THE EVOLUTION OF PLANTS
By C. Stuart Gager
Director of the Brooklyn Botanic Garden
In the conservatories of the Brooklyn Botanic Garden there is an exhibit designed to give a bird’s-eye view of the plant kingdom.[1] The specimens are arranged on a bench in the form of a tree, with a trunk and lateral branches (Fig. l). The trunk represents the main course of plant life through the ages; the branches are the great groups of plants. The plants now living on the earth are to be thought of as representing the tips of the branches of the genealogical tree.
Near the base of the trunk, on the lowest branch, are specimens of some of the simplest plants known. As we pass from these toward the other end of the bench we find plants of gradually increasing complexity, until we come to the orchids and composites at the topmost twigs.
Along the trunk of this family tree is a label indicating the changes met in a series of plants arranged in this order. The points where the branches leave the main trunk are “mile posts” calling attention to definite changes there represented. This long label is here reproduced, with the “mile-posts” in heavy-faced type. The names of the great groups of plants are in large and small capital letters:
Algae.
- Nearly all grow in water, with spores unprotected.
From water to land
Liverworts and Mosses.
- Have spores in a protecting spore case and grow on moist land. Have no true roots but have root-like organs (rhizoids).
From rhizoids to roots
Club Mosses.
- Have roots and water-conducting tissue and grow on drier land. Leaves small. Reproduce by spores. One spore case. Each spore develops into a small sexual plant (prothallus).
From small leaves to large leaves
Ferns.
- Spores usually all of one size and germinate on moist ground. No seeds.
From spores to seeds
Cycads.
- Two sizes of spores, which grow to small sexual plants that develop on the parent plants, thus producing seeds. Cycads have swimming sperms, as do all the preceding forms.
Conifers (cone-bearing plants).
- These and the flowering plants do not have swimming sperms. Cycads and conifers have naked seeds—that is, they are gymnosperms.
From cones to flowers
Anthophyta (flowering plants).
- Seeds enclosed in a covering—the fruit. Some seeds have two seed leaves {cotyledons), some only one, thus forming the following groups:
- Dicotyledons.
- Two seed leaves; leaves netted veined; parts of the flower in fives or fours.
- Monocotyledons.
- One seed leaf; leaves usually parallel veined; parts of the flowers in threes.
The above summary takes account of all the great groups shown in the exhibit except the fungi, which form one of the lower branches, just a little above the algae. The fungi resemble the algae in essential characters, except that none of them has the green coloring matter of plants, known as leaf-green or chlorophyll. They appear to be algae that have permanently lost this substance, but we do not yet know enough to speak with assurance as to the origin of the fungi.
_p139_plant_classification_and_phylogeny.png)
In contemplating this exhibit, or the plant kingdom itself, here represented in miniature or by samples, one is impressed by the fact that, amid the endless diversity of plant forms, it is possible to bring order out of apparent chaos. Men have been trying to do this with plants for more than two thousand years. What a long, hard struggle it has been to try to understand Nature! About 300 B.C. Theophrastus, a Greek botanist, a pupil of Aristotle, who was also a botanist, classified plants, according to their most obvious resemblances and differences, as trees, shrubs, half-shrubs, and herbs. This was a very superficial classification, but it took centuries of study by many keen minds to enable us to distinguish between essential and superficial or accidental differences and likenesses. According to the classification of Theophrastus, roses and apples fell into quite diverse groups, but in the modern classification they are placed in the same group. Other systems of classification are briefly indicated in Fig. 1.
Again, in contemplating this exhibit, one cannot help but ask himself the question, “How did all this orderly diversity come about? Have all of these various kinds of plants always existed? If not, which existed first? If they have not always existed, by what method were they created?”
It has been very natural for men to overlook the last question, and merely inquire, “By whom were they created?” This is a very proper question to ask, and full of absorbing interest, but if one has the scientific type of mind, he is not satisfied with this question, nor with any answer that may be given to it. We said above that one cannot help but ask himself the question, “How did all this orderly diversity come about?” But the scientist does not ask himself this question; he puts the question directly to Nature and seeks his answer there. He wishes to know not only who, but how.
