Mass.) Marine Biological Laboratory (Woods Hole.

Biological lectures delivered at the Marine biological laboratory of Wood's Hole ... 1890-[1899] (Volume 7) online

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various interglacial deposits. In the Leda clays at Montreal,
when fresh cuttings are being made, one may see leaves of the
common Vallisneria spiralis as perfect in all their structural
features as if freshly gathered from the adjacent river. But
the delicate structure has been so far influenced by its long
enclosure in the moist clay that desiccation leaves nothing but
a mere impression of the former organ. In the same formation
near Ottawa, fragments of perfectly preserved wood are enclosed
in nodules of clay, while at Toronto, from the same formation
again, we obtain the Osage orange with all its structure perfectly
preserved, and red cedar, which is not only perfect as to struc-
ture, but which possesses the characteristic shreddy bark, red
color, and distinctive odor. Such woods are sectioned in the
microtome precisely as if taken from a living plant.

While woods from the Pleistocene are often preserved as just
described, it more commonly happens in older formations that
the mass has been wholly or in part carbonized, or that it has
become so charged with mineral matter that it has passed into
a petrified state. These two forms of alteration commonly
accompany one another.

(a) Carbonization. Plant remains which are covered shortly
after decay arises, so that the latter continues under exclusion
of air, become converted into a mass of carbon which, accord-
ing to circumstances, retains the original structural features
more or less perfectly. This carbonization results from with-
drawal of the elements of water under the peculiar conditions
established, and if pressure and heat are subsequently brought
into operation, there results a compact coal, and possibly cer-
tain oily and gaseous products, as may be noted in the case



THE HISTORY OF PLANT LIFE. 25

of the Ohio shales, which are remarkable in some localities for
the vast number of spores of Protosalvinia, associated with oil,
and in such a state of preservation that they may be recognized
very readily. The extreme form of such alteration is to be
found in graphite, while the various forms of anthracite, bitu-
minous coal, lignite and peat indicate in inverse order the
successive stages through which the plant remains pass. It
follows from these considerations that, as a rule, we can expect
little structure to be exhibited in coaly masses, but we may
draw correct conclusions as to the general character of the
parts represented by taking into consideration the position in
the specimen and the original nature of the various tissues.
Thus we know that unmodified cellulose, such as constitutes the
soft parts of plants generally, contains carbon 44.55^), hydro-
gen 6. 14^, and oxygen 49.51^. Lignin, or the essential basis
of all woody structures, contains C. 62.25^, H. 5.93^, O.
36.82^, while cutin and suberin, the basis of cuticle, the walls
of spores and of cork, contain C. 73~74/&, H. io$fo, O. 16-17^).
A careful consideration of these figures leads to most important
and interesting conclusions, since the durability of parts, or
their ability to resist decay, as also the readiness with which
they pass into the form of carbonized remains, is in direct pro-
portion to the relative excess of carbon in the original structure.
Hence it is not difficult to understand why the more perishable
fundamental tissues of plants are so rarely preserved while the
woody and cortical parts are well preserved, and why the latter
particularly may remain as a shell of coal when all other parts
have been removed by decay or replaced by mineral matter.
In the light of these facts, also, the remarkable preservation of
many spores may be readily understood.

As carbonization proceeds, the material may become satu-
rated with water holding in solution small quantities of carbonate
of lime or of silica, in which case the entire structure becomes
gradually converted into a mass of calcite or of silica, as the case
may be, the displacement of the organic matter proceeding at
so gradual a rate that all the structural features are retained.
The structure is then represented by fine particles of carbon
disposed along the original lines of structure, while the



26 BIOLOGICAL LECTURES.

various cavities are rilled with the calcite, silica, or other
infiltrated material. 1

Plants from the early Palaeozoic are often most beautifully
preserved in this way, but cases occur in which the recrystalli-
zation of the infiltrated material brings about a redistribution
of the carbon particles in such a way as to produce a false
structure, as found most notably in the Celluloxylon primaevum
of Dawson, which was originally supposed to represent a purely
cellular plant, the component cells of which were of gigantic
size. More recent studies of this material, and comparison
with other well-determined plants in various conditions of
alteration and petrifaction, have afforded ample proof that the
apparent structure of Celluloxylon is nothing more nor less than
a condition incident to petrifaction, in which the carbon of the
original structure has been redistributed through the influence
of crystallization and deposited upon the surfaces of, or other-
wise between, the adjacent crystals.

