or on the other hand into brown and green tints of
marvellous splendour. Whoever has visited one of our sea-coast watering
places, must have admired the lovely forms of the Florideæ, which are
frequently dried on white paper and offered for sale.
Most of the Red Algæ are so delicate, that they are quite incapable of
being petrified; this is the case with the splendid Ptilotes, Plocamia,
Delesseria, etc. However, there are individual forms, like the Chondria
and Sphærococca, which possess a harder thallus, often almost as hard as
cartilage, and of these fossil remains have been preserved—principally
in the Silurian, Devonian, and Carboniferous strata, and later in the
oolites. It is probable that this class also had an important share in
the composition of the archilithic Algæ flora.
If we now again take into consideration the flora of the primordial
period, which was exclusively formed by the group of Algæ, we can see
that it is not improbable that its four subordinate classes had a share
in the composition of those submarine forests of the primæval oceans,
similar to that which the four types of vegetation—trees with trunks,
flowering shrubs, grass, and tender leaf-ferns and mosses—at present
take in the composition of our recent land forests.
We may suppose that the submarine tree forests of the primordial period
were formed by the huge Brown Algæ, or Fucoideæ. The many-coloured
flowers at the foot of these gigantic trees were represented by the gay
Red Algæ, or Florideæ. The green grass between was formed by the
hair-like bunches of Green Algæ, or Chloroalgæ. Finally, the tender
foliage of ferns and mosses, which at present cover the ground of our
forests, fill the crevices left by other plants, and even settle on the
trunks of the trees, at that time probably had representatives in the
moss and fern-like Siphoneæ, in the Caulerpa and Bryopsis, from among
the class of the primary Algæ, Protophyta, or Archephyceæ.
With regard to the relationships of the different classes of Algæ to one
another and to other plants, it is exceedingly probable that the Primary
Algæ, or Archephyceæ, as already remarked, form the common root of the
pedigree, not merely for the different classes of Algæ, but for the
whole vegetable kingdom. On this account they may with justice be
designated as primæval plants, or Protophyta.
Out of the naked vegetable Monera, in the beginning of the Laurentian
period, enclosed cytods were probably the first to arise (vol. i. p.
345), by the naked, structureless, albuminous substance of the Monera
becoming condensed in the form of a pellicle on the surface, or by
secreting a membrane. At a later period, out of these enclosed cytods
genuine vegetable cells probably arose, as a kernel or nucleus separated
itself in the interior from the surrounding cell-substance or plasma.
The three classes of Green Algæ, Brown Algæ, and Red Algæ, are perhaps
three distinct classes, which have arisen independently of one another
out of the common radical group of Primæval Algæ, and then developed
themselves further (each according to its kind), and have variously
branched off into orders and families. The Brown and Red Algæ possess no
close blood relationship to the other classes of the vegetable kingdom.
These latter have most probably arisen out of the Primæval Algæ, either
directly or by the intermediate step of the Green Algæ.
It is probable that Mosses (out of which, at a later time, Ferns
developed) proceeded from a group of Green Algæ, and that Fungi and
Lichens proceeded from a group of Primæval Algæ. The Phanerogamia
developed at a much later period out of Ferns.
As a second class of the Vegetable Kingdom we have above mentioned the
_Thread-plants_ (Inophyta). We understood by this term the two closely
related classes of _Lichens_ and _Fungi_. It is possible that these
Thallus plants have not arisen out of the Primæval Algæ, but out of one
or more Monera, which, independently of the latter, arose by spontaneous
generation. It appears conceivable that many of the lowest Fungi, as for
example, many ferment-causing fungi (forms of Micrococcus, etc.), owe
their origin to a number of different _archigonic_ Monera (that is,
Monera originating by spontaneous generation).
In any case the Thread-plants cannot be considered as the progenitors of
any of the higher vegetable classes. Lichens, as well as fungi, are
distinct from the higher plants in the composition of their soft bodies,
consisting as it does of a dense felt-work of very long, variously
interwoven, and peculiar threads or chains of cells—the so-called
_hyphæ_, on which account we distinguish them as a province under the
name Thread-plants. From their peculiar nature they could not leave any
important fossil remains, and consequently we can form only a very vague
guess at their palæontological development.
The first class of Thread-plants, the _Fungi_, exhibit a very close
relationship to the lowest Algæ; the Algo-fungi, or Phycomycetes (the
Saprolegniæ and Peronosporæ) in reality only differ from the
bladder-wracks and Siphoneæ (the Vaucheria and Caulerpa) mentioned
previously by the want of leaf-green, or chlorophyll. But, on the other
hand, all genuine Fungi have so many peculiarities, and deviate so much
from other plants, especially in their mode of taking food, that they
might be considered as an entirely distinct province of the vegetable
kingdom.
