Dicotyledonous Plant Families
All the great bulk of the flowering plants not included in the monocotyledons or the gymnosperms belong to about two hundred plant families that are included in the dicotyledons. In all of them the seed sends up two seed leaves, there are generally netted-veined leaves and the parts of the flower are in fours or fives or multiples of these numbers. In such a large aggregation of plant families there are three well-marked divisions, namely, those that bear no petals or sepals, those that do bear them and where they are separated to form individual sepals or petals, and those where the petals are united to form some sort of a tubular or at least connected corolla. These divisions are perhaps best shown thus:
(a) Apetalæ—Including families where the petals are never present, and in some there is even no calyx. Examples: walnut, hickory, willow, and oaks.
(b) Polypetalæ—Petals present but separate, not forming a tubular or connected corolla. Examples: buttercup, rose, pea, apple, geranium.
(c) Gamopetalæ—Petals united and forming some sort of a tubular or connected corolla. Examples: garden primrose, gentian, salvia, mint, snapdragon, and the daisy family.
Any attempt to describe the families contained in these three divisions of the dicotyledons would take all the rest of this book and crowd out other things about plant life that must not be omitted. All that can be done here is to outline briefly a few families in each division so that we shall have fixed in our minds what the general principles of plant classification are and how these are illustrated by well-known plants. There are many books that deal with this subject in great detail and to them the student should go for further elaboration of the subject. It is one of the most interesting phases of the study of botany, but it demands a longer and more intensive study than can be included here.
(a) Apetalæ—These include families of plants that are the simplest in structure of all the dicotyledons. In all of them there are no petals and in some both petals and sepals are lacking, leaving only essential organs. Taking first those families that have neither petals nor sepals we find that most of them bear their flowers in catkins, a flower cluster familiar enough in the pussy willow. Some of these families are the following:
Juglandaceæ—Trees with compound leaves, flowers in catkins and fruits inclosed in a thick husk. Examples: walnut and hickory. Six genera and over 30 species.
Salicaceæ—Shrubs or trees with simple leaves, flowers in catkins and capsular many-seeded fruits, but no nuts. Containing only willows and poplars. Two genera and over 200 species. ([Figure 93].)
Myricaceæ—Shrubs or trees with simple usually fragrant leaves, flowers in catkins and fruits one-seeded. The bayberries. Two genera and about 35 species. ([Figure 94].)
These and some other families of close relationship are the least developed, in their flower structure, of any of the dicotyledons. All of them bear only the essential organs of reproduction in their very simple flowers. In the walnuts and hickories the different sexes are in different flowers on the same plant, in the willows they are even on different plants, and in Myricaceæ they are often found both ways. All of these flowers are wind-pollinated, so that they bear no honey, usually have no odor, and of course their need for showy petals for attracting insects is nil, although some pollen-eating insects visit them.
Somewhat higher in the scale of plant life are those families that, while still lacking petals, do have sepals. Their flowers are for this reason much better protected against rain or other inclement conditions, which means that they are so much the more likely to reproduce their kind. At least two of these slightly more developed families cling to the habit of bearing some of their flowers in catkins, however. Other families are also found in other parts of the world, but in North America this group is represented by:
Betulaceæ—The birch, alder, hazelnut, and hornbeam. Both staminate and pistillate flowers in catkins. Fruit a small one-seeded nut or a winged samara in the birch. All wind-pollinated. Six genera and about seventy-five species, nearly all from north temperate zone.
Fagaceæ—The oaks, beech, and chestnut. All trees or shrubs with at least their staminate flowers in catkins. Fruit inclosed in a bur (chestnut and beech) or borne in a cup (the acorns of oaks). At least five genera and about 375 species, widely distributed.
The habit of bearing some or all their flowers in catkins which flower usually before the leaves appear, and of having such flowers wholly at the mercy of precarious winds, is, if not lost, at least much less frequent in the remaining families of the apetalæ. All the others, while still without petals, do have sepals and some of these are colored so that insect visitors are likely. There are too many of these families to be enumerated here, but two of the chief are:
Ulmaceæ—The elms and hackberry. Trees or shrubs with minute greenish or yellowish flowers crowded in small clusters or in spikes. Fruit a dry nut or one-seeded and winged; or in the hackberry a drupe, one of the first evidences of even a slightly fleshy fruit in the dicotyledons. About 13 genera and 140 species, widely distributed.