Each question is important, but the answers are likely to lead in different directions. One who was content merely to know who made the first telephone could never have invented nor helped to invent the radio; that could have been done only by one who insisted on knowing the how and the why—the structure, the mode of action, the underlying principles of the Bell telephone. In acquiring this knowledge it was not necessary for him to forget the inventor of the telephone, to deny his existence, nor to cease to admire him and his work. Moreover, by understanding the telephone he was in a position to understand its inventor more intelligently and to regard him with more admiration and reverence.
Do we enjoy the modern delicious varieties of fruits and vegetables and the wonderfully beautiful horticultural forms and colors of flowers? These were produced, not by Nature unaided, but by Nature aided by Man. The success of men in breeding the numerous horticultural varieties of plants depended upon their understanding the processes by which new forms of plant life come into existence.
But an understanding of the method by which the present condition of the plant kingdom was brought about has value from an entirely different point of view. It gives us a more intelligent comprehension of Nature, it widens our intellectual horizon, it reveals a world of law and order, not of caprice and chance, and it enables us better to understand ourselves and our relation to the world in which we live.
In the earlier periods of intellectual inquiry men endeavored to reach an understanding of Nature by philosophical speculation. But, as Mackenzie[2] has well said, just as the special sciences cannot furnish us with those ultimate explanations for which the human mind inevitably looks, so “no purely philosophical speculation can tell us about the particular structure of the world in which we find ourselves.” There is only one source of evidence and light, and that is the study of Nature itself. If we would know how the present condition of the plant world came about we must study plants. Let us, then, briefly present some of the more important general truths that have been brought to light by the study of plants.
1. Biogenesis. In the first place, studies of great thoroughness and accuracy have led biologists to reach the unanimous conclusion that every living thing comes into existence as the offspring of other living things somewhat similar to itself. There is no other method known by which living things now come into existence. This principle has been tersely stated by the familiar Latin motto, Omne vivum e vivo (all life from life).
To be sure, there is the ultimate question, How did the first living organisms come into existence? There has been much speculation on this question, some of it based upon painstaking experiment. From what we know of the geological history of the earth we are forced to visualize an early condition when life in any of the forms now known could not have existed. If that is true, then there must have been a time when living matter first came into existence and, of course, from non-living matter. At the Richmond meeting of the American Chemical Society in April, 1927, Dr. Victor C. Vaughan, discussing a chemical theory of the origin of species, noted that although no chemist has yet awakened dead matter into life, chemists have learned how to synthesize, out of inorganic matter, substances formerly found only in plants and animals. Calling attention to the recent discovery of particles smaller than bacteria that pass through a porcelain filter and grow and reproduce like living organisms, Dr. Vaughan contends that the lowest forms of life have come into existence by chemical processes.
Our present inquiry, however, concerns, not the origin of life, but the method by which the present condition of the plant world has been reached, granted the existence of living organisms to start with. However diverse existing organisms may be, the principle of biogenesis compels us to condude that they have come into existence by descent from preëxisting organisms.
2. Gradual change. Either conditions have always been as they now are, or else there has been a change throughout the vast aeons of geological time. The evidence on this subject consists of fossils found in the rocks of the earth’s crust. The more recently formed rocks (such as those of the Tertiary period) contain fossil remains of plants very similar to those now living—in fact species of the same genera. But as we go down the geological scale to older and older rocks all evidence of species now living gradually disappears. Moreover, we find abundant evidence in the older rocks of the existence of forms not represented in the fossils of the more recent rocks and not found at all in the vegetation of to-day. Obviously, there has been a profound change in the vegetation of the earth. Forms appear, persist for awhile, and then die out, giving place to new forms. Moreover, the geological and biological evidence forces us to the conclusion that this change has come about gradually.
3. Evolution. By studying the comparative anatomy of forms in successive geological periods we learn that they resemble one another just as they would if they were related to one another like parents and offspring. When we contemplate this fact in the light of the principle of biogenesis the only logical conclusion we can reach is that the plants of one geological period have been derived from those of a preceding period by a process of descent with gradual modification. This is what is meant by organic evolution.