Two full centuries have passed since the first observations
upon the occurrence of fossil plants were made. Yet palaeo-
botany has only recently attained to a position commensurate
with its importance as a branch of botanical science from which
most important and necessary data respecting the gradual evo-
lution of the higher plants may be gained. During the seven-
teenth century, when men were still experiencing the influence
of the Middle Ages, the discovery of plant and animal remains
in the crust of the earth was well calculated to call forth in all
seriousness the most remarkable explanations of their occur-
rence. And among other things we are thus told that such
evidence gave proof " that the whole terrestrial globe was
taken all to pieces and dissolved at the Deluge, the particles

1 A most instructive example of the extent to which a given material may be
involved in the process of petrifaction is afforded by Osmundites skidegatensis
Penh, from the Cretaceous of Vancouver Island. An analysis shows it to contain

Calcite 70.46%

Silica 12.15%

Combustible matter .... 17.36%

100.00%

The last constituent which disappears upon ignition, leaving a calcined mass,
probably represents the amount of carbon residue derived from the original plant.



THE HISTORY OF PLANT LIFE. 27


of stone, marble, and all solid fossils discovered taken up into

the water and there sustained together with seashells and
other animal and vegetable bodies ; and that the present earth
consists and was formed out of sand, earth, shells, and the rest
falling down again and subsiding from the water."

We are also informed that " The Deluge came forth at the
end of May when nuts are not ripe," because of the occurrence
of imperfectly formed hazel nuts in certain moss beds. Even
half a century later the idea that such remains were proof of a
deluge had in no way lost its force, since da Costa, who first
pointed out that Sigillarias and Stigmarias represented un-
known forms of life, and was, therefore, the first to indicate
the extinction of former types, firmly believed cones to be of
vegetable origin buried in the strata of the earth at the time
of the universal deluge recorded by Moses.

In these views we, no doubt, have an expression of the sur-
vival of primitive beliefs which are still current among certain
aboriginal people of eastern Asia.

And so for fully a century and a quarter the remains of
plants buried in the crust of the earth for millions of years
were matters of speculation without any adequate conception
of their real significance. Since the time of Witham, Sprengel,
and Goeppert there has been a constantly increasing interest in
the scientific study of plant remains and a correspondingly
greater appreciation of their true bearing upon the history of
plant life. Of the pioneers in this work, among English-speak-
ing people, probably no one has done more than Williamson in
England and Sir William Dawson in Canada to emphasize the
primary importance of the internal structure as a true guide to
relationship.

Our present knowledge of living forms leads us to the con-
clusion that there has been a more or less regular succession
of types from the most simple to the most complex ; and this
has been brought about, not by acts of special creation, but
by the gradual evolution or unfolding of continually higher
types in direct response to changed conditions of environment.
What those conditions were we are unable to say, but from our
present knowledge of plants in respect to their environment