Other plants live mostly upon inorganic food, upon simple combinations
which they render more complicated. They produce protoplasm by the
combination of water, carbonic acid, and ammonia. They take in carbonic
acid and give out oxygen. But the Fungi, like animals, live upon organic
food, consisting of complicated combinations of carbon, which they
receive from other organisms and assimilate. They inhale oxygen and
give out carbonic acid like animals. They also never form leaf-green, or
chlorophyll, which is so characteristic of most other plants. In like
manner they never produce starch. Hence many eminent botanists have
repeatedly proposed to remove the Fungi completely out of the vegetable
kingdom, and to regard them as a special and third kingdom, between that
of animals and plants. By this means our kingdom of Protista would be
considerably increased. The Fungi in this case would, in the first
place, be allied to the so-called “slime moulds,” or Myxomycetes (which,
however, never form any hyphæ). But as many Fungi propagate in a sexual
manner, and as most botanists, according to the prevalent opinion, look
upon Fungi as genuine plants, we shall here leave them in the vegetable
kingdom, and connect them with lichens, to which they are at all events
most nearly related.
The phyletic origin of Fungi will probably long remain obscure. The
close relationship already hinted at between the Phycomycetes and
Siphoneæ (especially between the Saprolegniæ and Vaucheriæ) suggests to
us that they are derived from the latter. Fungi would then have to be
considered as Algæ, which by adaptation to a parasitical life have
become very peculiarly transformed. Many facts, however, support the
supposition that the lowest fungi have originated independently from
archigonic Monera.
The second class of Inophyta, the _Lichens_ (Lichenes), are very
remarkable in relation to phylogeny; for the surprising discoveries of
late years have taught us that every Lichen is really composed of two
distinct plants—of a low form of Alga (Nostochaceæ, Chroococcaceæ), and
of a parasitic form of Fungus (Ascomycetes), which lives as a parasite
upon the former, and upon the nutritive substances prepared by it. The
green cells, containing chlorophyll (gonidia), which are found in every
lichen, belong to the Alga. But the colourless threads (hyphæ) which,
densely interwoven, form the principal mass of the body of Lichens,
belong to the parasitic Fungus. But in all cases the two forms of
plants—Fungus and Alga—which are always considered as members of two
quite distinct provinces of the vegetable kingdom, are so firmly united,
and so thoroughly interwoven, that nearly every one looks upon a Lichen
as a single organism.
Most Lichens form small, more or less formless or irregularly indented,
crust-like coverings to stones, bark of trees, etc. Their colour varies
through all possible tints, from the purest white to yellow, red, green,
brown, and the deepest black.
Many lichens are important in the economy of nature from the fact that
they can settle in the driest and most barren localities, especially on
naked rocks upon which no other plant can live. The hard black lava,
which covers many square miles of ground in volcanic regions, and which
for centuries frequently presents the most determined opposition to the
life of every kind of vegetation, is always first occupied by Lichens.
It is the white or grey Lichens (Stereocaulon) which, in the most
desolate and barren fields of lava, always begin to prepare the naked
rocky ground for cultivation, and conquer it for subsequent higher
vegetation. Their decaying bodies form the first mould in which mosses,
ferns, and flowering plants can afterwards take firm root. Hardy Lichens
are also less affected by the severity of climate than any other plants.
Hence the naked rocks, even in the highest mountains—for the most part
covered by eternal snow, on which no plant could thrive—are encrusted by
the dry bodies of Lichens.
Leaving now the Fungi, Lichens, and Algæ, which are comprised under the
name of Thallus plants, we enter upon the second sub-kingdom of the
vegetable kingdom, that of the _Prothallus plants_ (Prothallophyta),
which by some botanists are called phyllogonic Cryptogamia (in
contradistinction to the Thallus plants, or thallogonic Cryptogamia).
This sub-kingdom comprises the two provinces of _Mosses_ and _Ferns_.
Here we meet with (except in a few of the lowest forms) the separation
of the vegetable body into two different fundamental organs,
axial-organs (stem and root) and leaves (or lateral organs). In this the
Prothallus plants resemble the Flowering plants, and hence the two
groups have recently often been classed together as stemmed plants, or
Cormophytes.
But, on the other hand, Mosses and Ferns resemble the Thallus plants, in
the absence of the development of flowers and seeds, and even Linnæus
classed them with these, as Cryptogamia, in contradistinction to the
plants forming seeds; that is, flowering plants (Anthophyta or
Phanerogamia).
Under the name of “Prothallus plants” we combine the closely-related
Mosses and Ferns, because both exhibit a peculiar and characteristic
“alternation of generation” in the course of their individual
development. For every species exhibits two different generations, of
which the one is usually called the _Prothallium_, or _Fore-growth_, the
other is spoken of as the _Cormus_, or actual _Stem_ of the moss or
fern.
