PAST HISTORIES OF PLANT FAMILIES
X. The Lower Plants
In the plant world of to-day there are many families including immense
numbers of species whose organization is simpler than that of the groups
hitherto considered. Taken all together they form, in fact, a very large
proportion of the total number of living species, though the bulk of them
are of small size, and many are microscopic.
These “lower plants” include all the mosses, and the flat green
liverworts, the lichens, the toadstools, and all the innumerable moulds
and parasites causing plant diseases, the green weeds growing in water,
and all the seaweeds, large and small, in the sea, the minute green cells
growing in crevices of the bark of trees, and all the similar ones living
by millions in water. Truly a host of forms with an endless variety of
structures.
Yet when we turn to the fossil representatives of this formidable
multitude, we find but few. Indeed, of the fossil members of all these
groups taken together we know less that is of importance and real
interest than we do of any single family of those hitherto considered.
The reasons for this dearth of fossils of the lower types are not quite
apparent, but one which may have some bearing on it is the difficulty of
mineralization. It is self-evident that the more delicate and soft-walled
any structure is the less chance has it of being preserved without decay
long enough to be fossilized. As will have been understood from Chapter
II, even when the process of fossilization took place, geologically
speaking, rapidly, it can never have been actually accomplished quickly
as compared with the counter processes of decay. Hence all the lower
plants, with their soft tissue and lack of wood and strengthening cells,
seem on the face of it to stand but little chance of petrifaction.
There is much in this argument, but it is not a sufficient explanation of
the rarity of lower plant fossils. All through the preceding chapters
mention has been made of very delicate cells, such as pith, spores, and
even germinating spores (see fig. 47, p. 68), with their most delicate
outgrowing cells. If then such small and delicate elements from the
higher plants are preserved, why should not many of the lower plants
(some of which are large and sturdy) be found in the rocks?
As regards the first group, the mosses, it is probable that they did not
exist in the Palæozoic period, whence our most delicately preserved
fossils are derived. There seems much to support the view that they have
evolved comparatively recently although they are less highly organized
than the ferns. Quite recently experiments have been made with their near
allies the liverworts, and those which were placed for one year under
conditions similar to those under which plant petrifaction took place,
were found to be perfectly preserved at the end of the period; though
they would naturally decay rapidly under usual conditions. This shows
that Bryophyte cells are not peculiarly incapable of preservation as
fossils, and adds weight to the negative evidence of the rocks,
strengthening the presumption of their late origin.
That some of the lower plants, among the very lowest and simplest, can be
well preserved is shown in the case of the fossil fungi which often occur
in microscopic sections of palæozoic leaves, where they infest the higher
plants as similar parasitic species do to-day.
We must now bring forward the more important of the facts known about the
fossils of the various groups of lower plants.
Bryophytes.—_Mosses._ Of this family there are no specimens of any age
which are so preserved as to show their microscopical structure. Of
impressions there are a few from various beds which show, with more or
less uncertainty in most cases, stems and leaves of what appear to be
mosses similar to those now extant, but they nearly all lack the
fructifications which would determine them with certainty. These
impressions go by the name of _Muscites_, which is a dignified cloak for
ignorance in most cases. The few which are quite satisfactory as
impressions belong to comparatively recent rocks.
_Liverworts_ are similarly scanty, and there is nothing among them which
could throw any light on the living forms or their evolution. The more
common are of the same types as the recent ones, and are called
_Marchantites_, specimens of which have been found in beds of various
ages, chiefly, however, in the more recent periods of the earth’s
history.
It is of interest to note that among all the delicate tissue which is so
well preserved in the “coal balls” and other palæozoic petrifactions,
there are no specimens which give evidence of the existence of mosses at
that time. It is not unlikely that they may have evolved more recently
than the other groups of the “lower” plants.
Characeæ.—Members of this somewhat isolated family (Stoneworts) are
better known, as they frequently occur as fossil casts. This is probably
due to their character, for even while alive they tend to cover their
delicate stems and leaves, and even fruits, with a limy incrustation.
This assists fossilization to some degree, and fossil Charas are not
uncommon. Usually they are from the recently deposited rocks, and the
earliest true Charas date only to the middle of the Mesozoic.
An interesting occurrence is the petrifaction of masses of these plants
together, which indicate the existence of an ancient pool in which they
must have grown in abundance at one time. A case has been described where
masses of _Chara_ are petrified where they seem to have been growing, and
in their accumulations had gradually filled up the pond till they had
accumulated to a height of 8 feet.
The plants, however, have little importance from our present point of
view.
Fungi.—Of the higher fungi, namely, “toadstools”, we have no true
fossils. Some indications of them have been found in amber, but such
specimens are so unsatisfactory that they can hardly afford much
interest.
[Illustration: Fig. 119.—The Hyphæ of Fungi Parasitic on a Woody Tree
_c_, Cells of host; _h_, hyphæ of fungus, with dividing cell walls.]
The lower fungi, however, and in particular the microscopic and parasitic
forms, occur very frequently, and are found in the Coal Measure fossils.