4. Hypothesis, theory, fact. When men study the phenomena of nature they get ideas suggesting explanations of what they observe. These ideas are of the nature of guesses, but they are rational guesses, which are fully warranted by the contemplation of the facts observed. Such a logical guess is called a hypothesis, from a Greek work that means supposition or “guess”. The next step, of course, is to try to prove the correctness or incorrectness of the hypothesis. This may be done by reasoning out what consequences ought to follow if the hypothesis is correct, and then making further investigation to ascertain whether such consequences do follow in reality. If the hypothesis is found untenable it is abandoned. As Huxley once pointed out, the pathway of the history of science is strewn with abandoned hypotheses.
If a hypothesis is found to be valid, if the consequences postulated are realized in fact, then the hypothesis comes to be called a theory. A theory is a hypothesis that has been found to be in harmony with all the known facts, or with a vast majority of those facts. Eventually the conception called a theory may no longer be regarded as a theory but may be considered an actual fact or truth. Thus, our conception of matter as composed of atoms was at first a hypothesis; all that we knew suggested that idea. After undergoing rigid tests the atomic hypothesis became the atomic theory. Now we have, so to speak, handled atoms, and separated them into their component parts, so that atoms are no longer regarded as hypothetical or theoretical things but as actual facts.
So with the conception of the evolution of living things from earlier, simpler organisms to those now living. The idea was first a hypothesis, then a theory; and probably no living student of plants now doubts that evolution is a fact—that is, he believes that the present condition of the kingdoms of plants and of animals was attained by the process of evolution.
In the evolution of the plant kingdom the evidence available forces us to conclude that the earliest organisms were protein compounds endowed with these peculiar attributes:
1. Ability to take in matter from without and transform it into matter like themselves. This we call metabolism and nutrition.
2. Ability to grow— o increase in size and weight.
3. Ability to reproduce their kind.
4. Ability to detect changes in their surroundings and to react or readjust themselves to the changed conditions. This we speak of as ability to detect and to respond to stimuli.
It is also probable that the earliest forms of life were so simple that they could be regarded as neither plants nor animals, but merely as organisms or “living things.” Such organisms are well known today. The viruses, which cause the so-called virus diseases of plants, are possibly of this nature. They behave like living things, but they are so small that they cannot be seen with the most powerful microscope. This means that their greatest dimension is less than one-half the wave-length of light.
One group of organisms, known as slime molds (Myxomycetes. Fig. 2), at one stage of their existence so closely resemble the tiny animals (animalcules) known as Amoebae (singular, Amoeba) that they can hardly be told apart. Both are naked bits of protoplasm, capable of motion and locomotion. Zoölogists have regarded them as animals; botanists have contended that they are plants.
The similarity between animals and plants in their essential life processes has long been recognized by biologists and was forcibly presented by Claude Bernard in his classical Leçcons sur les Phénomènes de la Vie Communs aux Animaux et aux Végétaux (1878–79) . In the processes of respiration, digestion, cell-division, growth, reproduction, transmission of heritable characters, the possession of irritability and the power to detect and respond to stimuli, and in other physiological processes, they are essentially alike.
Early in the development of living things one group of plants acquired the ability to manufacture that wonderful substance, chlorophyll, which gives the green colour to all foliage, and these primitive chlorophyll-bearing organisms must be regarded as the ancestors of all plant life.
_p146_Pleurococcus_vulgaris.png)
Reproduced, by permission, from Gager’s Fundamentals of Botany, published by P. Blakiston’s Sons & Co.
The simplest chlorophyll-bearing plants to-day are the unicellular green algae, such, for example, as Pleurococcus (Fig. 3). These reproduce only by cell-division. Other green algae, such as green silk, or Spirogyra, reproduce by both cell-division and cell-fusion. The introduction of cell-fusion to the life histories of organisms laid the foundation for the development of sex, for cell-fusion is the essential process in sexual reproduction.
The modern representatives of the other great groups of chlorophyll-bearing plants, such as the mosses, ferns, club mosses, little club mosses, and conifers (Figs. 4–7), illustrate definite advances in evolutionary progress, but they do not form a genetical series—that is, they do not bear to each other the relation of ancestor and descendant. Some students incline to the opinion that all the great modern groups of plants have descended from one main hypothetical fern-like branch, the Primofilices, which can be traced back to the dawn of the fossil record but is now extinct. From the Primofilices there descended cycad-like forms (Cycadofilices, cycad-ferns), also now extinct, but known from
_p146_face_A_slime_mold.jpg)
Fig. 2.—A myxomycete or slime mold (Fuigo septica) in the plasmodium stage; a mass of protoplasm without cell wall.
