HISTORY OF THE PLANT KINGDOM
1. DAWN OF PLANT LIFE ON EARTH
“And the Earth was without form and void”
The quotation from the second sentence of the first chapter of Genesis
tells us more in eight words than could very well be said in as many
chapters. Not only have we biblical authority for this early absence of
life on the earth, but all the accumulated knowledge of the ages points
in the same direction. We have already seen that plants, because they
can take inorganic substances from the earth and air and transform them
into organic food, must in all probability have come on the earth before
animals which, directly or indirectly, all rely upon plants for their
food. Even those animals that eat only flesh devour other animals which
depend upon plants for food. It may safely be repeated, then, that upon
plants all animal life depends, and that, in the dim beginnings of
things on the earth, it must have been some form of plants that were the
first living things. Extremely simple unicellular animals, however, are
known to date from early times.
In the volume devoted to that subject in this series, you will find that
at the very earliest stages of what we know as our globe there was a
segregation of land and water somewhat different from our great oceans
and continents to-day in extent and area, but differing mostly in
this--that much of the water was fresh and very nearly boiling hot. We
have still the remnants of those great reservoirs of hot water, as our
hot springs and shooting geysers only too well prove. And if all plants
were as quickly killed by hot or boiling water as the common garden
geranium, we should not expect any plant life to have developed upon the
earth until all those great bodies of water had cooled. To have waited
for that would have been to delay the appearance of plants for no one
knows how many millions of years, and there is some fairly good evidence
that long before normal conditions of heat and cold were established
there already flourished certain kinds of plants. What those plants were
is something of a speculation, and indeed exactly what they were no one
knows. But in our present hot springs grow certain plants, microscopic
in size, but quite obviously related to the algæ or to certain
bacterialike organisms. They live with apparent comfort and reproduce
themselves freely in water so hot that no other form of life will
maintain itself. While there is no proof that these present plant
inhabitants of hot springs, common in the West, are descended from
infinitely ancient progenitors, it is a fair assumption that some
organism capable of growing in warm or hot water was the first living
thing to appear in a world otherwise “without form and void.”
This great question of how plants came on the earth, and particularly
how from these apparently simplest organisms our whole wonderful
vegetation has arisen, has always been one of the most interesting
things in the history of the world. There are many different ways of
studying this, and in the very earliest stages of plant development we
are forced to reason, not so much by actual records or buried skeletons
of the plants that probably existed then, for only a very few have ever
been found, but by our knowledge of the physical and chemical
requirements of unicellular plants, and those slightly more developed,
and of their individual life histories. It is, for instance, certain
that the first plants must have been aquatic, as no real land plants are
known for hundreds of thousands of years after the earth was quite
capable of maintaining plant life. The absolute necessity of water to
complete fertilization in nearly all cryptogams also makes it fairly
certain that water plants, and these of the simplest nonflowering type,
were the first living things to be found on the earth. And it is more
than a fair inference that these were inhabitants of warm or hot water.
Subsequently, as the water cooled, they may well have been not unlike
the green scum found on ponds to-day.
Of course, the actual origin of life itself is still as much of a riddle
as it was when the ancient philosophers began to speculate about it
years before the Christian era. Protoplasm, the unit or basis of all
life, while its composition and growth requirements are fairly well
known, has never been made in any laboratory. Nor have scientists ever
been able to decide what the combination of physical and chemical forces
must have been to originate it. But that from a perfectly sterile,
probably steaming hot globe, there did finally develop some form of
life, and that this must have been aquatic plant rather than animal
life, seems not only certain, but the only hypothesis upon which all
subsequent development of life must have been based. It is not necessary
to ascribe the origin of life to providential inspiration nor to the
meddling of strange and outlandish deities, as all savage tribes did and
some more civilized peoples still do. There can, however, be no escaping
the fact that life is more than the combination of physical and chemical
conditions which sustain it, and that its origin has never, and may
never be “explained” by merely describing the conditions which
unquestionably favored its appearance. In other words, the origin of
plant life throws us back upon things believed but incapable of proof,
and is none the less wonderful because we cannot yet understand the
probable progression of forces and materials to which it owes its
origin.
Assuming, then, and we must all start with this proposition, that
aquatic plants certainly, and warm or hot water plants probably, were
the first living things upon the earth, what are the next steps in the
history of the plant kingdom? The answer to that question involves a few
simple facts in geology and, particularly, in the making of fossils,
which must be understood before we can see those steps or their
significance. The geological changes which have resulted in the present
condition of the earth’s surface are described in the volume devoted to
that subject, and will not be repeated here. But some mention must be
made of the formation of fossil plants, particularly as it is upon the
evidence of these that the story of the development of plant life is
literally written in the rocks.
If a leaf or twig drops into shallow water with a clay or mud bottom, it
ultimately sinks, and if then a film of clay or silt is brought down by
freshets or what not, it will bury the leaf or twig, of course, filling
in every slight depression. If then the buried object were raised so
that it dried out and could be split open, we should find a perfect
impression of the veins and other outward characters of the leaf etched
in the clay. This is often such a perfect process that every detail of
the leaf is left in the mud impression, and only the opportunity for
this impression to become hardened into rock is needed for us to have a
fossil. For these are merely the final hardened rock stages of a process
that began as we have indicated, and the thousands of fossils that have
been dug out of the earth prove how common the conditions for their
formation must have been in certain periods of the world’s history.
But of the untold millions of fossils that have been made most have been
destroyed, for the geologists tell us that the earth’s crust has been
subjected to much upheaval. Mountain chains thrown up, inland seas
formed, great river systems carved out, and tremendous periods of
vulcanism or fire action have made the earth’s crust, at different
periods, a mighty restless place. And these changes, so slow that often
millions of years have elapsed before they were completed, have
sometimes been favorable to the making of fossils and sometimes to their
destruction. Darwin once wrote about the fossil record that he saw it
“as a history of the world imperfectly kept, and written in a changing
dialect; of this history we possess the last volume alone relating only
to two or three countries. Of this volume, only here and there a short
chapter has been preserved; and of each page, only here and there a few
lines. Each word of the slowly changing language, more or less different
in the successive chapters, may represent the forms of life, which are
entombed in our consecutive formations, and which falsely appear to have
been abruptly introduced.” And yet it is upon the evidence of this
fossil record that most of our knowledge of the history of the plant
kingdom is based.