Penetrating the tissues of the higher plants, their hosts, the parasitic
cells are often excellently preserved, and we may see their delicate
hyphæ wandering from cell to cell as in fig. 119, while sometimes there
are attached swollen cells which seem to be sporangia. From the Palæozoic
we get leaves with nests of spores of the fungus which had attacked and
spotted them as so many do to leaves to-day (see fig. 120). What is
specially noticeable about these plants is their similarity to the living
forms infesting the higher plants of the present day. Already in the
Palæozoic the sharp distinction existed between the highly organized
independent higher plants and their simple parasites. The higher plants
have changed profoundly since that time, stimulated by ever-changing
surroundings, but the parasites living within them are now much as they
were then, just sufficiently highly organized to rob and reproduce.
A form of fungus inhabitant which seems to be useful to the higher plant
appears also to have existed in Palæozoic times, viz. _Mycorhiza_. In the
roots of many living trees, particularly such as the Beech and its
allies, the cells of the outer layers are penetrated by many fungal forms
which live in association with the tree and do it some service at the
same time as gaining something for themselves. This curious, and as yet
incompletely understood physiological relation between the higher plants
and the fungi, existed so far back as the Palæozoic period, from which
roots have been described whose cells were packed with minute organisms
apparently identical with _Mycorhiza_.
[Illustration: Fig. 120.—Fossil Leaf _l_ with Nests of Infesting Fungal
Spores _f_ on its lower side]
Algæ.—_Green Algæ_ (pond weeds). Many impressions have been described as
algæ from time to time, numbers of which have since been shown to be a
variety of other things, sometimes not plants at all. Other impressions
may really be those of algæ, but hitherto they have added practically
nothing to our knowledge of the group.
Several genera of algæ coat themselves with calcareous matter while they
are alive, much in the same way as do the Charas, and of these, as is
natural, there are quite a number of fossil remains from Tertiary and
Mesozoic rocks. This is still more the case in the group of the _Red
Algæ_ (seaweeds), of which the calcareous-coated genera, such as
_Corallina_ and others, have many fossil representatives. These plants
appear so like corals in many cases that they were long held to be of
animal nature. The genus _Lithothamnion_ now grows attached to rocks, and
is thickly encrusted with calcareous matter. A good many species of this
genus have been described among fossils, particularly from the Tertiary
and Cretaceous rocks. As the plant grew in association with animal
corals, it is not always very easy to separate it from them.
Brown Algæ (seaweeds) have often been described as fossils. This is very
natural, as so many fossils have been found in marine deposits, and when
among them there is anything showing a dark, wavy impression, it is
usually described as a seaweed. And possibly it may be one, but such an
impression does not lead to much advance in knowledge. From the early
Palæozoic rocks of both Europe and America a large fossil plant is known
from the partially petrified structure of its stem. There seem to be
several species, or at least different varieties of this, known under the
generic name _Nematophycus_. Specimens of this genus are found to have
several anatomical characters common to the big living seaweeds of the
_Laminaria_ type, and it is very possible that the fossils represent an
early member of that group. In none of these petrified specimens,
however, is there any indication of the microscopic structure of
reproductive organs, so that the exact nature of the fossils is not
determinable. It is probable that though perhaps allied to the Laminarias
they belong to an entirely extinct group.
An interesting and even amusing chapter might be written on all the
fossils which look like algæ and even have been described as such. The
minute river systems that form in the moist mud of a foreshore, if
preserved in the rocks (as they often are, with the ripples and raindrops
of the past), look extraordinarily like seaweeds—as do also countless
impressions and trails of animals. In this portion of the study of
fossils it is better to have a healthy scepticism than an illuminating
imagination.
Diatoms, with their hard siliceous shells, are naturally well preserved
as fossils (see fig. 121), for even if the protoplasm decays the mineral
coats remain practically unchanged.
Diatoms to-day exist in great numbers, both in the cold water of the
polar regions and in the heat of hot springs. Often, in the latter, one
can see them actually being turned into fossils. In the Yellowstone Park
they are accumulating in vast numbers over large areas, and in some
places have collected to a thickness of 6 feet. At the bottoms of
freshwater lakes they may form an almost pure mud of fine texture, while
on the floor of deep oceans there is an ooze of diatoms which have been
separated from the calcareous shells by their greater powers of
resistance to solution by salt water.
[Illustration: Fig. 121.—Diatom showing the Double Siliceous Coat]
There are enormous numbers of species now living, and of fossils from the
Tertiary and Upper Mesozoic rocks; but, strangely enough, though so
numerous and so widely distributed, both now and in these past periods,
they have not been found in the earlier rocks.
In one way the diatoms differ from ordinary fossils. In the latter the
soft tissues of the plant have been replaced by stone, while in the
former the living cell was enclosed in a siliceous case which does not
decompose, thus resembling more the fossils of animal shells.
Bacteria are so very minute that it is impossible to recognize them in
ordinary cases. In the matrix of the best-preserved fossils are always
minute crystals and granules that may simulate bacterial shapes
perfectly. _Bacillus_ and _Micrococcus_ of various species have been
described by French writers, but they do not carry conviction.
As was stated at the beginning of the chapter, from all the fossils of
all the lower-plant families we cannot learn much of prime importance for
the present purpose. Yet, as the history of plants would be incomplete
without mention of the little that is known, the foregoing pages have
been added.