The first and original generation, the Fore-growth, or Prothallus, also
called Protonema, still remains in that lower stage of elaboration
manifested throughout life by all Thallus plants; that is to say, stem
and leaf-organs have as yet not differentiated, and the entire cell-mass
of the Fore-growth corresponds to a simple thallus. The second and more
perfect generation of mosses and ferns—the Stem, or Cormus—develops a
much more highly elaborate body, which has differentiated into stalk and
leaf (as in the case of flowering plants), except in the lowest mosses,
where this generation also remains in the lower stage of the thallus.
With the exception of these latter forms the first generation of Mosses
and Ferns (the thallus-shaped Fore-growth) always produces a second
generation with stem and leaves; the latter in its turn produces the
thallus of the first generation, and so on. Thus, in this case, as in
the ordinary cases of alternation of generation in animals, the first
generation is like the third, fifth, etc., the second like the fourth,
sixth, etc. (Compare vol. i. p. 206.)
Of the two main classes of Prothallus plants, the Mosses in general are
at a much lower stage of development than the Ferns, and their lowest
forms (especially in an anatomical respect) form the transition from the
Thallus plants through the Algæ to Ferns. The genealogical connection of
Mosses and Ferns which is indicated by this fact can, however, be
inferred only from the case of the most imperfect forms of the two
classes; for the more perfect and higher groups of mosses and ferns do
not stand in any close relation to one another, and develop in
completely opposite directions. In any case Mosses have arisen directly
out of Thallus plants, and probably out of Green Algæ.
Ferns, on the other hand, are probably derived from extinct unknown
Mosses, which were very nearly related to the lowest liverworts of the
present day. In the history of creation, Ferns are of greater importance
than Mosses.
The branch of _Mosses_ (Muscinæ, also called Musci, or Bryophyta)
contains the lower and more imperfect plants of the group of
Prothallophytes, which as yet do not possess vessels. Their bodies are
mostly so tender and perishable that they are very ill-suited for being
preserved in a recognizable state as fossils. Hence the fossil remains
of all classes of Mosses are rare and insignificant. It is probable that
Mosses developed in very early times out of the Thallus plants, or, to
be more precise, out of the Green Algæ. It is probable that in the
primordial period there existed aquatic forms of transition from the
latter to Mosses, and in the primary period to those living on land. The
Mosses of the present day—out of the gradually differentiating
development of which comparative anatomy may draw some inferences as to
their genealogy—are divided into two different classes, namely: (1)
Liverworts; (2) Leafy Mosses.
The first and oldest class of Mosses, which is directly allied to the
Green Algæ, or Confervæ, is formed by the _Liverworts_ (Hepaticæ, or
Thallobrya). The mosses belonging to them are, for the most part, small
and insignificant in form, and are little known. Their lowest forms
still possess, in both generations, a simple thallus like the Thallus
plants; as for example, the Ricciæ and Marchantiaceæ. But the more
highly developed liverworts, the Jungermanniaceæ and those akin to them,
gradually commence to differentiate stem and leaf, and their most
highly-developed forms are closely allied to leaf-mosses. By this
transitional series the liverworts show their direct derivation from
the Thallophytes, and more especially from the Green Algæ.
The Mosses, which are generally the only ones known to the
uninitiated—and which, in fact, form the principal portion of the whole
branch—belong to the second class, or _Leafy Mosses_ (Musci frondosi,
called Musci in a narrow sense, also Phyllobrya). Among them are most of
those pretty little plants which, united in dense groups, form the
bright glossy carpet of moss in our woods, or which, in company with
liverworts and lichens, cover the bark of trees. As reservoirs,
carefully storing up moisture, they are of the greatest importance in
the economy of nature. Wherever man mercilessly cuts down and destroys
forests, there, as a consequence, disappear the leafy mosses which
covered the bark of the trees, or, protected by their shade, clothed the
ground, and filled the spaces between the larger plants. Together with
the leafy mosses disappear the useful reservoirs which stored up rain
and dew for times of drought. Thus arises a disastrous dryness of the
ground, which prevents the growth of any rich vegetation. In the greater
part of Southern Europe—in Greece, Italy, Sicily, and Spain—mosses have
been destroyed by the inconsiderate extirpation of forests, and the
ground has thereby been robbed of its most useful stores of moisture;
once flourishing and rich tracts of land have been changed into dry and
barren wastes. Unfortunately in Germany, also, this rude barbarism is
beginning to prevail more and more. It is probable that the small
frondose mosses have played this exceedingly important part in nature
for a very long time, possibly from the beginning of the primary period.
But as their tender bodies are as little suited as those of all other
mosses for being preserved in a fossil state, palæontology can give us
no information about this.