The difficulty of getting any true picture of the beginnings of plant
life is great, for those earliest stages of development were
unquestionably water-inhabiting plants, whose tissue is mostly too soft
and too easily decayed to make fossil impressions of them likely to be
preserved. Yet fossil algæ have been found in rock strata so old that no
fern or flowering plant had yet made its appearance. It is not too much
to picture the world then as peopled only by cryptogamous plants of
simple structure, living in the water, and land plants which to-day make
up the bulk of our vegetation as not yet developed. Furthermore, there
is in these earliest stages no trace of plants with any kind of a
vascular system, such as all ferns and flowering plants possess. No
stretch of our imagination can readily picture the earth as it in all
probability was in that period, with no trees or vines or flowers, the
land wholly bare of vegetation, and in the water, along its sterile
shores, only unicellular or slightly more developed, wholly nonflowering
plants. The conditions supporting such plant life existed for many
millions of years, and some geologists have claimed that this period of
time exceeded all the subsequent ones combined, so that algæ and some
other unicellular plant types are the oldest in the world; and they
still exist in enormous numbers.
Much later than this, fossil algæ of comparatively complex structure
have been found, showing by their frequency and more highly developed
characters a more advanced stage in the development of the plant
kingdom. So common were these various types of what we now call
seaweeds, although most of them apparently lived in fresh water, and so
widespread was their occurrence, that this Pre-Cambrian period has often
been called the reign of algæ. As yet no other plants had been developed
and none of these ancient types had invaded the land, which for millions
of years more must have been entirely without vegetation.
2. THE DEVELOPMENT OF LAND PLANTS
After the reign of algæ and other cryptogamous water plants, our
knowledge of which is so unsatisfactory because of the incompleteness of
the fossil record, there appeared the first evidences of plants that
were able to live with “one foot on the land and the other in the
water,” so to speak. How many transitional stages there may have been,
and what relation any of these may bear to existing plants, is not
known, or is, at any rate, so little understood that it is a disputed
point. But somewhere about this period there did appear plants capable
of living at least part of their life on the land, and possessing in
their vascular system a structure of enormous advantage over their
predecessors. It is pure speculation as to what this first land plant
was derived from, or from what particular group of water-inhabiting
plants it took some of its characteristics. Its appearance, in any case,
was a dramatic event of the first importance. Not the least interesting
feature of it is that the very plants of which we have indisputable
evidences of being the first land plants have come down through the ages
to the present day. For it is practically impossible to separate our
modern representative from its ancient ancestor, despite all the
tremendous changes that have been going on both in the history of the
earth’s crust and in the vicissitudes of the vegetation in meeting those
changes.
There is the best of evidence that these first land plants were of the
club moss family, which are relatives of the ferns. One of them,
representing our present _Lycopodium Selago_ (Figure 107) so closely as
to be practically indistinguishable, is a common type, as revealed to us
in the fossil record. The present plant inhabits rich, moist, and mostly
evergreen woods in the northern part of the globe, and is common in the
Adirondack and White Mountains.
[Illustration: FIG. 107.--CLUB MOSS
(_Lycopodium Selago_)
A club moss which has come down through the ages almost unchanged from
the days when coal was being formed. Grows to-day in the north temperate
zone, particularly in mountains.]
Nor was the earth peopled wholly by this ancestor of _Lycopodium
Selago_, for we find at this time, or just after it, a great development
of plants of this type. Some of these were giant, treelike club-mosses
that have been so well preserved as fossils that even their internal
structure and spore-bearing characters are well known. Many other
strange relatives of our modern club mosses flourished in those days,
some of which have wholly disappeared, as have all the treelike forms.
These highly organized club mosses, quite unlike any modern
representative of the family, appear to have been crowded off the earth
by other and subsequent types of vegetation, while _Lycopodium Selago_,
and about thirty related species, have persisted to the present day; not
precisely in all cases as they were in this dawn of a land flora, but in
many cases with modern structure and reproductive processes so close to
the ancestral types as to be nearly identical.
Perhaps nothing gives one a better impression of the tremendous time
that must have elapsed before the appearance of these ancient club
mosses than the very slight modifications from their ancient condition
which their structure at the present time exhibits. While nothing is
certainly known of their origin, when they first appeared they were
plants with a well-developed vascular system, having stems and leaves
quite unlike any of their predecessors’, and a reproductive process
almost precisely like their modern descendants. In other words, if they
have changed so slightly in all the millions of years since our
rock-written records of them first occur, what an infinitely greater
period must have elapsed down the dim vista of the ages before their
appearance. Of this period, with the exception of fossil algæ, we know
practically nothing, and, worst of all, the actual transition from a
wholly water-inhabiting flora to these certainly land-inhabiting club
mosses may never be known. For, added to the difficulty of water plants
being preserved as fossils, already mentioned, is the fact that as they
are the oldest, they are found in the deepest strata and, consequently,
the hardest to find; and due to changes in the earth’s crust, these
ancient fossil-bearing strata have often been much disturbed.
The conclusion appears to be indicated that the origin of a land flora
came about with the appearance of these ancient club mosses, which are
not mosses in our present-day interpretation of those plants, and that
at about the same period many other plants also were found, the whole
vegetation resembling nothing that exists at the present time, but many
of the different kinds of this ancient flora showed unmistakable
evidence of being the progenitors of many plants that exist to-day. What
these were, and particularly what they accomplished, both in the history
of the plant kingdom and in making the world habitable for man who did
not come for millions of years after they were preparing the way, will
be considered in
3. CARBONIFEROUS PLANTS AND THE FORMATION OF COAL
The carboniferous time, or the period when the earth was covered with
huge forests of strange shrubs and trees, most of which were unlike
their modern successors, apparently had a climate so nearly uniform and
seasonless that fossil remains of these plants have been found
throughout the world. Even in the Arctic the rock strata show the
flourishing of forests that must have needed a climate very different
from the frigid condition there to-day, and furnishing indisputable
evidence of a warm, most probably frostless, climate practically
throughout the world.
