leaves, and the opening and closing of flowers, both regulated by the
amount of light, apparently acting on them as it acts on our nervous
system; in the detection of subtle differences in light, which escape
the human eye, by plants; in their general sensitiveness to external
influences, even in the diseases which attack them, the study of
which Sir James Paget has commended to pathologists, we have the
rudiments of attributes and powers which reach their full development
in the higher animals, and therefore a series of fundamental
correspondences between plant and animal which point to the merging
of their apparent differences in one community of origin.
In fine, that which was once thought special to one is found to
be common to both, and to this there is no exception. Not only is
there correspondence in external form in the lower life groups, but,
fundamentally, plants and animals are alike in internal structure and
in the discharge of the mysterious process of nutrition (although
this forms a convenient line of separation) and of reproduction.
All, from the lowest to the highest, have their unity and kinship in
ancestral life which was neither plant nor animal.
Of course, the difficulty of classifying vanishes in the higher
forms; the lowest plants are allied to the lowest animals, but the
higher the plant the more it diverges from the animal, which is
evidence that in the succession of life the highest plants do not
pass into the lower animals. Descent is not lineal, but lateral;
the relations between the two kingdoms are represented by two lines
starting from a common point and spreading in different directions.
Even the “lower” and “higher” are relative terms; the organization of
the amœba is as complete for its purpose, as is that of the man for
his purpose, the modification in the complex forms being due to the
division of functions which are performed in every part by the simple
forms.
Although the foregoing and numberless other facts, together with the
law of continuity, alike forbid the drawing of any hard and fast
lines, and involve the conclusion, to borrow Professor Huxley’s
words, “that the difference between animal and plant is one of
degree rather than of kind, and that the problem whether, in a
given case, an organism is an animal or a plant may be essentially
insoluble,” there exists, exceptions notwithstanding, a broad
distinction in the mode of nutrition.
“All things the world which fill
Of but one stuff are spun,”
and this stuff, the basis of all life, the formative power, is a
semi-fluid, sticky material, full of numberless minute granules in
ceaseless and rapid motion, to which the name “protoplasm” (Gr.
_protos_, first; _plasma_, formed) has been given. It consists of
four of the elementary substances, carbon, hydrogen, oxygen, and
nitrogen, complexly united in the compound called _protein_, which is
closely identical with the albumen or white of an egg. These are the
_essential_ elements, but a few others enter into the chemistry of
life, with slight resulting differences in the _incidental_ elements
in animals and plants. As water is necessary to all vital processes,
a very large proportion enters into living matter.
But there is this fundamental and significant difference between the
two kingdoms. The plant possesses the mysterious power of weaving
the visible out of the invisible; of converting the lifeless into
the living. This it does in virtue of the chlorophyll, or green
coloring matter, which is found united with definite portions of the
protoplasm-mass, of which it is a modification, the exact nature
being unknown. The water and the carbonic acid which the plant
absorbs through the numberless stomata or mouth-pores in its leaves
or integument are, when the sunlight falls upon them, broken up by
the chlorophyll, which sets free the oxygen, and locks together the
hydrogen and carbon, converting this hydro-carbon into the simple and
complex cells and tissues of the plant, with their store of energy
for service to itself and other organisms. Animals, a few low forms
excepted, can not do this; they are powerless to convert water,
salts, gases, or any other inorganic substances, into organic; they
are able only to assimilate the matter thus supplied by the plant,
nourishing themselves therewith either directly, by eating the plant,
or indirectly, by eating some plant-feeding animal.
In other words, the plant manufactures protein from the mineral
world, and the animal obtains the protein ready-made; the plant
converts the simple into the complex; and this the animal, by
combining it with oxygen, consumes, using up the energy it thereby
obtains in doing work. So the plant is the origin of all the energy
possessed by living things, but why it can by virtue of the sunshine
convert the stable inorganic into the unstable organic, while the
animal can not, we do not know. Neither do we know whether plant
preceded animal, or _vice versâ_, in life’s beginnings, although
the evidence seems to point in favor of the priority of the plant.
Structurally the lowest animal is below the lowest plant, since it is
a speck of formless, colorless protoplasm, whereas the protoplasm of
the lowest plant is organized to the extent that it has formed for
itself an outer layer or membraneous coat called the cell-wall. For
example, the vegetable character of yeast-granules is determined,
apart from their mode of nutrition, by the protoplasm being inclosed
within a cellulose coat, and the animal character of the amœba,
not because of contractile or locomotive power or of inability to
manufacture protein from inorganic matter, but by the absence of any
such covering. Upon this Haeckel remarks that the vegetable cells
sealed their fate when inclosed within a hard thick cellular shell,
being thereby less accessible to external influence, and less able to
combine for the construction of nervous and muscular tissues than the
animal.