We learn from the science of petrifactions much more than we do in the
case of Mosses of the importance which the second branch of Prothallus
plants—that is, Ferns—have had in the history of the vegetable world.
Ferns, or more strictly speaking, the “plants of the fern tribe”
(Filicineæ, or Pterideæ, also called Pteridophyta, or Vascular
Cryptogams), formed during an extremely long period, namely, during the
whole primary or palæolithic period, the principal portion of the
vegetable world, so that we may without hesitation call it the _era of
Fern Forests_. From the beginning of the Devonian period, in which
organisms living on land appeared for the first time, namely, during the
deposits of the Devonian, Carboniferous, and Permian strata, plants like
Ferns predominated so much over all others, that we are justified in
giving this name to that period. In the stratifications just mentioned,
but above all, in the immense layers of coal of the Carboniferous or
coal period, we find such numerous and occasionally well preserved
remains of Ferns, that we can form a tolerable vivid picture of the very
peculiar land flora of the palæolithic period. In the year 1855 the
total number of the then known palæolithic species of plants amounted to
about a thousand, and among these there were no less than 872 Ferns.
Among the remaining 128 species were 77 Gymnosperms (pines and
palm-ferns), 40 Thallus plants (mostly Algæ), and about 20 not
accurately definable Cormophyta (stem-plants).
As already remarked, Ferns probably developed out of the lower
liverworts in the beginning of the primary period. In their
organization Ferns rise considerably above Mosses, and in their more
highly developed forms even approach the flowering plants. In Mosses, as
in Thallus plants, the entire body is composed of almost equi-formal
cells, little if at all differentiated; but in the tissues of Ferns we
find those peculiarly differentiated strings of cells which are called
the vessels of plants, and which are universally met with in flowering
plants. Hence Ferns are sometimes united as “vascular Cryptogams” with
Phanerogams, and the group so formed is contrasted as that of the
“vascular plants” with “cellular plants,”—that is, with “cellular
cryptogams” (Mosses and Thallus plants). This very important process in
the organization of plants—the formation of vessels—first occurred,
therefore, in the Devonian period, consequently in the beginning of the
second and smaller half of the organic history of the earth.
The branch of Ferns, or Filicinæ, is divided into five distinct classes:
(1) Frondose Ferns, or Pteridæ; (2) Reed Ferns, or Calamariæ; (3)
Aquatic Ferns, or Rhizocarpeæ; (4) Snakes Tongues, or Ophioglossæ; and
(5) Scale Ferns, or Lepidophyta. By far the most important of these five
classes, and also the richest in forms, were first the Frondose Ferns,
and then the Scale-ferns, which formed the principal portion of the
palæolithic forests. The Reed Ferns, on the other hand, had at that time
already somewhat diminished in number; and of the Aquatic Ferns, we do
not even know with certainty whether they then existed. It is difficult
for us to form any idea of the very peculiar character of those gloomy
palæolithic fern forests, in which the whole of the gay abundance of
flowers of our present flora was entirely wanting, and which were not
enlivened by any birds. Of the flowering plants there then existed only
the two lowest classes, the pines and palm ferns, with naked seeds,
whose simple and insignificant blossoms scarcely deserve the name of
flowers.
The phylogeny of Ferns, and of the Gymnosperms which have developed out
of them, has been made especially clear by the excellent investigations
which Edward Strasburger published in 1872, on “The Coniferæ and
Gnetaceæ,” as also “On Azolla.” This thoughtful naturalist and Charles
Martins, of Montpellier, are among the few botanists who have thoroughly
understood the fundamental value of the Theory of Descent, and the
mechanical-causal connection between ontogeny and phylogeny. The
majority of botanists do not even yet know the important difference
between homology and analogy, between the morphological and
physiological comparison of parts—which has long since been recognized
in zoology—but Strasburger has employed this distinction and the
principle of evolution in his “Comparative Anatomy of the Gymnosperms,”
in order to sketch the outlines of the blood relationship of this
important group of plants.
The class among Ferns which has developed most directly out of the
Liverworts is the class of real Ferns, in the narrow sense of the word,
the _Frondose Ferns_ (Filices, or Phyllopterides, also called Pteridæ).
In the present flora of the temperate zones this class forms only a
subordinate part, for it is in most cases represented only by low forms
without trunks. But in the torrid zones, especially in the moist,
steaming forests of tropical regions, this class presents us with the
lofty palm-like _fern trees_. These beautiful tree-ferns of the present
day, which form the chief ornament of our hot-houses, can however give
us but a faint idea of the stately and splendid frondose ferns of the
primary period, whose mighty trunks, densely crowded together, then
formed entire forests. These trunks, accumulated in super-incumbent
masses, are found in the coal seams of the Carboniferous period, and
between them, in an excellent state of preservation, are found the
impressions of the elegant fan-shaped leaves, crowning the top of the
trunk in an umbrella-like bush. The varied outlines and the
feather-like forms of these fronds, the elegant shape of the branching
veins or bunches of vessels in their tender foliage, can still be as
distinctly recognized in the impressions of the palæolithic fronds as in
the fronds of ferns of the present day. In many cases even the clusters
of fruit, which are distributed on the lower surface of the fronds, are
distinctly preserved. After the Carboniferous period, the predominance
of frondose ferns diminished, and towards the end of the secondary
period they played almost as subordinate a part as they do at the
present time.