[Illustration: FIG. 108.--HORSETAIL
(_Equisetum hyemale_)
A modern horsetail or scouring rush, common in the north temperate zone.
Ancestors of these formed huge forests at the time that coal was being
formed.]
The giant club mosses have already been mentioned, with their
persistence to the present day in much reduced number, and vastly
reduced sizes. No one can picture the grandeur of those ancient forests,
peopled with queer animals long since extinct and with dragon flies
known to have a wing-spread of two feet or more. But with the club
mosses were giant horsetails, which in somewhat changed form have also
come down to our times, but in much reduced stature and frequently are
familiar enough as weedy plants along railway embankments, and
sometimes in more natural environments. Most of our modern
representatives of the genus _Equisetum_ (Figure 108), or horsetails,
are low herbs, but one South American kind still retains the ancient
habit of growing to considerable heights, as specimens up to twenty feet
high are known. Related to the ancient treelike horsetails were queer
vines with slender twining stems, which, judging from their fossil
remains, must have been very common. Both the ancestors of our club
mosses and the horsetails must have occupied vast swampy areas, as their
stem structure indicates a fondness for water, to which, as we have
already seen, their still more ancient ancestors were always confined.
Vigorously competing with these plants for occupancy of those great
swamps were vast quantities of plants that have been called cycadlike
ferns from their likeness to ferns on the one hand and plants like the
so-called sago palm on the other. The sago palm, or _Cycas revoluta_, is
a modern representative of these ancient forms, and retains the
remarkable characteristic of having its male fertilizing cell capable of
movement as we know to be the case in nearly all cryptogamous plants.
Yet _Cycas_, with its related genera, which are found in nearly all the
warmer parts of the earth to-day, are true flowering plants which bear
cones. We see, therefore, in these old cycadlike ferns one of the first,
almost experimental, evidences of the seed habit, and consequently the
breaking away from the spore habit which overwhelmingly characterized
the reproductive processes of its ancient associates.
The inhabitants of higher parts of that dim, mysterious world, of which
we know only that part revealed in the fossil record, were largely
ancestors of our modern cone-bearing evergreens. They are known as
_Cordaitales_ and have long since disappeared. Forming forests of huge
size and making long, slender trunks with a crown of leaves at the top
not unlike some modern conifers to which they are, of course, related,
these progenitors of our pines and spruces must have been striking
objects of that strange landscape. Rooted stumps of these ancient trees
have been uncovered, and their narrow leaves, often three feet long, are
common as fossils. By some our present conifers and the _Cordaitales_
are both thought to be descendants from a still more ancient group, of
which the existence is only conjectured.
We can, perhaps, best summarize our sketch of the plant life existing at
the close of this period by saying that all the forms show unmistakable
evidence of being crytogamous so far as their reproductive processes are
concerned, or else, as in the progenitors of our conifers and cycads,
the beginnings of a definite seed habit are indicated. Most of the
lowland representatives of this flora were cryptogamous in their
characters and ancestry, while some of them, and nearly all the flora of
drier sites, appear to have shown the beginnings of flower production.
Some of these flowers, which are always cones, are unmistakable as such,
and pollen in tremendous quantities has been found among their buried
remains. These cones belong to trees that are actual gymnosperms or
obvious ancestors of them, for no herbs are known as yet. Nor are
angiospermous flowering plants known from this period, nor have any ever
been found in strata millions of years younger than the fossil-bearing
strata of this age of the ancestors of our modern ferns, conifers, or
cycads.
Nor must we picture the development of these different plant inhabitants
of that time as passing from one to another in orderly sequence, for
that would give us the impression of a regular progression from simple
to complex, which may or may not be the truth. There appears to be such
a sequence, and the internal structure of the remains of many of these
ancient types of plant life have greatly aided our ability to understand
their relationships. But with the possibility of various reproductive
processes or other structures appearing in quite unrelated forms, and
with the comparative paucity of the fossil record in mind, no one can
say for certain what are the true lines of descent. The necessity for
water in the reproductive act of nearly all the crytogams, the origin of
the vascular structure, and the consequent ability to live upon the
land, and finally the production of a conelike flower structure with
pollen, and all that that implies, are all found during this period.
To the vegetation inhabiting the swamps during this period, man owes a
debt perhaps as great as to our modern food plants, for it is upon this,
and some later plant remains, that we rely for coal. This period has
been well called the Carboniferous, for its chief claim to attention,
outside the realms of botanical research, was the deposition of those
great collections of plant remains, which, as coal, contain as high as
90 per cent of carbon and furnish the fuel of the world. This is
scarcely the place or time to go into the composition of different kinds
of coal, but some mention of the conditions under which these ancient
swampy forests were transformed into that valuable substance may well
conclude the account of a vegetation period the history of which has in
large part been found written in the very strata from which coal itself
is derived.
In the lowest and wettest parts of those forests there occurred, just as
there may occur to-day, a large accumulation of fallen trees and other
vegetable refuse. In the ordinary way these would simply rot, due to the
work of insects and the fungi of decay, and in a few score years there
would be nothing to show. “Dust to dust” would be, and is, the history
of so many living things that it is only some machinery for arresting
this process which will give us very different results. In the case of
coal formation the original impetus appears to be certain microscopic
organisms, probably saprophytic fungi, fossil remains of which have been
identified, which work upon the fallen mass of vegetation and start its
decay, but which can only do so while their prey is still within the
influence of the air. The initial stages of decay must, therefore, have
been going on while the water was low enough for these organisms to
work. But in many parts of that ancient landscape the water level was a
fluctuating quantity, due to local conditions or to changes in the
earth’s crust. So that many times partially decomposed vegetation masses
would become submerged, stopping the work of these organisms of decay
that demand air, but providing the only conditions under which certain
others could complete the transformation. These bacterial organisms that
will work only when deprived of air continue the process, but in a
different way. For one thing, the lack of air delays decomposition or
almost stops it, as witness the resistance of logs under water, some of
which are known to be hundreds of years old. And forest stumps off the
coast of Cape May, in New Jersey, are in nearly as perfect a state as
when first submerged, over 40,000 years ago. In the production of coal
these anærobic (living without air) bacteria release oxygen and hydrogen
from the partly decayed mass, leaving as a residue a substance known as
peat, which is largely carbon. The transformation of peat into coal
depends upon requisite pressure of the strata that may be laid down on
top of the peat bed, and probably upon chemical changes that go on after
such covering strata have been laid down.