This plasmodium grew on moist decaying wood in a glass jar and was photographed after it had “crawled up” the inner surface of the jar in the manner of the microscopic animal Amoeba. Its color was bright orange.
_p146_face_B_Osmunda_cinnamomea.jpg)
Fig 5.—Plants of the cinnamon fern (Osmunda cinnamomea), showing foliage leaves and (in the center) spore-bearing leaves.
Note that here the leafy plant is spore-bearing (the sporophyte), whereas in the moss (Fig. 4) the leafy plant is egg-bearing and sperm-bearing.
Reproduced, by permission, from Gager’s Fundamentals of Botany, published by P. Blakiston’s Sons & Co.
_p147_face_Selaginella_amoena.jpg)
Fig. 6.—A little club-moss (Selaginella amoena). The spore-bearing leaves are aggregated in cones borne at the tips of leafy branches of the sporophyte.
Reproduced, by permission, from Gager’s Fundamentals of Botany, published by P. Blakiston’s Sons & Co.
abundant fossil remains. They have also been called Pteridosperms, or seed-bearing ferns (Fig. 8) . From these descended cycad-like plants, which had features resembling those of modern _p147_Polytrichum_commune.png)
Fig. 4.—Hair-cap moss (Polytrichum commune). A, male plant; B, same, reproducing vegetatively, growing from the tip of another; C, female plant bearing a spore-case on a long, slender stalk. This spore-bearing phase of the plant (sporophyte) is developed from an egg-cell after it had been fertilized by a sperm from a male plant.
Reproduced, by permission, from Gager’s Fundamentals of Botany, published by P. Blakiston’s Sons & Co. flowering-plants of primitive type and which are called Pro-angiosperms. From this stock are descended the modern cycads (Figs. 9 and 10) and the two great groups of flowering plants—those with two seed-leaves (dicotyledons—magnolias (Fig. 12), buttercups, roses, bell-flowers, dandelions, etc.), and those with one seed-leaf (monocotyledons—lilies (Fig. 14), grasses,
_p148_seed-fern_Neuropteris_heterophylla.png)
Fig. 8.—A seed-bearing fern (Neuropteris heterophylla), one of the Pteridospermae or Cycadofilices. The leaves resemble those of the royal fern (Osmunda regalis), and some of those shown bear seeds. Note that the unexpanded leaves are circinately coiled, as in modern true ferns. The two lower left-hand figures show fronds with seeds attached. Restoration by Miss Janet Robertson. (After D. H. Scott, Extinct Plants and Problems of Evolution, with the permission of the author and The Macmillan Company, Ltd.).
_p148_face_Pinus_sylvestris.jpg)
Fig. 7.—Scotch pine (Pinus sylvestris). Terminal parts of leafy branches, with spore-bearing leaves in “cones” at the tips of lateral branches. As the cones mature during the first year their stalks bend down. The cones at the right are one year old. The pine tree is the sporophyte, corresponding to the stalked spore case of the moss (Fig. 4).
Reproduced, by permission, from Gager’s Fundamentals of Botany, published by P. Blakiston’s Sons & Co.
_p149_pre-face_ovuliferous_leaf_of_Cycas_revoluta.jpg)
Fig. 9.—A spore-bearing leaf )megasporophyll) from a female cycad (Cycas revoluta), bearing six young spore cases (ovules) containing “large” spores (megapores).
The spores have developed, within the spore cases, into the egg-bearing phase of the plant (gametophyte). The spore-bearing phase of the next generation begins its development within the tissues of the egg-bearing phase, thus forming the embryo, which resumes its growth when the seed germinates. The “small” spores (microspores) are borne in microsporophylls on male plants.
Reproduced, by permission, from Gager’s Fundamentals of Botany, published by P. Blakiston’s Sons and Co.
_p149_face_cones_of_Macrozamia_moorei.jpg)
Fig. 10—A cycad (Macrozamia Moorei). Upper end of the stem, showing the bases of the crown of foliage leaves and two lateral branches, each having at its tip a cone of spore-bearing leaves.