But since the function creates the organ, and where function is not
localized there is no variation of parts, life probably began in
formless combinations having no visible distinction of parts. And as
the cell is the first step in organization, it is the fundamental
structure of living things, “it marks only where the vital tides have
been or how they have acted,” the lowest organisms consisting of one
cell only, and the higher consisting of many cells, which, increasing
in complexity or diversity of form adapted to their different
functions at later stages, are modified into the special tissues,
with resulting unlikeness in parts or organs, of which all plants and
animals are composed. Every variation in structure is, therefore, due
to cellular changes, and every living thing is propagated in one way
or another by cells, by their self-division or multiplication; or
by gemmation, _i. e._, throwing off buds; or by the union of like
cells; or, in more complex mode, by the spontaneous or aided union of
unlike cells, as the sperm-cell of the male with the germ-cell of the
female, giving rise to a seed or egg from which grows offspring more
or less like its parents.
In both plant and animal the cell-contents usually, although here
again exceptions occur in some of the lowest organisms, exhibit
a rounded body called the _nucleus_, which itself often incloses
another body called the _nucleolus_, the functions performed by
both of which in cell development are obscure. That even thus much
is known of cell structure may awaken wonder when it is remembered
that we are dealing with bodies for the most part beyond the range
of our unaided vision. Bacon truly says that “the complexity of
nature exceeds the subtlety of man”; the infinite divisibility
and indivisibility of matter is apparent in the organic as in the
inorganic; and size counts for little; the oak and pine, the acacia
and the rose, are lower in scale of life than the thistle and the
daisy; the elephant is 150,000 times heavier than the mouse, but the
egg of the one is nearly as large as that of the other, and it has
been calculated that if one molecule in the nucleus of the ovum of a
mammal were to be lost in every second of time, the whole would not
be exhausted in seventeen years.
These molecules are the sufficing material media of transmission of
resemblances, both striking and subtle, between parent and offspring;
and of the vast sum total of inherited tendencies, good or bad, which
are the product of no one generation, but which reach us charged
with the gathered force of countless ancestral experiences.
“Born into life! man grows
Forth from his parents’ stem,
And blends their bloods, as those
Of theirs are blent in them;
So each new man strikes root into a far fore-time.”
CLASSIFICATION OF PLANTS
--LOUIS FIGUIER
Every plant which grows on the surface of the earth or in the waters
constitutes a distinct individuality. The careful examination and
comparison of a certain number of these individuals of the vegetable
world will lead to the admission that a great many are quite
identical in some of their characteristics, while others possess no
character in common. Examine the individual plants, for instance,
which compose a field of oats; in each the root, the stem, the
flowers, the fruit, present the same identical characters. The seed
of any one whatever of these plants will yield other plants like
those of the field. Every individual in the field belongs therefore
to the same _species_--to the species Avena sativa.
The species, then, is a collection of all the individuals which
resemble each other, and which will reproduce other individuals like
themselves.
These species may present, as the result of diverse influences, such
as change of climate or cultivation, differences more or less marked,
more or less persistent, which withdraw them from the original type.
To these, according to their importance, botanists give the name of
_varieties_ and _sub-varieties_. The wheat-plant, the vine, the pear,
the apple, and most of our cultivated legumes, all yield, under the
influence of culture extending over a long series of years, plants
altogether different from the original in their exterior; but they
preserve, one and all, the essential characters of the species. They
are _varieties_ of the wheat-plant, of the vine, of the pear, of the
apple.
The assemblage of a certain number of distinct species presenting the
same general characteristics, the same disposition of organs, the
same structure of flower and fruit, constitutes a group to which the
name of _genus_ is applied. Rosa canina, R. villosa, and R. Sabini
are three different species of the same group--the genus Rosa. The
words _oak_, _poplar_, _barley_, are collective common names, which
served, long before botanical science existed, to designate certain
groups of plants. These are true generic names of popular creation,
which botanists have accepted because they were the result of exact
observation. “A man of observant eye and quick intelligence,” says
Auguste Pyramus de Candolle, “would observe certain groups in the
vegetable kingdom which we call genera before discerning the species.”
The germs of botanical science are to be sought for in the
rudimentary state in very remote antiquity. In the sacred writings we
meet with constant allusions to the vegetable world. The cultivators
of the science among the early Greeks and Romans were not botanists,
but Rhizotomæ, or root-cutters, since they directed their attention
to the roots in search of medicinal properties. Aristotle of
Stagira, who lived in the fourth century before our era, may be
regarded as the founder of botany; Mithridates, and the younger Juba,
King of Mauritania, were among its cultivators. They established
botanic gardens, some probably from love of the science, others of
them in order to cultivate the deadly plants from which poisonous
juices were obtained. Nicander of Colophon, Cato, Varro, Columella,
Virgil, Pedanius Dioscorides of Cilicia, and lastly, the elder Pliny,
all dwell upon the wonders of vegetation; and war, notwithstanding
its desolating tendencies, was made to promote the interests of
science.