The fact that coal is sometimes found only in thin veins, with layers of
shale and other material between, tells us that its origin must often
have been a precarious affair, where alternate emergence and submergence
would permit first the vegetation to develop and then its transformation
to peat, followed by the deposition of fine sands or silt covering the
bed. Several such cycles occurred, sometimes separated by untold ages of
time, or again by much briefer periods. Certain mines, however, contain
over 200 feet of solid coal. The length of time necessary for such a
vast accumulation, or how many generations of these ancient plants went
into their making, is beyond calculation. With the mining of coal
running into the hundreds of millions of tons yearly, we get some idea
of how great were those Carboniferous forests, and how extensive they
were is proved from the widely separated localities in which coal mines
are found.
The Carboniferous age of fern, cycad, and conifer ancestors was by no
means a quiet, orderly period, as from geological evidence it appears to
have been much subject to alternate emergence and submergence of great
tracts of land. Compared with what followed, it actually was a period of
comparative quietness, however, and it must, in at least most parts of
the world, have permitted the slow development of certain of its plant
groups to a state of perfection never reached since. This is
particularly true of the ancient relatives of our club mosses and
horsetails.
Perhaps one of the most obvious questions to ask about these plants is
how long ago they lived, and upon the answer to such a question depend
many others. What, for instance, is the position of the Carboniferous as
compared to what preceded it and came after? How old is the earth and
when did life first appear on it? The evidence upon which such questions
are answered comes from the estimates of physicists as to the age of the
earth; from students of fossil animals and plants; from astronomers,
from geologists and other students. A compromise of these different
estimates, and one that has consequently been widely accepted, gives the
age of the earth, dating from the time of its having a definite crust
with land and water masses, as somewhere near a hundred million years.
Such figures are beyond our comprehension and consequently mean almost
nothing, but the proportion in time of the different stages of the
development of plants may be stated with greater certainty. Taking the
total age of the earth as 100 per cent, the period when there is no
record of life of any sort may be set down as about 45 per cent of the
total, the reign of algæ and development of land plants about 8 per
cent, the carboniferous or coal-forming plants about 28 per cent, which
leaves only 19 per cent from that distant time to the present. And many
things happened in this comparatively brief fifth of the plant world’s
history, among them the origin of some plants that have come straight
down to us, without discoverable change.
[Illustration: FIG. 109.--GINKGO OR MAIDENHAIR TREE
(_Ginkgo biloba_)
Found in most fossil strata and in a practically unchanged condition
from the upper part of the Carboniferous to the most recent fossil
records. Now unknown as a wild tree and preserved for us through its
cultivation in ancient temple gardens in eastern Asia.]
We could hardly leave the Carboniferous time without at least brief
mention of the ginkgo tree (Figure 109), or, as some call it, the
maidenhair tree. From the upper strata of the Carboniferous it is
common, as it is in practically all subsequent fossil accumulations down
to the most recent. And yet the tree has never been found wild, although
its frequency in temple gardens in China and Japan, always as a
cultivated tree, suggests that its disappearance as a wild plant must
have come since the priests began preserving it, which can be only a
matter of a few thousand years at most. In other words, we have just
missed seeing in the ginkgo what has so many times happened to these
very ancient types of vegetation, namely, their final extinction. This
must have occurred within historic times, and, judging by its frequent
use as a temple tree in eastern Asia, that region was its last outpost
after its long journey from the dim past. Thousands of other ancient
plants have completely disappeared, and one cycad from New South Wales
is at this moment putting up a losing fight against modern competitors,
but in the ginkgo tree the actual twilight and extinction of its wild
existence has missed observations by modern plant geographers by only a
brief period. It is almost as though we had waited all our life to see
some great event and then missed it by a few moments. Fortunately the
tree is now common in cultivation, and not the least interesting feature
of it is the fact that its male fertilizing cell retains its power of
movement, which dates back to its early associates. Among modern
flowering plants only the ginkgo and the relatives of the sago palm or
cycads retain this relic of an overwhelmingly cryptogamous ancestry.
The end of the Carboniferous or coal-forming ages was marked by great
changes in the earth’s surface, some of them cataclysmic in their
effects. What they were in detail is described in the volume on geology
and need not be repeated here. What happened to the development of the
plant kingdom after this will be considered in the next section of this
chapter.
4. MORE RECENT ANCESTORS OF OUR MODERN FLORA
The vegetation at the ending of Carboniferous times was much affected by
the great changes in the earth’s surface which happened then. The
thrusting up of great mountain chains, the slow encroachment of
continental glaciers, and the other phenomena characterizing that period
could not but be reflected in the plant population. For one thing the
giant club mosses and horsetails were much reduced in extent and finally
disappeared, leaving only the immediate ancestors of our present-day
forms. _Cordaitales_ gave place to trees not unlike some of the modern
yew trees. True ferns as well as the cycadlike ferns with seeds appear
to have lived side by side with true cycads, which subsequently
supplanted their obvious cycadlike fern ancestors. There was an obvious
dwindling of ancient Carboniferous forms, some of which, however,
persisted in considerable numbers. Many other plants existed then, some
of which died out there, and some of which still survive in descendants,
particularly among our conifers and ferns. But there happened toward the
upper end of this period an event in the history of the plant kingdom so
dramatic, of such far-reaching results, that its appearance might be
likened to the overthrow of the Czar in Russian history or to the
downfall of the Kaiser in Germany. For with it dawned a new era for the
plant world, the effects of which we see all about us to-day.