28 BIOLOGICAL LECTURES.

we are permitted to draw certain inferences as to the general
nature of the controlling forces. That such changes may, in
particular cases, be brought about quickly, so that the results
become more or less apparent within the space of an average
life, and thus form the basis of broader generalizations, is well
known ; but in the main they have progressed but slowly, and
enormous intervals of time have been required for the transi-
tion from one type to another. We are not permitted to con-
ceive that this line of descent takes the form of a strictly lineal
succession. On the other hand, there is strong evidence in
support of the view that our mental figure must be that of a
deliquescent tree in which the main line of descent, or stem,
is dissolved into a series of great limbs or primary divisions.
These again branch repeatedly, until in the final division all
direct connection with the original stem is lost. As certain of
these lesser branches retain a full measure of vigor and persist
in their development to the very end, so others attain their
highest development at a comparatively early stage, enter upon
a period of decline, and shortly disappear, thereby introducing
a further important disturbance of the natural arrangement
whereby it becomes increasingly difficult to determine the pre-
cise order of development and relationship between any two
members. And so with plants as a whole. Certain side lines
of descent have not yet attained their full development ; others
are now far along in their period of decline, which may have
had its origin in very remote geological times ; while yet others
have long since disappeared altogether, leaving no visible sign
of their former presence. And thus the symmetry of the bio-
logical tree is disturbed to such an extent and in such a way
that the remaining members appear to have little in common,
and sometimes even stand forth as isolated groups which seem
to justify the ancient idea of special creation. Fortunately the
situation is saved, in the first instance, by an appreciation of
the fact that, within certain limits, each individual reproduces
in the course of its life history the history of the group.
From this it follows that, however divergent species or groups
may appear in their fully matured state, the relationship may be
ascertained through their embryological phases and through



THE HISTORY OF PLANT LIFE. 29

their most elementary members ; and thus it becomes possible
to ascertain at what points suppression has occurred and the
normal succession been broken.

We are perhaps not far wrong in the assumption that the
general line of descent began with the green algae as the first
clearly defined type. From these aquatic forms, which were,
no doubt, at first dominant, if not the exclusive forms, am-
phibious types appeared, leading eventually to terrestrial forms,
as represented by the mosses and liverworts, plants which
clearly show their derivation from an aquatic ancestry when
they enter upon their reproductive phase, and especially in the
development of an algoid protonema. But. here we encounter
one of the so-called missing links, since, although the approach
of the alga to the moss, and of the moss to the alga, is well
defined, the intermediate stages are as yet unknown. And so
again the thallus of the liverwort reappears in the prothallus
of the fern, and as these latter lead on to higher types, we find
undoubted evidence of relationship, the exact bond of which is
as yet wanting. One of the most significant facts, however,
is to be found in the evidence of an aquatic ancestry, which
extends through all grades of development in terrestrial plants
until the Cycads are reached ; I refer to the occurrence of
motile spermatozoids, the significance of which can scarcely be
misunderstood.

It is highly probable that further light respecting these ob-
scure problems will be gained as our knowledge of living forms
advances ; but as many of the existing gaps in the biological
tree have resulted from suppressions which must have occurred
in remote periods of the earth's history, it is peculiarly within
the province of palaeobotany to discover the necessary data, and
thus supply the missing links in the chain of plant life, in order
that we may have a clear and complete explanation of the rela-
tions of the various great groups of plants. And it is among
those forms which are now extinct that our search must be
prosecuted with the greatest hope of success. That this is
within the limits of possibility has been abundantly shown by
the investigations of recent years, and possibly no better illus-
tration could be had than the evidence derived from extinct



30 BIOLOGICAL LECTURES.

forms as presented by Dr. Jeffrey in his recent admirable
memoir on " The Development, Structure, and Affinities of the
Genus Equisetum."

Among the most interesting and promising of all botanical
problems is the theory relative to the origin of the sporophyte
as propounded by Bower a few years since, and more recently
emphasized by Campbell, who has himself done so much to
advance our knowledge in this direction. While the evidence
to be derived from living plants points with great force to the
possible correctness of Bower's hypothesis, it is not yet so
complete as to justify us in regarding the law as fully estab-
lished. But it is in the solution of problems of precisely this
nature that palaeobotany would prove of the highest importance,
and it is probably not too much to expect that the study of fos-
sil plants, particularly of types now extinct, may eventually
enlarge our views upon this as upon other important problems.