Polygonaceæ—The buckwheat, knotweeds, common dock, and many other genera. Sepals often colored white or pink so the flowers are sometimes at least insect-pollinated. Flowers small and crowded in various clusters, often in a spike. Fruit an achene, a dry fruit familiar enough in the buckwheat. About 40 genera and over 800 species, mostly herbs or vines here, but often trees in the tropics.
From here on plant families leave, with some exceptions, the greenish or otherwise inconspicuous flower color, and somewhere about here they begin to rely more upon insect fertilization for the perpetuation of their kind. None of those so far mentioned have any petals to their flowers, but in the pink family or Caryophyllaceæ we find the first evidences on any considerable scale of the presence of sepals and petals, the latter usually beautifully colored. Familiar representatives of this family are the pink, carnation, chickweed, corn cockle, and the stichwort. There are over 50 genera and 1,000 species, nearly all in temperate regions.
The apetalous families appear to show a development from catkin-bearing trees with the sexes separated, and with neither petals nor sepals, through the Polygonaceæ, with often colored sepals, and the beginnings of insect fertilization. In Caryophyllaceæ, the most highly developed of them all, there are, besides the sepals, often or usually petals, and the reliance on insect fertilization is nearly complete. There are many transitional stages which cannot be included here, but they show step by step the development of the apetalous families from perfectly naked reproductive organs to the next larger group, the polypetalæ, where the process of increasingly complex flower development will now be sketched.
(b) Polypetalæ—In this large group of plant families the petals are free and quite separate, but as if they had not yet lost all the characters of the apetalæ, some families show incompletely the general characteristics of their more stable neighbors. There are, for instance, no petals in many species of the buttercup family, none in the sweet-gum tree nor in the maples, and a few others. But in spite of occasional exceptions this large group of polypetalous families do usually bear separate petals and sepals, and are among the most important of all the plant families. As they number over a hundred and contain thousands of species, all that can be done here is to mention a few typical or important ones. Just as in the apetalæ the families in this large group appear to show definite stages in development from simpler to more complex forms. But the steps are harder to trace and what appears simple characters in some plants are very complex in others.
While all the families in this group have separate petals some of them show a tendency to have united sepals, a character of perhaps some advantage and certainly very common among the still more developed gamopetalæ. Some of the families that have separate sepals agree in having the stamens inserted below the ovary. Of these the following three families may serve as types.
Nymphæaceæ—The water lilies. Aquatic plants with usually large showy flowers in which the calyx, corolla, and stamens often merge one into the other so that it is sometimes difficult to know where one series ends and the other begins. Five genera and 45 species throughout the world.
Ranunculaceæ—Buttercup family. Includes buttercups, clematis, columbine, meadow rue, golden seal, marsh-marigold, hepatica, and scores of other native plants. All herbs, except a few semiwoody vines like clematis. Sepals always present, and where no petals are found, as in marsh-marigold, colored like them. Highly irregular flowers are not uncommon, as in columbine and monkshood. The fruits are berrylike in some genera and in others dry capsules. Thirty-five genera and over 1,000 species throughout the world, but most abundant in temperate regions.
Lauraceæ—Laurel or sassafras family. Includes besides them the guava and cinnamon and camphor trees, all tropical, and the native spice-bush. All trees and shrubs with small, yellow, or greenish-yellow flowers and usually aromatic juice. Fruit a one-seeded drupe or a berry. About 40 genera and over 1,000 species, nearly all tropical, but a few in the United States.
At this point, in the sequence of plant families, there are two or three families that bear quite different fruits than any heretofore noted, and in one of them, at any rate, the four petals are in the form of a cross. So uniformly is this true that the family was for years known as the Cruciferæ, but is better known as Brassicaceæ, from Brassica, the generic name of the mustard. This large mustard family mostly has fruits known as a silique or silicle, which are pods that split into two valves; and yellow or white, rarely pinkish flowers. The juice is always somewhat acrid, familiar through the pleasant pungent taste of water-cress, but none of the family is poisonous. There are over 200 genera and nearly 2,000 species of wide distribution, and common representatives include the cress, mustard, horse-radish, garden stock, sweet alyssum, cabbage, cauliflower, brussels sprouts, radish, and turnip.