To the Arabians of the Twelfth Century we are next indebted for our
knowledge of botany. After them the darkness of the Middle Ages sets
in, and it is only since the illustrious Venetian, Marco Polo, came
to examine and describe the wonders of the East that the darkness has
been dispelled. He examined the treasures of Asia and the east coast
of Africa, described many plants of India and the Indian Ocean, and
from his day to the present our knowledge of the names of plants, as
well as of their structure and physiology, has been continually on
the increase.
The science of botany, as now understood, can not be held, however,
to date further back than two centuries. In the year 1682 Nehemiah
Grew published his _Anatomy of Plants_. In 1684 the French botanist
Tournefort, then professor of botany at the Jardin des Plantes,
published his _Elements of Botany_, being the first attempt to
define the exact limits of genera in vegetables. Most of the genera
established by Tournefort remain, proving the correctness of the
formula from which he deduced their common characters. Tournefort
succeeded to a large extent in unraveling the chaos into which the
science of botany had been plunged from the days of Theophrastus
and Dioscorides. Separating genera and species according to
their characteristics, he described no less than 698 genera and
10,146 species. He published, at the same time, a system for the
classification of plants, eminently attractive, especially if we
connect it with the times in which it appeared. The French botanist
directed the attention of observers, probably for the first time, to
those parts of plants most likely to excite admiration, namely, the
different forms of the corolla.
In selecting the form of the corolla as the basis of his
classification, Tournefort has, perhaps, contributed more to the
progress of botany than any other savant of any age. The task of
instruction was rendered a pleasure by thus taking, as a subject of
scientific inquiry, the most attractive part of the plant. He soon
made adepts of those who had hitherto only contemplated flowers as
the source of an agreeable sensation.
The system of Tournefort for the classification of plants met with
great favor among his contemporaries, on account of its simplicity.
Nevertheless, in its application, this system presented many
difficulties. The form of the corolla is not always so exactly
appreciable that the class to which that plant belongs can be settled
from that character alone. But the gravest defect of the system is,
that by it the vegetable world is divided into two classes, namely,
Herbaceous Plants and Trees--a division which has no existence in
nature. The division destroys the natural analogies, for the size
of a plant has no bearing upon its organization and structure. In
conclusion, the continually increasing number of new species, which
were unknown in Tournefort’s time, tests, in the strongest manner,
the defects of his system of distribution. The greater number of
vegetable species discovered since Tournefort’s time could not
be placed in either of his classes. This defect soon became very
apparent, and the system fell by degrees out of favor with botanists
even among his own countrymen, with whom it had found most admirers.
In England the study of plants had taken a more philosophical
direction. About the middle of the Seventeenth Century the microscope
was first applied to the study of the organs of plants; and in 1661
spiral vessels were detected by Henshaw in the walnut tree, and
shortly afterward the cellular tissues were examined by Hooke. These
discoveries were followed by the publication of two works on the
minute anatomy of plants by Malpighi and Grew. They examined the
various forms of cellular tissues and intercellular passages in their
minutest details, and with an exactness which causes their works
still to be recognized as the groundwork of all physiological botany.
The real nature of the sexual organs in plants was demonstrated by
Grew; the important difference between the seeds with one and those
with two cotyledons was first pointed out by him. Clear and distinct
ideas of the causes of vegetable phenomena were gradually developed,
and a solid foundation laid on which the best theories of vegetation
have been formed by subsequent botanists.
About the time when Tournefort was engaged in arranging his
system of plants, and when Grew had completed his microscopical
observations, John Ray was driven from his collegiate employments
at Cambridge by differences of opinion with the ruling powers of
his university. He sought and found consolation in the study of
natural history, to which he was ardently attached, and for which
his powers of observation, capacious mind, and extensive learning
so highly qualified him. Profiting by the discoveries of Grew and
other vegetable anatomists, in 1686 he published the first volume
of his _Historia Plantarum_, in which are embodied all the facts
connected with the structure and organs of plants, with an exposition
of the philosophy of classification, the merits of which are better
appreciated now than they were in his own days.
Ray was careful to guard his readers against the supposition that
classification was other than a means of identification. He argued
that there was no line of demarcation in nature between one group
or order, or even genus, and another, or that any system could be
perfect.