Reproduced, by permission, from Gager’s Fundamentals of Botany, published by P. Blakiston’s Sons & Co.
orchids, etc.). This, in bare outline, is the monophyletic hypothesis (Fig. 11).
Other students, on what they believe to be equally good evidence, postulate two primitive main branches, appearing as distinct at the dawn of the fossil record—the club-moss stock, or Lycopsida, and the fern stock, or Pteropsida, from which have descended the modern conifers and flowering plants and their ancestors. This is one of the polyphyletic hypotheses (Fig. 13).
The solution of the question of the “family-tree” of plant life may be roughly likened to the task of putting together a picture-puzzle, in which many of the pieces are not understood and some are perhaps temporarily or permanently lost. If we had a museum collection of specimens of all the kinds of plants that have ever lived, botanists believe that such specimens could be so arranged as to represent their genetic relations and to give us a true picture of the evolutionary development of the present plant world. But probably no such collection can ever be made. We are continually finding, with more or less certainty, where this or that piece belongs in the picture, and lost pieces are continually being discovered as fossils in the rocks or as facts disclosed in the laboratory and field.
Again—to use once more the illustration afforded by the picture puzzle—it is the difficulty of the problem that fascinates the scientist, and it is the modicum of his success that lures him on to further research; he finds his reward in the quest and in the satisfaction of making some contribution, however slight, to the ultimate, but probably unattainable success.
It is one thing, however, to accept evolution as a fact and quite another thing to explain the method of evolution—how this gradual change or series of changes has been brought
_p150_phylogeny_of_plants.png)
Fig. 11.—Hypothetical genealogical tree showing the ancestral line of the modern plant groups (orders) according to a monophyletic hypothesis. (Compare Fig. 13.)
Reproduced, by permission, from Gager’s Fundamentals of Botany, published by P. Blakiston’s Sons & Co.
_p150_face_Magnolia_flower.jpg)
Fig. 12.—Flower of a species of Magnolia, illustrating a primitive type of dicotyledonous flower structure, in that the stamens (leaves bearing “small” spores) are spirally arranged. The carpels (above the stamens) are spore-bearing leaves carrying “large” spores. Here the spore-bearing leaves are surrounded by a floral envelope of petals and sepals, thus making a true flower, in contrast to the cone of the pines and the organs in lower plants.
Reproduced, by permission, from Gager’s Fundamentals of Botany, published by P. Blakiston’s Sons & Co.
_p151_phylogeny_of_plants.png)
Fig. 13.—Hypothetical genealogical tree, showing the ancestral line of the modern plant orders according to a polyphyletic hypothesis. (Compare Fig. 11.)
Reproduced, by permission, from Gager’s Fundamentals of Botany, published by P. Blakiston’s Sons & Co.
about. The theory of Natural Selection, proposed in 1859 by Charles Darwin, is the most fruitful of several that have _p152_Lilium_martagon.png)
Fig. 14.—Turk’s cap lily (Lilium Martagon). One of the monocotyledons—the group of plants having one seed-leaf or cotyledon. There is evidence that the monocotyledons form the most recently evolved group of plants. been proposed. It has recently been discussed so fully and so frequently in daily newspapers and in popular and technical periodicals and books that it need only be mentioned here. That new plant forms may be derived from preëxisting forms by the process of descent with modification has been demonstrated by actua1 experiments, culminating in the classical work of the Dutch botanist, de Vries. The method by which this may be brought about has been outlined by de Vries in his mutation theory. The mechanism of mutation and of inheritance has been worked out in detail by Gregor Mendel and more recent students of genetics, who have extended and elaborated the pioneer work done on this problem by Mendel.
In summary it may be said that the plant kingdom presents itself to us as a multitude of organisms of various degrees of complexity, ranging from one-celled algae to multi-celled organisms such as orchids and chrysanthemums. The present vegetation of the earth differs profoundly from that of preceding geological ages; and amid all the present and past diversity of form there is evidence that leads to only one conclusion, namely, that the various forms of plant life are genetically related—that the newer and more complex types have been derived by descent (with modification) from older and simpler types. A mechanism has been worked out along lines suggested particularly by Darwin, de Vries, and Mendel, which offers a partial but rational explanation as to how these evolutionary changes may have been and probably have been accomplished.