The Calamariæ, Ophioglossæ, and Rhizocarpeæ seem to have developed as
three diverging branches out of the Frondose Ferns, or Pteridæ. The
Calamariæ, or Calamophyta, have remained at the lowest level among these
three classes. The Calamariæ comprise three different orders, of which
only one now exists, namely, the Horse-tails (Equisetaceæ). The two
other orders, the Giant Reeds (Calamiteæ), and the Star-leaf Reeds
(Asterophylliteæ), are long since extinct. All Calamariæ are
characterized by a hollow and jointed stalk, stem, or trunk, upon which
the branches and leaves (in cases where they exist) are set so as to
encircle the jointed stem in whorls. The hollow joints of the stalk are
separated from one another by partition walls. In Horse-tails and
Calamiteæ the surface is traversed by longitudinal ribs running
parallel, as in the case of a fluted column, and the outer skin contains
so much silicious earth in the living forms, that it is used for
cleansing and polishing. In the Asterophylliteæ, the star-shaped whorls
of leaves were more strongly developed than in the two other orders.
There exist, at present, of the Calamariæ only the insignificant
Horse-tails (Equisetum), which grow in marshes and on moors; but during
the whole of the primary and secondary periods they were represented by
great trees of the genus Equisetites. There existed, at the same time,
the closely related order of the Giant Reeds (Calamites), whose strong
trunks grew to a height of about fifty feet. The order of the
Asterophyllites, on the other hand, contained smaller and prettier
plants, of a very peculiar form, and belongs exclusively to the primary
period.
Among all Ferns, the history of the third class, that of the _Root_, or
_Aquatic Ferns_ (Rhizocarpeæ, or Hydropteridæ), is least known to us. In
their structure these ferns, which live in fresh water, are on the one
hand allied to the frond ferns, and on the other to the scaly ferns, but
they are more closely related to the latter. Among them are the but
little known moss ferns (Salvinia), clover ferns (Marsilea), and pill
ferns (Pilularia) of our fresh waters; further, the large Azolla which
floats in tropical ponds. Most of the aquatic ferns are of a delicate
nature, and hence ill-suited for being petrified. This is probably the
reason of their fossil remains being so scarce, and of the oldest of
those known to us having been found in the Jura system. It is probable,
however, that the class is much older, and that it was already
developed during the palæolithic period out of other ferns by adaptation
to an aquatic life.
The fourth class of ferns is formed by the _Tongue Ferns_ (Ophioglossæ,
or Glossopterides). These ferns, to which belongs the Botrychium, as
well as the Ophioglossum (adder’s-tongue) of our native genera, were
formerly considered as forming but a small sub-division of the frondose
ferns. But they deserve to form a special class, because they represent
important transitional forms from the Pterideæ and Lepidophytes towards
higher plants, and must be regarded as among the direct progenitors of
the flowering plants.
The fifth and last class is formed by the _Scale Ferns_ (Lepidophytes,
or Selagines). In the same way as the Ophioglossæ arose out of the
frondose forms, the scale ferns arose out of the Ophioglossæ. They were
more highly developed than all other ferns, and form the transition to
flowering plants, which must have developed out of them. Next to the
frondose ferns they took the largest part in the composition of the
palæolithic fern forests. This class also contains, as does the class of
reed ferns, three nearly related but still very different orders, of
which only one now exists, the two others having become extinct towards
the end of the Carboniferous period. The scaled ferns still existing
belong to the order of the club-mosses (Lycopodiaceæ). They are mostly
small, pretty moss-like plants, whose tender, many-branched stalk creeps
in curves on the ground like a snake, and is densely encompassed and
covered by small scaly leaves. The pretty creeping Lycopodium of our
woods, which mountain tourists twine round their hats, is known to all,
as also the still more delicate Selaginella, which under the name of
creeping moss is used to adorn the soil of our hot-houses in the form of
a thick carpet. The largest _club-mosses_ of the present day are found
in the Sunda Islands, where their stalks rise to the height of
twenty-five feet, and attain half a foot in thickness. But in the
primary and secondary periods even larger trees of this kind were widely
distributed, the most ancient of which probably were the progenitors of
the pines (Lycopodites). The most important dimensions were, however,
attained by the class of scale trees (Lepidodendreæ), and by the seal
trees (Sigillarieæ). These two orders, with a few species, appear in the
Devonian period, but do not attain their immense and astonishing
development until the Carboniferous period, and become extinct towards
the end of it, or in the Permian period directly following upon it. The
scale trees, or Lepidodendreæ, were probably more closely related to
club-mosses than to Sigillarieæ. They grew into splendid, straight,
unbranching trunks which divided at the top into numerous forked
branches. They bore a large crown of scaly leaves, and like the trunk
were marked in elegant spiral lines by the scars left at the base of the
leaf stalks which had fallen off. We know of scale-marked trees from
forty to sixty feet in length, and from twelve to fifteen feet in
diameter at the root. Some trunks are said to be even more than a
hundred feet in length. In the coal are found still larger accumulations
of the no less highly developed but more slender trunks of the
remarkable seal trees, Sigillarieæ, which in many places form the
principal part of coal seams. Their roots were formerly described as
quite a distinct vegetable form (under the name of Stigmaria). The
Sigillarieæ are in many respects very like the scale-trees, but differ
from them and from ferns in general in many ways. They were possibly
closely related to the extinct Devonian _Lycopterideæ_, combining
characteristic peculiarities of the club-mosses and the frondose ferns,
which Strasburger considers as the hypothetical primary form of
flowering plants.