Between the Brassicaceæ and the following families there are many others that cannot be mentioned here. Somewhat farther along in the sequence are a group of families, large and important, and all having their stamens inserted around or even above the ovary, and in which the sepals are partly or wholly united. They include some of our most beautiful flowers and useful fruits. Of the many closely related families that agree in these characters the two most important are:
Rosaceæ—Rose family. In the broad sense including, besides the rose, the strawberry, blackberry, apple, pear, peach, plum, besides many herbs with wholly dry fruits. There are always five petals, five lobes to the partly united calyx, but numerous stamens. They may be herbs, shrubs, or trees, with simple or compound leaves, but these are nearly always alternately arranged. There are over 100 genera and nearly 2,000 species. Because of the size of the Rosaceæ and differences in fruit, the apple and its relatives are often included in a separate family, the Malaceæ ([Figure 96]), and the peaches and plums in Amygdalaceæ. The general structure of the flower is sufficiently uniform, however, for them all to have been included in Rosaceæ ([Figure 97]).
A prickly shrub related to the apple, which, with the plums, cherries, pears, strawberry, blackberry and hundreds of other plants are all grouped in the Rosaceæ or rose family.
Papilionaceæ—Pea family. A large family having characteristic pealike flowers, a description of which has already been given in Chapter I. They all bear legumes, a pod that splits down one side, not both, as in Brassicaceæ. Besides the pea, the bean, vetch, alfalfa, lentil, locust tree, and dozens of valuable timber trees in the tropics belong here. Flowers all showy and absolutely dependent upon insects for fertilization. Seeds highly nutritious in many genera, and the roots of nearly all have bacteria associated with them. (See chapter on Food Habits of Plants.) Three hundred and twenty-five genera and over 5,000 species of wide distribution, but most frequent in the tropics.
There are many other smaller families in different parts of the world which hover, as it were, about these two giant plant families that make definite landmarks in the scheme of plant classification. The character of having partly united sepals and numerous stamens inserted around, or even above the ovary, give to all the flowers of the Rosaceæ and related plants a general family resemblance that is very striking. The pea family, and its relatives, also have flower and fruit characters of remarkable uniformity, considering the tremendous number of species.
From this point on to the end of the polypetalæ there are scores of plant families, all agreeing in having a compound ovary, that is, one that is more than one-celled, and in also having quite distinct and separate sepals. Their agreement in these characters, however, ends all other evidences of relationship, and it is beyond the scope of this book to go into the details of each, or even a few of them. A list with some brief notes on those most important must suffice here.
Geraniaceæ—Geranium family. Includes also the common garden as well as the wild geranium. Fruit splitting into five parts. Leaves always divided or even dissected; 12 genera and 470 species, all herbs.
Anacardiaceæ—Includes the tree of heaven, sumac, poison ivy, and in the tropics, the mahogany, all trees, shrubs, or vines; 60 genera and 500 species, mostly in the tropics.
Malvaceæ—Mallow family, including, besides the marshmallow, the rose of Sharon, and cotton. They all have the stamens united into a column or tube which surrounds the style. About 40 genera and 900 species of herbs, shrubs, or trees of wide distribution.
Cactaceæ—Cactus family. Nearly all desert plants, with no leaves or practically none, and greenish stems that function as leaves and also store water. Of the greatest variety of form and always bearing numerous petals and fleshy fruits, of which the prickly pear is familiar enough. Of 40 genera and over 1,000 species, all, but a handful, come from North and South America.
Umbelliferæ (often called Ammiaceæ)—The climax of the polypetalous families, and nearly always bearing flowers in umbels. There are usually many flowers, sometimes several hundred in each cluster. Familiar examples are parsley, celery, parsnip, Queen Anne’s lace, and many others. The seeds often contain an aromatic oil, as in caraway, and some are violently poisonous, as the water hemlock. About 250 genera and over 2,000 species, all herbs, widely distributed, but most common in temperate regions.
We have seen from the foregoing the probable development of dicotyledonous plants from those simplest ones, where, as in the pines, there is merely a naked ovule between scales, through the catkin-bearing trees, without petals or sepals, and all wind-pollinated, to families where just an inconspicuous and, subsequently, a colored calyx is found, and after this the dawn of those plants that have complete and perfect flowers. Among the latter all those so far noted have separate petals, but after the Ammiaceæ, or carrot family, there appears a new character, setting off practically all other dicotyledonous plants from those already treated. This new character—and hints of it are found before it reaches the perfection found in the subsequently described families—is that of the petals being united to form some sort of a connected or, more often, a tubular corolla. The petals are represented merely by the lobes of the corolla, mostly four or five, and in many families of this group, known as the gamopetalæ, literally, united petals, this tubular corolla is irregular and often beautifully formed. In salvia, for instance, there is a hoodlike upper part overhanging the lower tubular part. Other familiar examples of these irregular corollas are the garden snapdragon, Oswego tea, skullcap, pentstemon, and many others.