Somewhere in the rock strata of this period we find the first
angiosperm, or plant that matures its seed in a closed ovary, and with
the origin of that habit there began such a development of plants of
this type that its impetus has not yet been lost. It is impossible to
tell at this distance from the origin of that first angiosperm from what
it developed, nor how many ages it may have existed before the accident
of its preservation as a fossil revealed its presence. It is certainly
not without significance that it bore conelike fruits, such as all its
associates and predecessors among flowering plants had done, but its
possession of large, showy petals is the first evidence of a flower
characteristic that was destined to make our present vegetation the
lovely thing it is. This exceedingly interesting plant was a _Magnolia_
(Figure 110), or so like our present plants of that genus as to be their
obvious ancestor. Somewhere here, too, must have arisen the insect
fertilization of flowers which we have seen to be such an important part
of flower economy at the present time. Most of this ancient magnolia’s
associates must have relied on wind pollination for seed production, as
many modern plants still do, but the origin of insect fertilization
appears to have come with the appearance of the first really
petaliferous flower.
[Illustration: FIG. 110.--COMMON LAUREL MAGNOLIA
(_Magnolia virginiana_)
The fossil record tells us that probably the first flowering plant was
some ancestor of magnolia.]
With this _Magnolia_ were found other flowering plants which soon
increased tremendously in numbers of individuals and differences of
structure, such as our sassafras, the tulip tree, the poplar tree, and
some others. All of these are trees or shrubs and we do not yet know
whether herbs grew in the strange surroundings of that ancient forest or
not. Their soft tissue may have prevented their preservation as fossils,
but, at any rate, no herb has left its rock-written record from as early
in this period as the trees and shrubs. All of these ancient trees have
been recorded only in the northern hemisphere and it may be true that
this part of the earth was the cradle of all those hosts of the
flowering plants that now number over 150,000 species.
There must have been a mighty struggle for occupancy of the really
desirable plant sites soon after the rise of these immediate ancestors
of our modern plants. For there is every evidence of the progressive
dwindling of those still more ancient holdovers from the Carboniferous,
and the steady encroachment of the newly arisen and obviously vigorous
young race. As we get higher up in the strata, or, in other words,
nearer to the present, there are literally thousands of these immediate
ancestors of our modern flora, and it is not very long before herbs,
particularly grasses and sedges, begin to be common, together with other
monocotyledonous plants such as palms. One not unlike our coconut palm
has been found in some of these strata in France.
While this period records the origin of hundreds, and there are probably
thousands of unrecorded species which are very near our modern
descendants of them, it was also a period when the earth’s crust was in
an almost constant state of restlessness. Ice periods, huge inland seas,
great volcanic upheavals, and the thrusting up of mountain chains such
as the Alps, Himalayas, and some others, were only a few of the
disturbances to the orderly procession of this wholly new type of
vegetation that doomed the older kinds and subsequently conquered the
world. The spread of this new element in the plant kingdom was greatly
helped and sometimes greatly hindered by land connections between
continents, now separated by the oceans. The giant redwoods, now
isolated in a few localities on our Pacific Coast, were found then
nearly throughout the world. Because of these changes of land areas and
some others of even greater influence on plant growth, such as climate,
there was a constant shuffling of floral and, of course, animal
elements, so that by the end of this period the new type of flora had
spread throughout the world, but with here and there very local
occurrence of certain genera and families, some of which have persisted
to the present day. As we shall see in the last chapter of this book,
certain whole families are confined to restricted areas; the cactus and
pineapple family, and the genus _Helianthus_, or sunflowers, for
instance, are, with one or two trifling exceptions, wholly American. And
we have already seen how many food and other useful plants were first
found here by the Spaniards--chocolate, tobacco, corn, the potato, and
others.
It would fill the rest of this book to enumerate the plants that
flourished toward the end of this period, and, in fact, it might almost
be said that the flora of those days was not very different from our
own, only it was distributed in different ways and mixed in very
different proportions. With the disappearance or partial dwindling of
more ancient groups, the rise of the plants that immediately preceded
our own ushered in a new era in the history of the plant kingdom.
[Illustration: FOSSIL AND LIVING ALGÆ COMPARED. C. A living algal pool
colony near the Great Fountain Geyser, Yellowstone Park. (_After
Walcott._) B. Fossil calcareous algæ. _Cryptozoön proliferum_ Hall, from
the Cryptozoön ledge in Lester Park, near Saratoga Springs, N. Y. These
algæ, which are among the oldest plants of the earth, grew in
cabbage-shaped heads on the bottom of the ancient Cambrian sea and
deposited lime in their tissue. The ledge has been planed down by the
action of a great glacier which cut the plants across, showing their
concentric interior structure. (_Photographed by H. P. Cushing. Pictures
and explanations of them from “The Origin and Evolution of Life,” by
Professor Henry Fairfield Osborn, who kindly permitted their
reproduction here._) (_Courtesy of Brooklyn Botanic Garden._)]
[Illustration: PROBABLE LANDSCAPE IN THE CARBONIFEROUS AGE. ABOUT THE
TIME COAL WAS IN THE MAKING. (_After Patonie_) 1. Tree fern 2. Giant
ancestors of our horsetails. 3 and 4. Ancestors of our club mosses. 5.
_Cordaites_, a primitive type, or perhaps even the ancestor of our
modern evergreens. At this time no herbs and no plants with petals, were
known, nor for ages after this period. (_Courtesy of Brooklyn Botanic
Garden._)]
At the end of this period an event of commanding interest occurred,
because it happened only some 40,000 years ago. With it came the
encroachment from the north and south poles of the last of the great
continental glaciers. There had been many before, stretching over a past
period of time, but as the last of these great ice invasions it is the
most interesting to us. It crowded all these temperate and even
subtropical plants that then grew up in the far north toward the
equator, and scraped clear of vegetation every part of the earth which
it covered. In the volume on geology you will find an account of the
extent and thickness of this great ice sheet, which ultimately receded
to its present home. As it went back the plants crowded forward to
occupy the freshly released land, the far northern or glacial first,
followed by waves of other kinds. Some of the glacial or northern plants
were left on the tops of the highest mountains, where to-day they
persist in complete isolation, nearly all their friends and associates
of that greatest of all winters having left them for points farther
north. Many students of plant geography think that wave of plant life
creeping northward to occupy the region uncovered by the retreating ice
is still going on, and recent studies appear to show in at least one
isolated mountain in the Adirondacks that the survivals of the ice age
which have been isolated on its rocky peak ever since are in
considerable danger of being crowded out by invaders from the lowlands.