In leaving the dense forests of the primary period, which were
principally composed of frond ferns (Lepidodendreæ and Sigillarieæ), we
pass onwards to the no less characteristic pine forests of the secondary
period. Thus we leave the domain of the Cryptogamia, the plants forming
neither flowers nor seeds, and enter the second main division of the
vegetable kingdom, namely, the sub-kingdom of the _Phanerogamia_,
_flowering plants_ forming seeds. This division, so rich in forms,
containing the principal portion of the present vegetable world, and
especially the majority of plants living on land, is certainly of a much
more recent date than the division of Cryptogamia. For it can have
developed out of the latter only in the course of the palæolithic
period. We can with full assurance maintain that, during the whole
archilithic period, hence during the first and longer half of the
organic history of the earth, no flowering plants as yet existed, and
that they first developed during the primary period out of Cryptogamia
of the fern kind. The anatomical and embryological relation of
Phanerogamia to the latter is so close, that from it we can with
certainty infer their genealogical connection, that is, their true blood
relationship. Flowering plants cannot have directly arisen out of
thallus plants, nor out of mosses; but only out of ferns, or Filicines.
Most probably the scaled ferns, or Lepidophyta, and more especially
amongst these the Lycopodiaceæ, forms closely related to the
Selaginella of the present day, have been the direct progenitors of the
Phanerogamia.
On account of its anatomical structure and its embryological
development, the sub-kingdom of the Phanerogamia has for a long time
been divided into two large branches, into the _Gymnosperms_, or plants
with naked seeds, and the _Angiosperms_, or plants with enclosed seeds.
The latter are in every respect more perfect and more highly organized
than the former, and developed out of them only at a late date during
the secondary period. The Gymnosperms, both anatomically and
embryologically, form the transition group from Ferns to Angiosperms.
The lower, more imperfect, and the older of the two main classes of
flowering plants, that of the _Archispermeæ_, or _Gymnosperms_ (with
naked seeds), attained its most varied development and widest
distribution during the mesolithic or secondary epoch. It was no less
characteristic of this period, than was the fern group of the preceding
primary, and the Angiosperms of the succeeding tertiary, epoch. Hence we
might call the secondary epoch that of Gymnosperms, or after its most
important representatives, the era of Pine Forests. The Gymnosperms are
divided into three classes: the Coniferæ, Cycadeæ, and Gnetaceæ. We find
fossil remains of the pines, or Conifers, and of the Cycads, even in
coal, and must infer from this that the transition from scaled ferns to
Gymnosperms took place during the Coal, or possibly even in the Devonian
period. However, the Gymnosperms play but a very subordinate part during
the whole of the primary epoch, and do not predominate over Ferns until
the beginning of the secondary epoch.
Of the two classes of Gymnosperms just mentioned, that of the _Palm
Ferns_ (Zamiæ, or Cycadeæ) stands at the lowest stage, and is directly
allied to ferns, as the name implies, so that some botanists have
actually included them in the fern group. In their external form they
resemble palms, as well as tree ferns (or tree-like frond ferns), and
are adorned by a crown of feathery leaves, which is placed either on a
thick, short trunk, or on a slender, simple trunk like a pillar. At the
present day this class, once so rich in forms, is but scantily
represented by a few forms living in the torrid zones, namely, by the
coniferous ferns (Zamia), the thick-trunked bread-tree (Encephalartos),
and the slender-trunked Caffir bread-tree (Cycas). They may frequently
be seen in hot-houses, and are generally mistaken for palms. A much
greater variety of forms than occurs among the still existing palm ferns
(Cycadeæ) is presented by the extinct and fossil Cycads, which occurred
in great numbers more towards the middle of the secondary period, during
the Jura, and which at that time principally determined the character of
the forests.