(c) Gamopetalæ—The earlier families among those generally having united petals seem not yet quite sure of their new character, for a few of them hark back to the condition of having, in some genera, quite separate petals. One of the first families in this series, the Ericaceæ ([Figure 98]), or heath family, has several genera in which this is true, notably in the Labrador tea and the sand myrtle among native plants, and some foreign relatives. The Ericaceæ are almost exclusively shrubs or trees, but some of our native sorts, such as trailing arbutus and wintergreen, are practically herblike, although they are, strictly speaking, woody plants. The family is remarkable for containing beautiful flowered garden plants, such as the hundreds of species of South African heaths, the heather, the azaleas and rhododendrons, and our beautiful native Rhodora, about which Emerson wrote one of his most beautiful poems. The flowers in the heath family are often perfectly regular and bell-shaped, but sometimes irregular, as in azaleas and several other genera. Nearly all the family rely on microscopic organisms to get their food, and some close relatives, like the Indian pipe, are saprophytes. There are over 70 genera and 1,200 species widely distributed. Central Asia is the home of most rhododendrons and azaleas, scores of species being found in the upper reaches of the Himalayas.
The Ericaceæ are typical of many families in the first group of the gamopetalæ, in that all of them, with a few exceptions like the cranberry, have a superior ovary. That is, the petals and sepals arise from the base or below the ovary, and consequently the mature fruit in such plants is never crowned with the remains of the withered calyx, as blueberries always are and all other gamopetalæ that have an inferior ovary. The character of having an inferior or superior ovary separates the gamopetalæ into two large groups of families, the heath family and many others, with superior ovary, and a few but numerically very important families that always have an inferior ovary.
Taking first the families that all have a superior ovary, we must, for lack of space, exclude most of them from here. A few of the most important, or typical, after the Ericaceæ, are:
Primulaceæ—Primrose family. All herbs in which the stamens are as many as the lobes of the corolla and inserted on them. The flowers are quite regular. They all have some form of a capsule for fruit, which in most generally split lengthwise. Familiar examples include the garden primrose (not the wild evening primrose), yellow loosestrife, the star flower, pimpernel, shooting star, and the beautiful cyclamens. A few members of the family are slightly luminous in the dark, apparently an attraction to night-flying insects. About 28 genera and over 400 species, mostly from the northern hemisphere, a few in temperate South America and South Africa.
Gentianaceæ—Gentian family. Over 700 species in 70 genera, all bitter herbs, with opposite leaves, quite without teeth and beautiful, sometimes fringed, always regular flowers. In this and related families the stamens are of the same number as the lobes of the corolla, and always alternate with them. Gentian and sea or marsh pinks are our best-known native representatives, while some related plants are medicinal.
There are many other families in this part of the scheme of plant classification that have minor differences among themselves, but agree pretty generally in the number and position of their stamens, their superior ovary, and, on the whole, in the regular flower. Irregular and regular flowers may be recognized at once by cutting them lengthwise through the middle. In regular flowers there would be as much on one side as on the other of the dividing line, and in irregular ones quite obviously more on one side than the other. The character of all the genera in a family having irregular flowers begins to occur here with greater and sometimes exclusive frequency. In the mint family, or Lamiaceæ, nearly all of its 160 genera and over 3,000 species have two-lipped or irregular flowers. The garden salvia well illustrates the type.
The Lamiaceæ or Labiatæ, as they are often called from the two-lipped corolla, are herbs locally, but in the tropics often shrubs or trees. Almost universally they have four-sided stems and opposite leaves without stipules. The flowers may be solitary, much more often they are crowded into various kinds of clusters. The four stamens are borne on the corolla tube, and nearly always there are two long and two short ones. The family is universally fertilized by insects, and some of the flowers are wonderfully adapted for this end. Common examples, besides the salvia, are mint, thyme, skullcap, hyssop, bugle, blue curls, catnip, hedge-nettle, coleus, and Oswego tea. Most of the genera contain heavy odorous oils in their foliage, from which oil of mint, pennyroyal, lavender, rosemary, marjoram, savory and balm are the best known. These volatile oils give to members of the family their characteristic and often very beautiful odors.