If we now turn to plants as we find them in the rocks, it is
to be observed that the succession displayed by living forms is
there essentially repeated and thereby confirmed. A glance at
the geological succession of the earth's crust shows that there
are four great periods in which geological time may be reck-
oned, and that these periods conform in the main to epochs in
the development of plant life. In the earliest or Eozoic time,
chiefly represented in northeastern America by the great
Laurentian formation which gives the dominant physical as-
pect to the northern watershed of the St. Lawrence River
throughout the greater portion of its length, there are few and
trustworthy evidences of former plant life to be obtained ;
that is to say, we find in those rocks no well-defined plant
remains. On the other hand, the occurrence in the Palaeozoic
of plants of a somewhat high degree of organization leads to
the inference that they represent a line of descent which must
have had its origin very early in Eozoic time. But if definite
remains are wanting, there is nevertheless evidence in the
abundance of graphite which occurs in the Laurentian forma-
tion, commonly interstratified with gneiss and attaining a ver-
tical depth upwards of six hundred feet or more, of former
vegetation ; for, as Prestwich very correctly observes, there is



THE HISTORY OF PLANT LIFE. 31

good reason for the belief that the various forms of carbon now
found in the crust of the earth must have been derived in the
first instance from the atmosphere through the agency of green
plants. Sir William Dawson has also expressed the opinion that
if the Laurentian graphite may be taken as representing former
plant life, it indicates its occurrence in vast profusion, and this
was no doubt the case. To these views we may also add the
opinion of the late Sterry Hunt, that the great Laurentian
beds of iron ore had their origin in plant decay, precisely as
they are formed at the present day.

During those early times the water resting upon the earth's
surface was of a rather high temperature. At the same time
the atmosphere was charged with a relatively high percentage
of carbon dioxide, in consequence of which its temperature was
considerably higher than at present. These considerations
lead to the conclusion that the earlier and aquatic vegetation
of the general character of the Chlorophyceae with which we
are familiar to-day, but whose delicate structure did not admit
of permanent preservation flourished under conditions now
fairly represented by hot springs, where vegetation thrives at
temperatures upwards of 93 C. If, furthermore, plants began
to emerge from their aquatic condition in later Eozoic time,
they must have been brought under an environment essentially
similar to that of the tropics of to-day. Certainly this was the
case until late in Palaeozoic time.

We are thus led to the belief that the flora of Eozoic time
was not only very abundant, but that it consisted, perhaps
wholly, of aquatic thallophytes comparable with the marine and
fresh-water algae of to-day ; that those plants were capable of
photosynthesis, and that along certain lines they attained a
somewhat high degree of development.

One cannot fail to remark upon the peculiar absence of plant
life during Huronian and Cambrian times, while evidences of
animal life are abundant. This may have its explanation in
the complete destruction of the more perishable plant struc-
tures, consequent upon the great disturbances which character-
ized the close of the Eozoic and the opening of the Palaeozoic
period. That plant life must have been maintained in full



32 BIOLOGICAL LECTURES.

vigor during that time is evident from the nature of the
remains which appear in later formations, and the first well-
defined occurrence of terrestrial forms in the Siluro-Cambrian
or Ordovician would go far to show that plants passed through
their amphibious stage at least as early as the Huronian, thus
preceding the corresponding phase in the progress of animal
life by a very long period of time, since in the latter case this
change was not effected until the later Carboniferous and
Permian. That the Eozoic and early Palaeozoic must have
witnessed the development of high types of plants is abun-
dantly evident from the abrupt occurrence, in the Upper Silu-
rian and Lower Devonian, of gigantic marine algae, of which
there are no equally large representatives in later formations ;
while in the Devonian, also, there occur for the first time plants
which are comparable in their reproductive structures with the
more modern Pilularia, and a brief consideration of the leading
features of some of these plants may assist us in gaining a more
complete appreciation of the statements already made.

In his Old Red Sandstone, Hugh Miller described certain
peculiar discoid bodies found in the Devonian rocks of Scotland
at Blairgowrie, Myreton, and other localities. In 1831 these
objects were described by Dr. Fleming under the name of
Parka decipiens, and both by him and subsequent observers,
during a period of sixty years, they were regarded as represent-
ing the spawn of JVIollusca, the eggs of frogs or other animals,
but always without suspicion that they might have been derived
from plants ; and it was reserved for Dawson and Penhallow, in
1891, to prove clearly their vegetable nature. These bodies
consist of carbonized discs, or their impressions, about 5-6 mm.
in diameter and grouped in oval masses varying in size from
3.5x5.3 cm. to 13x20 mm. In their more complete forms
these masses show the presence of an external covering,
and there is also some indication that a stalk may have been
present. No stems or leaves can with certainty be considered
to belong to these bodies, since as yet no organic union between
such organs has been observed, but there is reason to believe,
from the associated structures, that they belonged to plants with
a creeping stem and upright leaves, while the recent finding