The class of _Pines_, or _coniferous trees_ (Coniferæ), has preserved
down to our day a greater variety of forms than have the palm ferns.
Even at the present time the trees belonging to it—cypresses, juniper
trees, and trees of life (Thuja), the box and ginko trees (Salisburya),
the araucaria and cedars, but above all the genus Pinus, which is so
rich in forms, with its numerous and important species, spruces, pines,
firs, larches, etc.—still play a very important part in the most
different parts of the earth, and almost of themselves constitute
extensive forests. Yet this development of pines seems but weak in
comparison with the predominance which the class had attained over other
plants during the early secondary period, that of the Trias. At that
time mighty coniferous trees—with but proportionately few genera and
species, but standing together in immense masses of individuals—formed
the principal part of the mesolithic forests. This fact justifies us in
calling the secondary period the “era of the pine forests,” although the
remains of Cycadeæ predominate over those of coniferous trees in the
Jura period.[2]
From the pine forests of the mesolithic, or secondary period, we pass on
into the leafy forests of the cænolithic, or tertiary period, and we
arrive thus at the consideration of the sixth and last class of the
vegetable kingdom, that of the _Metaspermæ_, _Angiospermæ_, or _plants
with enclosed seeds_. The first certain and undoubted fossils of plants
with enclosed seeds are found in the strata of the chalk system, and
indeed we here find, side by side, remains of the two classes into which
the main class of Angiosperms is generally divided, namely, the _one
seed-lobed plants_, or _monocotylæ_, and the _two seed-lobed plants_, or
_dicotylæ_. However, the whole group probably originated at an earlier
period during the Trias. For we know of a number of doubtful and not
accurately definable fossil remains of plants from the Oolitic and Trias
(sic) periods, which some botanists consider to be Monocotylæ, whilst
others consider them as Gymnosperms. In regard to the two classes of
plants with enclosed seeds, the Monocotylæ and Dicotylæ, it is
exceedingly probable that the Dicotyledons developed out of the
Gnetaceæ, but that the Monocotyledons developed later out of a branch of
the dicotyledons.
The class of _one seed-lobed plants_ (Monocotylæ, or Monocotyledons,
also called Endogenæ) comprises those flowering plants whose seeds
possess but one germ leaf or seed lobe (cotyledon). Each whorl of its
flower contains in most cases _three_ leaves, and it is very probable
that the mother plants of all Monocotyledons possessed a regular triple
blossom. The leaves are mostly simple, and traversed by simple, straight
bunches of vessels or “nerves.” To this class belong the extensive
families of the rushes, grasses, lilies, irids, and orchids, further a
number of indigenous aquatic plants, the water-onions, sea grasses,
etc., and finally the splendid and highly developed families of the
Aroideæ and Pandaneæ, the bananas and palms. On the whole, the class of
Monocotyledons—in spite of the great variety of forms which it
developed, both in the tertiary and the present period—is much more
simply organized than the class of the Dicotyledons, and its history of
development also offers much less of interest. As their fossil remains
are for the most part difficult to recognize, it still remains at
present an open question in which of the three great secondary
periods—the Trias, Jura, or chalk period—the Monocotyledons originated.
At all events they existed in the chalk period as surely as did the
Dicotyledons.
[Illustration:
_Haeckel History of Creation._
_PL. V._
Relative lengths of the 5 Epochs in percentages.
Quarternary Epoch 0.5
Tertiary Epoch 2.3
Secondary Epoch 11.5
Primary Epoch 32.1
Primordial Epoch 53.6
-----
_Total_ 100.0
Single-stemmed or
MONOPHYLETIC PEDIGREE
of the
VEGETABLE KINGDOM
based on Palæontology.]
The second class of plants with enclosed seeds, the _two seed-lobed_
(Dicotylæ, or Dicotyledons, also called Exogenæ) presents much greater
historical and anatomical interest in the development of its subordinate
groups. The flowering plants of this class generally possess, as their
name indicates, two seed lobes or germ leaves (cotyledons). The number
of leaves composing its blossom is generally not three, as in most
Monocotyledons, but four, five, or a multiple of those numbers. Their
leaves, moreover, are generally more highly differentiated and more
composite than those of the Monocotyledons; they are traversed by
crooked, branching bunches of vessels or “veins.” To this class belong
most of the leafed trees, and as they predominate in the tertiary period
as well as, at present, over the Gymnosperms and Ferns, we may call the
cænolithic period that of leafed forests.