There are many other families of plants, some with irregular and others with regular flowers, that appear to group themselves around the Lamiaceæ, all of which agree, in spite of individual differences, in having a superior ovary. The remaining families of the gamopetalæ, however, always have an inferior ovary, usually obvious by the insertion of petals above the ovary, and in the fruit often conspicuous by the remains of the withered calyx still clinging to the top of the fruit. Only two of the scores of families, with inferior ovary and gamopetalous corollas, will be mentioned here, both of which are important.
Rubiaceæ—Madder family. Common examples are the creeping bedstraw, the sweet woodruff, partridge berry, button bush, and bluets or quaker-ladies. All, except one of these, are herbs, but in the tropics the Rubiaceæ are nearly all shrubs or trees. Among those are the coffee, quinine, and ipecac. All the family have opposite leaves (a few verticillate) and stipules, regular flowers, with stamens as many as the corolla lobes, and alternate with them. The fruits are a drupe, berry, or capsule. Over 340 genera and 6,000 species are known almost throughout the earth.
Compositæ or Carduaceæ—The daisy family and the largest and most complex of all the plant families. As the culmination of the scheme of plant classification, they show the greatest perfection in the arrangements for cross-fertilization. For a description of their flower structure, see Chapter I, page 44, and Figures 43-45. Some of the Compositæ have no ray flowers, others are all ray flowers, but the great bulk of them bear both tubular and ray flowers in a single head. This may be single, or more commonly it is arranged in various kinds of clusters. Each head is surrounded at its base by one or more series of usually tightly overlapping bracts, incorrectly called a calyx by the unobservant. The Compositæ include over 900 genera and 11,000 species from all parts of the world. Most of them in America are herbs. Daisy, dandelion, dahlia, chrysanthemum, sunflower, boneset, chicory, lettuce, and scores of other examples could be cited, all herbs. In the tropics the Compositæ are more often trees and shrubs. The family contains many economic plants such as arnica, chamomile, artichokes, inula, and many others.
This all too brief account of the grouping of plants in families, and the sequence of these from the comparatively simple naked-seeded pines, through monocotyledons, the apetalous dicotyledons, followed by the polypetalous dicotyledons, and culminating in the Compositæ among the gamopetalous families, gives us merely a hint of what are the characters upon which plants are divided. While the details are necessarily omitted, the gradual development from naked-seeded plants, wholly at the mercy of the elements, up to those which are marvelously provided with contrivances to insure cross-fertilization, has been traced. Perhaps no other phase of botanical study offers such a rich opportunity as this, for upon the solution of some of the problems of plant classification depends the answer to many questions about the history of the earth and man’s ability to live on it. Certain of these plant families have lived on the earth hundreds of thousands of years before man first came. Others have apparently arisen comparatively recently. Many botanists believe that all the monocotyledons should be placed after the dicotyledons, as the latter may be a more ancient type than the former. How these different plant structures, some very ancient and others more recent, help to show us some of the history of the earth, will be treated, among many other evidences of a plant nature, in the chapter on “The History of the Plant Kingdom.”
CHAPTER V
USES OF PLANTS TO MAN
FOR perhaps the largest number of readers the chief value of plants is what they furnish in the way of food, clothing, fuel, and so forth, and from this standpoint alone the study of them is more than worth while. It is unnecessary here to enumerate all the thousand and one things that we get from plants, and no attempt will be made to do so in the following pages. But certain plants like wheat, corn, cotton, jute, rubber producers, and tobacco have so shaped the life of the people, so absolutely dictated the development of whole regions of the earth’s surface that their stories are part of the history of mankind. What our cotton fields of the South, the wheat and corn fields of the Middle West, the jute in India, and the coconut palm and sugar cane in the tropics have done to dictate the economic destiny of those regions is common knowledge. Hundreds of less important plants throughout the world contribute their quota to the huge debt that man owes to the plant world. Probably no other feature of plant life offers such attractions as the study of man’s uses of plants, which is known as Economic Botany, and for which our Government maintains a large staff of experts. Some of the publications of this bureau are textbooks of the greatest value to those who grow or import plants or their products. What that amounts to in the aggregate no one can readily estimate. It certainly exceeds all other commerce combined.