Not all the geological changes which have remodeled the earth’s surface
have been mentioned in this brief history of those plants that preceded
our own, nor have anything like all the plants occurring in the
different strata been even hinted at. But the thing which has been
stressed and for us to fix in our minds is that all our present
vegetation literally has its roots deep down in the earth. Some, as
_Lycopodium Selago_, go back no one knows how many millions of years;
others, like the flowering herbs, are much more recent. We come to
understand how recent we are and what a comparatively brief flash in the
pan all our modern development both of plants and in man has been since
the last glacial period only by looking for a moment at what has
happened in the past. In the account of the Carboniferous plants we
found that there remained after that period only about 19 per cent of
the earth’s age in which all the changes since then could have come
about. If, as may well be possible, this period has been about
19,000,000 years, then the mere 40,000 years since the last Ice Age
seems a brief period indeed. As some one has written, to contrast all
man’s historic period, back to the days of the most ancient Chinese
manuscript, with that long journey from the dim past which the plant
world has slowly accomplished, is to realize that we are “as the
flashing of a meteor through the sea of night.”
Fossil plants then, and this delving into the dead past of the plant
world, reveals to us as nothing else can how much the modern plant
kingdom is literally built upon a mighty race of ancestors. Some
perished as did the _Cordaitales_, but left descendants who themselves
gave rise to other groups that survive to-day. To look over a list of
the fossil plant genera of the different strata is to visualize a drama
the like of which no one living will ever see replayed, the results of
which are recorded all over the world with its changing panorama of
vegetation.
5. PRESENT-DAY PLANTS AND WHERE THEY COME FROM
There are living to-day somewhere about 150,000 species of flowering
plants; half a hundred conifers or gymnosperms; about 3,500 ferns; 500
club mosses; over 70,000 bacteria, fungi, and lichens, and probably over
20,000 species of algæ. The estimates of the last three divisions are
more or less uncertain, as many species are still being discovered.
From what has just been read regarding the plants of earlier periods it
is at once clear how completely the flowering plants have conquered all
their ancient forbears, and what a pitifully small remnant of once proud
and ancient forests are now represented by our club mosses and
horsetails. That process of crowding, of the dominance of one kind to
the exclusion or even extinction of others is still going on, and, as we
shall see in the last chapter, often on a great scale.
If the period just after plants were first known on the earth may be
called the Reign of Algæ, and subsequent periods were typified by still
other kinds of plants, then our present period is _par excellence_ the
reign of flowering plants. In numbers of individuals they are still far
outclassed by such cryptogams as the fungi, bacteria, algæ, etc., but
their dominating influence in the plant world is unquestioned.
While all our present vegetation must have been derived from preexisting
types, all of it is not necessarily directly descended from species
which from their fossil records we know to have existed in geological
periods older than our own. While the fossil record of the times
immediately preceding the last ice age is a much more complete one than
for many other periods, it fails to account directly for the great bulk
of our varied flora of to-day. While ferns in great variety,
gymnosperms, and hundreds of flowering plants are known quite
definitely, they total only a minute fraction of these groups to-day.
Even granting the always imperfect nature of the fossil record, and we
have seen what an accident the formation of a plant fossil may be, and
it is common knowledge how few comparatively have ever been
recovered--even granting all this, there still remains a large part of
our present flora of which the origin probably dates from comparatively
recent times. So overwhelmingly true is this that of the Compositæ, or
daisy, family, now numbering over 11,000 species, scarcely a handful of
fossil species have been found. And in all collections of fossils the
woody plants far outnumber the herbs, perhaps because of the greater
probability of their being thus preserved rather than to any actual
scarcity of herbs in the upper strata. And yet herbs to-day outnumber
woody species over two to one. While it is true, then, that our present
flora must have been derived from preexisting races, it is also true
that much of it is apparently derived from plants that do not date very
far back into the past. A few main _types_ of flowering plants
unquestionably are to be linked with fossil genera, but these types have
now branched out into a wealth of detail that may not have existed and
is certainly not recorded in the fossil record.
Some of these types stand out with remarkable clearness, notably
magnolia, willows, poplars, walnuts, birches, oaks, figs, sassafras and
its relatives, the rosales, the pea family, the spurges, maples, grapes,
linden, myrtle, ginseng, and some others. All these, and in not very
different aspect from their modern representatives, have been found in
the fossils of the different and usually more recent strata before the
last Ice Age. But the total fossil record of even these well-known
genera is only a fraction of their modern development, and we are
constantly confronted with the apparent dilemma of accounting for a
present wealth of forms based upon an obvious poverty of ancestry. While
the whole race of flowering plants is certainly a new one, as such
things are reckoned fossilwise, there has been a fecundity in the origin
of new species among these lusty upstarts that is simply amazing. How
that, in part at least, has been accomplished will be considered in the
final section of this chapter. Not only among these present-day plants,
but all through the story of the development of the plant kingdom, we
have been reading and writing of the changes of form and structure, some
of which have been of far-reaching consequences. It is clear enough that
if new types of vegetation and different races of plants have come into
being and so modified the complexion of the plant kingdom, those changes
must have first arisen in individuals which had within them some
capacity for change, and furthermore the ability to use the change to
their advantage. While, as we have seen, the losses have been
tremendous, no one, with even this brief history of their development in
mind, can doubt that there has been progress toward our present
perfection of plant life.
6. HOW PLANTS CHANGE THEIR CHARACTERS AND BECOME NEW SPECIES
It was with something very different in mind than the changing of plant
characters that Cardinal Newman once said: “To live is to change, and to
be perfect is to change often.” And yet nothing better expresses the
facts of plants’ ability to change and the results of it than this reply
of a great churchman to critics who could not or would not understand
the truth of his now famous reply.
It is perhaps best to begin any discussion of the changes in plants by
remembering a few simple facts regarding changes in ourselves. “Like
father like son” is something more than an old saw which we repeat for
centuries without stopping to think whether it is true or only half
true. As with so many speeches of the sort, this is just precisely a
half truth, for while sons are more apt to be like their fathers than
other men, we all have within us the capacity, whether expressed or not,
to change very considerably. In other words, all living things may be
said to be a reflection or, perhaps better, the result of two divergent
tendencies, one of which tends to make like produce like, and the other
to produce something different.