Although the majority of Dicotyledons belong to the most highly
developed and most perfect plants, still the lowest division of them is
directly allied to the Gymnosperms, and particularly to the Gnetaceæ. In
the lower Dicotyledons, as in the case of the Monocotyledons, calyx and
corolla are as yet not differentiated. Hence they are called _Apetalous_
(Monochlamydeæ, or Apetalæ). This sub-class must therefore doubtless be
looked upon as the original group of the Angiosperms, and existed
probably even during the Trias and Jura periods. Among them are most of
the leafed trees bearing catkins—birches and alders, willows and
poplars, beeches and oaks; further, the plants of the nettle
kind—nettles, hemp, and hops, figs, mulberries, and elms; finally,
plants like the spurges, laurels, and amaranth.
It was not until the chalk period that the second and more perfect class
of the Dicotyledons appeared, namely, the _group with corollas_
(Dichlamydeæ, or Corollifloræ). These arose out of the Apetalæ from the
simple cover of the blossoms of the latter becoming differentiated into
calyx and corolla. The sub-class of the Corollifloræ is again divided
into two large main divisions or legions, each of which contains a large
number of different orders, families, genera, and species. The first
legion bears the name of star-flowers, or Diapetalæ, the second that of
the bell-flowers, or Gamopetalæ.
The lower and less perfect of the two legions of the Corollifloræ are
the star-flowers (also called Diapetalæ or Dialypetalæ). To them belong
the extensive families of the Umbelliferæ, or umbrella-worts (wild
carrot, etc.), the Cruciferæ, or cruciform blossoms (cabbage, etc.);
further, the Ranunculaceæ (buttercups) and Crassulaceæ, the Mallows and
Geraniums, and, besides many others, the large group of Roses (which
comprise, besides roses, most of our fruit trees), and the Pea-blossoms
(containing, among others, beans, clover, genista, acacia, and mimosa).
In all these Diapetalæ the blossom-leaves remain separate, and never
grow together, as is the case in the Gamopetalæ. These latter developed
first in the tertiary period out of the Diapetalæ, whereas the Diapetalæ
appeared in the chalk period together with the Apetalæ.
The highest and most perfect group of the vegetable kingdom is formed by
the second division of the Corollifloræ, namely, the legion of
bell-flowers (Gamopetalæ, also called Monopetalæ or Sympetalæ). In this
group the blossom-leaves, which in other plants generally remain
separate, grow regularly together into a more or less bell-like,
funnel-shaped, or tubular flower. To them belong, among others, the
Bell-flowers and Convolvulus, Primroses and Heaths, Gentian and
Honeysuckle, further the family of the Olives (olive trees, privet,
elder, and ash), and finally, besides many other families, the
extensive division of the Lip-blossoms (Labiatæ) and the Composites. In
these last the differentiation and perfection of the Phanerogamic
blossoms attain their highest stage of development, and we must
therefore place them at the head of the vegetable kingdom, as the most
perfect of all plants. In accordance with this, the legion of the
Gamopetalæ appear in the organic history of the earth later than all the
main groups of the vegetable kingdom—in fact, not until the cænolithic
or tertiary epoch. In the earliest tertiary period the legion is still
very rare, but it gradually increases in the mid-tertiary, and attains
its full development only in the latest tertiary and the quaternary
period.
Now if, having reached our own time, we look back upon the _whole
history of the development of the vegetable kingdom_, we cannot but
perceive in it a _grand confirmation of the Theory of Descent_. The two
great principles of organic development which have been pointed out as
the necessary results of natural selection in the Struggle for Life,
namely, the laws of _differentiation_ and _perfecting_, manifest
themselves everywhere in the development of the larger and smaller
groups of the natural system of plants. In each larger or smaller period
of the organic history of the earth, the vegetable kingdom increases
both in _variety_ and _perfection_, as a glance at Plate IV. will
clearly show. During the whole of the long primordial period there
existed only the lowest and most imperfect group, that of the Algæ. To
these are added, in the primary period, the higher and more perfect
Cryptogamia, especially the main-class of Ferns. During the coal period
the Phanerogamia begin to develop out of the latter; at first, however,
they are represented only by the lower main-class, that of Gymnosperms.
It was not until the secondary period that the higher main-class, that
of Angiosperms, arose out of them. Of these also there existed at first
only the lower groups without distinct corollas, the Monocotyledons and
the Apetalæ. It was not until the chalk period that the higher
Corollifloræ developed out of the latter. But even this most highly
developed group is represented, in the chalk period, only by the lower
stage of Star-flowers, or Diapetalæ, and only at quite a late date, in
the tertiary period, did the more highly developed Bell-blossoms,
Gamopetalæ, arise out of them, which at the same time are the most
perfect of all flowering plants. Thus, in each succeeding later division
of the organic history of the earth the vegetable kingdom gradually rose
to a higher degree of perfection and variety.