While he enumerated the true uses of classification, Ray also
laid the foundations of the natural system, which has since been
universally adopted by botanists. He separated flowerless from
flowering plants, and he divided these again into Monocotyledonous
and Dicotyledonous plants.
Forty years after the publication of Tournefort’s system, and while
Ray was yet pursuing his philosophical investigations, the Linnæan
system appeared. This new mode of distributing vegetable species was
hailed with admiration. Its author, Charles von Linnæus, reigned
supreme and without a rival till the end of the Eighteenth Century,
and even in our days his partisans are neither few nor powerless. In
Germany, for instance, more than one botanical work of character has
for foundation the system of Linnæus, and many school-gardens are
arranged after his classification.
The system of Linnæus rests upon the consideration of the organs
of fecundation--organs almost overlooked until then, but whose
physiological functions have since been ably demonstrated. He
introduced in 1736 a salutary and much-wanted reform into botanical
language and nomenclature, defining most rigorously the terms used to
express the various modifications and characters of the organs, and
reducing the name of each plant to two words, the first designating
the genus, the second designating a species of the genus. Before
his time, in fact, it was necessary to follow the name of the genus
through a whole sentence in order to characterize the species, and
in proportion as the number of species increased, the sentences
were lengthened until it seemed as if they would never come to an
end. It was like the confusion which would arise in society if, in
place of using the baptismal name and surname, we were to suppress
the baptismal name, and substitute for it an enumeration of many
qualities distinctive of the individual; as if, for example, in place
of saying Pierre Durand or Louis Durand, we said Durand the great
sportsman, or any other phraseology applicable to the qualities of
the individual. Nevertheless the Linnæan or binary nomenclature is
one of the great titles to that glory which has been awarded to
its immortal author. In the scheme of the Linnæan system it has
been found possible to describe all plants discovered since his
time--an irrefragable proof of the great merits of this artificial
classification of species.
This classification of plants has received the name of the artificial
system, because it groups the species according to a small number
and not from the whole of their characteristics; in short, it rather
permits one class to be distinguished from another than makes each
known in an intimate manner. It insists much upon their differences,
little upon their resemblances. Between species thus compared, only
one essential analogy may exist. The rush takes place beside the
barberry, because each of these plants has six stamens and only
one style. The vine is ranged beside the periwinkle, because they
each have five stamens and one style. The carrot is allied to the
gooseberry, etc. There may not be between the plants thus compared
any natural bond, but only some trace of resemblance in a particular
part of the organization, which may be found also in a number of very
different plants.
Linnæus was endowed with too sound a judgment, with a tact too
exquisite, not to feel the defects of this artificial mode of
classification. He detected by the force of his genius the existence
of vegetable groups superior to genera, and connected them by a large
number of characteristics. He called this group a _natural order_,
and it has since his time been called a “natural family.” He also
tried to distribute plants after a natural classification--that is to
say, into families. After the death, and during the life, of Linnæus,
botanists endeavored to discover upon what principle he had founded
his _natural orders_--that is to say, they sought to find the key to
the hidden principle of his orders; but no one has succeeded. Linnæus
himself does not appear to have had very fixed views on the subject.
He created his orders by a sort of instinct which belongs only to
the man of genius; by that kind of semi-divination which the man of
learning acquires who possesses vast and profound knowledge of the
objects which he passes his life in observing.
In a letter we find the following passage: “You ask me for the
characters of my orders. My dear Giseke, I assure you that I know not
how to give them.”
Magnol, professor of botany to the School of Medicine, in his work
entitled _Prodromus Historiæ Generalis Plantarum_ (1689), is the
first author who uses the happy term “family” to designate natural
groups of vegetable genera. M. Flourens speaks of the preface to
this little book of a hundred pages as calculated to immortalize
the author, as in it was first solved a very difficult problem. The
following lines are taken from this much-admired preface: “Having
examined the methods most in use,” says Magnol, “and found that
of Morison insufficient and very defective, and that of Ray much
too difficult, I think I can perceive in plants a certain affinity
between them, so that they might be ranged in divers _families_, as
we class animals. This apparent analogy between animals and plants
has induced me to arrange them in certain families, and, as it
appeared to me impossible to draw the characters of these families
from the single organ of fructification, I have selected principally
the most noted characteristics I have met with, such as the root,
the stem, the flower, the seeds. There is also found among plants _a
certain similitude_, a certain affinity, as it were, which does not
exist in any of the parts considered separately, but only as a whole.
I have no doubt, for instance, but that the characters of families
might be taken from the first leaf of the germ as it issued from the
seed. I have followed the order that the parts of plants follow in
which are found the principal and distinctive characters of families,
but without limiting myself to any one single part, for I have often
considered many of them together.”