In particular the fact to stress in such problems as those here discussed is that they cannot be solved by philosophical speculation; they can be solved only by first-hand study of plants themselves. In our quest of the elusive thing we call truth, whether in science, religion, politics, or any other department of human thought, the most conspicuous historical feature we note is change, revision, and continued research for new and more reliable information and interpretation. What one generation ties to, the next rejects, but not in toto. A residuum remains, which we believe represents the truth. Some progress is made by each generation. The discovery of new facts may necessitate the radical revision or even the abandonment of old ideas, but the only things that should cause us grave concern would be the cessation of the discovery of such facts (for each revision takes us one step nearer to ultimate truth) and the closing of our minds, by prejudice or otherwise, so that we could not entertain new truths nor revise our old conceptions in the light of new and revitalizing evidence.
REFERENCES
- Babcock and Clausen. Genetics in Relation to Agriculture. 2d ed., New York, 1927.
- Berry, Edward Wilber. Tree Ancestors. A Glimpse into the Past. Baltimore, 1925.
- Bower, F. O. Plant Life on Land. Cambridge (Eng.), 1911.
- Bower, F. O. The Origin of a Land Flora. Cambridge (Eng.), 1908.
- Campbell, D. H. Plant Life and Evolution. New York, 1911.
- Chamberlin, T. C., and Others. Fifty Years of Darwinism, etc. New York, 1909.
- Darwin, Charles. The Origin of Species by Means of Natural Selection. 1st ed., London, 1859.
- Darwin, Frances. The Life and Letters of Charles Darwin. New York, 1901.
- Darwin, Frances. More letters of Charles Darwin. New York, 1903.
- Gager, C. Stuart. Fundamentals of Botany. Philadelphia, 1916.
- Gager, C. Stuart. Heredity and Evolution in Plants. Philadelphia, 1920.
- Gager, C. Stuart. The Relation Between Science and Theology: How to Think About It. Chicago and London, 1925.
- Gager, C. Stuart. General Botany. Philadelphia, 1926.
- Knowlton, Frank Hall. Plants of the Past. Princeton, 1927.
- Lotsy, J. P. Evolution by Means of Hybridization. The Hague, 1916.
- Mendel, Gregor. Experiments in Plant Hybridization. Eng. trans. By Royal Horticultural Society (In Bateson, W., Mendel’s Principles of Heredity. Cambridge, 1909). The original paper, Versuche über Pflanzen-hybriden was published in Verhandlungen des naturforschenden Vereins in Brünn. Abhandlungen Band IV, 1865. Brünn, 1866.
- Morgan, T. H. A Critique of the Theory of Evolution. Princeton Univ. Press, 1916.
- Morgan, T. H. The Physical Basis of Heredity. Philadelphia, 1919.
- Newman, H. H. Evolution, Genetics, and Eugenics. Chicago, 1925.
- Scott, D. H. Studies in Fossil Botany. 2d ed., London, 1909.
- Scott, D. H. The Evolution of Plants. New York and London, 1911.
- Scott, D. H. Extinct Plants and Problems of Evolution. London, 1924.
- Seward, A. C. Links With the Past in the Plant World. Cambridge (Eng.), 1911.
- Vries, Hugo de. The Mutation Theory. Eng. trans, by Farmer and Darbishire. Chicago, 1909.
- Vries, Hugo de. Intracellular Pangenesis. Eng. trans, by C. Stuart Gager, Chicago, 1910.
- White, Orland E. (In Gager’s General Botany. Chapters XL–XLII.) Philadelphia, 1926.
“There is a grandeur in this view of life, with its several powers, having been originally breathed by the Creator into a few forms or into one; and that, whilst this planet has gone cycling on according to the fixed law of gravity, from so simple a beginning, endless forms most beautiful and most wonderful have been and are being evolved.”—Darwin.
“Nature! We are surrounded and embraced by her; powerless to separate ourselves from her, and powerless to penetrate beyond her.
"She is ever shaping new forms; what is, has never yet been; what has been, comes not again. Everything is new, and yet naught but the old.”—Goethe.
- ↑ The exhibit here described was planned and installed by Dr. Alfred Gundersen, Curator of Plants, Brooklyn Botanic Garden.
- ↑ Mackenzie, J. F., Elements of Constructive Philosophy, p. 308.