Upon the ability of like to produce like rests the continuity of those
plant groups, well exemplified by _Lycopodium Selago_ and the ginkgo,
which, through all the changing panorama of the history of the plant
world, have steadily produced individuals so close to the ancestral type
as to be essentially indistinguishable from it. It is upon the
possession of this ability that all the different races of plants depend
for the unchanged perpetuation of their kind. And, as we shall presently
see, it is also upon this very ability that the new forms that do arise,
rely for holding fast to their differences.
While it is true, then, that like tends to produce like, it is also and
perhaps even more true that they do not precisely do so. In fact, they
never do absolutely, and it is the degree of divergence from the type
that different plants or animals exhibit, which is the measure of their
ability to vary, or “produce something different.” Upon this capacity to
vary, from whatever cause, rest all the changes which have occurred in
the plant world, and, as we have seen in previous chapters, that has
been by no means an insignificant affair. We know, in fact, that while
one plant of _Lycopodium Selago_, than which scarcely any other now
living has had greater opportunity to become fixed in its characters, is
much like another, no two of them are actually identical. Nor are any
two plants of the same species ever precisely alike, any more than two
children, even of the same parents, are. The tendency for like to
produce like is matched then, or sometimes exceeded, by an almost
equally strong tendency to vary.
Heredity on the one hand and variation on the other are the two forces
upon which the origin of new species or kinds of plants is based. Both
of these work in rather definite ways, some of which are fairly well
understood, but many of which are still among the things that scientists
are striving to clear up. As the capacity to inherit characters from one
generation to another reflects itself in the generally stable conditions
which the plant world exhibits, while the capacity to vary is the only
source of new forms, it is quite naturally the variations of plants from
one generation to another which have been most studied. And the study of
variation in plants is not the simple thing we might assume it to be,
having in mind only the well-known fact that no two organisms are
exactly alike. Wherein do they differ? Are their perhaps temporary or
even quite casual differences passed on to their progeny? These and
many other questions about variation make it at once the most
complicated and often one of the most fruitful subjects of plant
research. It is clear enough that with the bewildering variety of
different plants in nature it is next to impossible not only to record
accurately the amount of variation or its probability of being handed
on, but least of all to arrive at any clue as to the origin of that
variation. Because of this, and still more because practically no one
has ever seen the actual origin of a new species in nature, for we only
see the finished product, practically all our knowledge of the laws of
variation has been derived from studying cultivated plants. The ease of
controlling them and of recording thousands of observations of their
characteristics has made the work of the plant breeder, and others who
study variation in the vegetable world, much more of an exact science
to-day than the mass of often interesting but usually unrelated data
that crowd the pages of older botanical literature.
One of the main facts about variation is that it is itself a very
variable thing, and the nature of those “fluctuating variations” which
are so common in nature well illustrate the point. Within what we know
as a species there are many individuals that vary one way or the other
from a fairly central, we might almost call them normal, mass of
individuals which are typical of that species. Nearly all these forms on
the fringe of the species due to the environmental changes and not to
changes of hereditary constitution will, if left to themselves, tend
through their progeny to become more like the central mass as time goes
on, while their position, or some other equally nontypical edge of the
fringe, will be taken up by other variants from the average conditions.
The amount of this fluctuating variability among plants is beyond
calculation, and its action has often been likened to the swinging of a
pendulum, which of course spends twice the time passing through the
center of its arc, that it does on the limits of it. This very nearly
expresses the proportion of fluctuating variants to the mass of typical
individuals in many species of plants. In many others, often peculiarly
unstable species, the number of individuals at the fringe is very large
indeed. Sometimes there may be one or more that do hold their
characteristics, in which case we know that they are not true
environmental variations but have actually a different constitution.
These will be considered presently under another and different sort of
variability. But, speaking generally, these fluctuating or environmental
variants are merely forms of the species, and, other things being equal,
they will not actually originate new species.
It should be emphasized here, and before we go further in our discussion
of variation, that species and varieties are after all largely creations
of the mind of man rather than the reflection of actuality in nature.
When we speak of a “species” it is merely a term which through usage by
botanists becomes the symbol of a group of plants more like one another
than like anything else. It is obvious that it is therefore necessarily
an inaccurate designation of the actual conditions found among plants,
which might almost be considered as all belonging to one great group of
which, for our convenience in referring to them, we mark off units
(families, genera, species, etc.), much as units are marked off on a
rule. Species, then, and varieties of plants, notwithstanding the
utmost refinement of method used in designating and describing them, and
this is historically the most ancient and the most widely developed
phase of botanical science, cannot reflect the true conditions, and for
a number of reasons. The chief one is that species differ in usually
several characters one from another and in large genera there is often a
bewildering recombination of characters of the genus in the species
belonging to it. Species and varieties are concepts of convenience, nay
of absolute necessity, in talking or writing about plants, but hardly
expressions of exact truth.
With this in mind we can appreciate the position of those plant breeders
who insist that the basis of differences in all plants are the simplest,
so-called, factor expressions, which can be isolated and studied with
some approach to exactitude in experimental cultures. A factor may be
defined as the hereditary determiner or base, which, either singly or in
conjunction with other factors, is expressed as a character, such as
tallness in peas, or brown eyes in human beings. Such studies have built
up a body of information about variation in plants that show it to be of
several kinds and with different chances of being passed on from one
generation to another.
It was noted in the paragraph before the last that fluctuating variants
were sometimes so far off the usual that they might almost be considered
distinct forms or varieties. Many such changes appear to be the result
of different conditions of the local environment, due to changed
conditions rather than to any internal difference in the constitution of
the plant itself. A familiar illustration of environmental variation
may be seen in lima beans. In any considerable number of plants one
often finds smaller and larger pods, either sparsely or well filled with
beans. If the beans from the small-podded, few-seeded variants are
planted they will produce, apparently quite indiscriminately, large and
small podded progeny, just as there will result a mixed progeny if only
beans from the well-filled and large-podded kind are sown. In other
words the plant fluctuates about a general average which typifies the
usual or mass characteristics of the species. One should not, however,
regard _all_ variations of the character in lima beans, or any other
plant as environmental or fluctuating ones, for some of them may be due
to differences in hereditary constitution. And these could only be
determined by breeding tests.