THE HISTORY OF PLANT LIFE. 33

of leaf-like bodies similar to those of Marsilia seems to suggest
the possibility of foliage of that type. Our present knowledge,
however, centers in the carbonized discs, from which important
data have been obtained. Boiling them out in nitric acid they
yield two kinds of spores. The macrospores measure 40 /* in
diameter and are therefore slightly larger (34 /u) than the spores
of Lycopodium. The somewhat elongated microspores are 1 5 p
in diameter. In addition, there are to be found numerous
compound cellular bodies with a central carbonized mass.
The obvious remains of spores, from which as a center there
extends a tissue in various stages of development, leave no
doubt as to the fact that these structures represent prothalli in
various stages of growth. From these facts the conclusion is
a direct and justifiable one that Parka was a heterosporous
plant with its separate male and female sporangia enclosed in
a common sporocarp, and therefore comparable among modern
plants with Pilularia. Beyond this nothing definite can be
stated, hence for the present the specific name decipiens, as
originally applied by Fleming, is retained.

In 1855 Sir William Dawson discovered, in the Devonian
sandstones of Gaspe, the remains of a gigantic alga, to which
he subsequently gave the generic name of Nematophyton. Since
then plants of the same type have been found in Germany,
England, Scotland, and various parts of the United States,
thus indicating the very wide distribution of plants having a
common ancestry. At the present time, for purposes of con-
venience, eight different species are distinguished, but it is
altogether probable that these may in reality represent only
four separate species at the most.

As represented by specimens of N. Logani in the Peter
Redpath Museum of McGill College, these plants were some-
times eighteen inches to two feet or more in diameter, and
specimens recently found in New York State show a recover-
able length of twenty-four feet. These facts seem to indicate
that the complete plant attained to great size, far exceeding
anything known among the arborescent forms of algae at the
present day. No foliage or fruit has been found, and our
knowledge of these plants rests entirely upon the details of



34 BIOLOGICAL LECTURES.

internal structures, which are often most beautifully preserved.
The main features are as follows :

The stem exhibits a concentric structure comparable with
that which is shown by the larger Laminariae, a feature which
in the first instance' led to a misinterpretation of their true
nature, as it appeared to indicate an exogenous stem. This
view was still further strengthened by the appearance of cer-
tain radial canals simulating medullary rays, but of very irregu-
lar width and indeterminate length, and wholly lacking definite,
limiting walls. Such spaces reappear in N. Ortoni, N. Crassum,
and others, where they take the form of nearly isodiametric
openings in the body of the structure, through which they are
scattered without any definite order or apparent connection
with the larger elements of the organism. They nevertheless
appear to be intimately connected with the smaller and second-
ary elements, as will be shown presently, and it is therefore
quite possible that, as suggested by Barber, they were designed
to perform a certain role in the internal aeration of the plant.

The principal structure consists of tubular, non-septate cells
having a diameter upv/ards of 67 /i, but of indeterminate
length and loosely interlaced, so that their general direction
coincides with the axis of growth. These large cells of the
medulla branch into smaller hyphae which form an intercellular
plexus. The hyphae are chiefly from 4-6 ^ in diameter, though
sometimes much less. They appear, in some cases at least
(N. Storriei), to be septate, and their derivation from the large
cells of the medulla appears to occur chiefly in the immediate
region of the medullary spaces which they occupy in the form
of a very loose web. The majority of the species exhibit
minor differences only, but in N. Ortoni we observe the occur-



Online LibraryMass.) Marine Biological Laboratory (Woods HoleBiological lectures delivered at the Marine biological laboratory of Wood's Hole ... 1890-[1899] (Volume 7) → online text (page 3 of 25)