Environmental variations are as frequent as the ever-changing conditions
of plant growth may determine, and it is common knowledge that such
diversity of the environment and the variants resulting from it are
extremely frequent.
A much more fruitful source of new forms of plant life results from
natural cross-fertilization, which, as we saw in an earlier chapter, is
the nearly universal condition in the plant world. If species and
varieties can be distinguished only by factor differences, as the plant
breeders no doubt correctly insist, it becomes obvious enough that we
have in cross-fertilization to consider not alone the factor differences
of the pistillate or female, but also of the staminate or male
contribution to the union, and how these are reflected in the progeny.
Our knowledge of this has practically all been based on work done on
cultivated plants under control conditions and it shows some
interesting developments which occur from crossing.
If garden peas with, let us say, reddish-purple flowers are crossed with
white-flowered ones, the progeny will not be a mixture of these colors
but all reddish-purple. If all danger of subsequent cross-fertilization
is excluded this first generation of reddish-purple progeny will
themselves produce reddish-purple and white progeny in the ratio of
three to one. But the extraordinary part of it is that in the third
generation all the white and about one-third of the reddish-purple
plants will breed true to color. The balance of the reddish-purple
plants, which comprise about two-thirds of the second generation, will,
if their seeds are germinated, produce colored as against white-flowered
progeny in the three-to-one ratio. In other words, these artificial
crosses, made by the plant breeder, and this splitting up of hybrids
which has been many times verified, are seen to be very fertile causes
of the origin of new forms of plant life, if only the factor and
character differences in the ancestry be sufficiently complex. With no
two plants precisely alike, with cross-fertilization so nearly
universal, and with all characters, not a single character or factor
expression, as in control conditions likely to be affected by the cross,
it may be seen how fruitful a source of new forms this natural crossing
may be. It is, in fact, not surprising that plants vary, but that the
force of heredity will hold them into such recognizable categories that
the red maple, or white ash, or blue cohosh are, with thousands of other
species, after all fairly definite designations without which talking
and writing about plants would be all but impossible. Some of our most
beautiful garden plants have arisen either as the result of natural
crossing, or crosses deliberately made by the plant breeder. The scores
of forms of the common garden lilac have mostly come about by such
crosses, although many other garden plants have arisen by still another
kind of variability.
The effects of crossing which have been so briefly noted were not
understood, as indeed the cause of them is still unknown, before 1865,
when Gregor Mendel, an Augustinian monk, published the results of his
work on peas, which furnished the basis for all subsequent work on this
kind of variability. His work was neglected until 1900, when what is now
known as Mendel’s law, involving the Mendelian ratio already noted, was
rediscovered by three independent workers. It is now practically
universally accepted as the way in which natural or induced hybrids
transmit their characters.
There remains still another type of variability which has been noticed
from very early days, and received the name _sport_, because quite
suddenly, from a crop of otherwise similar specimens, one or a few
plants showed marked and permanently transmitted differences from the
average condition. Such sudden offshoots, which occur rather frequently
in many plants, are known as mutants, the process as mutation. Hugo de
Vries, a Dutch botanist of world-wide fame, was the chief modern figure
who drew attention to mutants, and explained how they differ from
fluctuating variants in that while these tend to revert to the average
or mass conditions the mutant, once it appeared, held true to type. A
well-known example of mutation is the cabbage, brussels sprouts,
cauliflower, and kohl-rabi, all of which are sports or mutants from a
weedy seaside plant of the mustard family, native in Europe. Since
their appearance, hundreds of years ago, they have held their essential
characters. If they had been environmental variants they would in all
probability have reverted to their weedy ancestor. Hundreds of sports or
mutants have been recognized and isolated, so that many of our most
valuable garden plants have arisen through this ability of plants to
vary in often sudden and rather startling degree. The gardener and
horticulturist, from long observation and a keen sight for valuable
novelties, have always known that sports are fruitful sources of new
forms of plants, but De Vries first scientifically studied them and
worked out the principles by which they apparently react. The cause of
them is still unknown.
While the cause of mutants has not yet been revealed, we have already
seen that the two remaining kinds of variability are due to changes in
the environment, or to crossing. Charles Darwin when he published his
“Origin of Species,” than which no other book has so completely
revolutionized modern thought, did not state the cause of those
variations of which he was our greatest observer. He did state the now
universally accepted law of the “Survival of the Fittest” which explains
how, once these variations make their appearance, the inexorable
conflict of nature would automatically weed out the unfit. We have seen
all through the course of this history of the plant kingdom how whole
types of vegetation have been overthrown to give way to other types
better fitted to survive. That process is going on with just as
inexorable results to-day as it has down through the ages. While Darwin
never claimed that such a purely selective process could initiate new
species, many of his partisans who waged battle for him during the first
years of the tremendous opposition his views encountered, did so claim,
and probably wrongly. The actual cause of the origin of new species,
except those demonstrated to result from new combinations of already
known characters, through crossing, cannot be explained through the
natural selection of plants or animals which exhibit favorable
variations. We see their effects; it is obvious enough, that those of
value tend for the survival of plants having such variations, and it was
natural enough that older students of the problem should mistake these
effects for the cause of them. The process of selective elimination
constantly going on does tend to fix certain favorable variations and
untold millions of plants have had their day in the past due to their
possession of such, and the killing off of their less fortunately
provided associates. We speak of this great march from the simplest
organism up to our most complex plants and animals as their evolution,
but we must never forget that it has gone step by step, by one or the
other methods by which we have seen that plants vary, or perchance by
some undetected method, and that while the results of it are for all to
see, the causes of that infinitely slow and quite often wayward
variation are not understood. Upon such a conception our modern plant
life is seen to be a development of plants that have gone before, that
all existing life is derived from preexisting, and not from providential
interposition or special creations. All through the long marches of
plant evolution there appears to be a definite and final goal toward
which it tends, but we do not know the direction, least of all the
object, of that goal. In fact, there may be many goals, just as there
are the diversity of ambitions among human beings. In tracing the
present ascendancy of our flowering plants from their links with the
past perhaps we shall find no better statement of their present
condition or destiny than to repeat Cardinal Newman’s reply: “To live is
to change, and to be perfect is to change often.”