Families of Gymnosperms

CHAPTER VI FAMILIES OF GYMNOSPERMS The gymnosperms consist mostly of shrubs and trees, some of gigantic size. They were once the predominant vegetation of the earth but have long been gradually declining in numbers (see frontispiece). They are of ancient lineage, perhaps from as far back as Cambrian times, some 400 million years ago, but reached the climax of their abundance in the Pennsylvanian (Upper Car- boniferous) period about 250 million years ago. Since that time they have gradually become fewer, many species now being entirely extinct. While other orders waned, the Coniferales greatly increased in number and size of individuals, and this has become the predominant order of the class. General Appearance. In the general appearance of the vegetative portion some gymnosperms resemble large, coarse ferns, some are suggestive of certain kinds of palms, while most of those in the North Temperate Zone are forest trees the pines, spruces, firs, etc., with which most of us are familiar. The great majority are evergreen, Larix spp. being an exception. Gymnosperms are the most ancient of the seed plants. There is no doubt that they evolved from pteridophytes, probably heterosporous ferns, and the seven orders (three of them extinct) fall naturally into two groups or subclasses as indicated by Chamberlain. 1 These two lines may have originated separately or as one that soon branched. Fossil evidence leaves the ques- tion unsettled. Classification. The following classification is used by Chamberlain in the work already cited: Cycadophytes Order Cycadofilicales (extinct) Order Bennettitales (extinct) Order Cycadales CHAMBERLAIN, Press, 1935. CHARLES J., " Gymnosperms, " 77 Stanford University 78 A TEXTBOOK OF SYSTEMATIC BOTANY PLEIOCENC MIOCENE OLI0OCENE EOCENE UPPFRCRETACfOWS LOWfRCRHACEOUS UPPER JURASSIC MIOWE JURASSIC RHAETIC LOWER TfUASStt (Z EC H STEIN ROTHLIEGENDE3 (STEPHANIAN) (URALIAN) OR I; MIS5ISSIPP1AN (KULM; UPPER DEVDHMN MIDDLE DEVONIAN LOWERDEVONIAN A FIG. 21. conjectural position of the various gymnosperm groups in geo- logical time. The horizons were compiled by Dr. A. C. No6 from various sources. The comparative amount of space does not equal the comparative amount of time. (From C. J. Chamberlain.) FAMILIES OF GYMNOSPERMS 79 Coniferophytes Order Cordaitales (extinct) Order Ginkgpales (nearly extinct) Order Coniferales Order Gnetales (nearly extinct) A comparison of the size and general appearance of the Cycadophytes and the Coniferophytes is shown in Fig. 22. In general, the former is characterized by an unbranched trunk with large compound leaves and the latter by a branching trunk with small, narrow, simple leaves. ORDER I. CYCADOFILICALES This extinct order is probably the oldest of the gymnosperms. Fossil remains are found in Devonian strata, and the origin of the group may have been still earlier. The climax of abundance and specialization occurred in the Carboniferous period, after which the decline was rapid. Many of the fern-like fossils commonly found in coal are Cycadofilicales, as shown by their production of seed, which caused the English botanists to call them "seed ferns" (Pteridospermae). Vegetative Structures. The most primitive of the group lacked the broad fronds characteristic of the ferns of today, having only small, narrow, branch- ing leaf stalks, in which respect they were much like the primitive true ferns of the same period. This suggests either that this ancient order of gymnosperms came from primitive ferns or that the two had FIG. 22. The habit and comparative size of members of the Cycadophytes and Coniferophytes. (From C. J. Chamberlain.) a common ancestry and followed a parallel course in leaf development until, in the Carboniferous period, broad fronds were the rule in both. The fibrovascular system varied greatly with the species. lu some it was like that of our common ferns; in others the stem had 80 A TEXTBOOK OF SYSTEMATIC BOTANY a cortex surrounding a stele of phloem and xylem with a cambium layer between and a pith in the center. Reproduction. In reproduction two kinds of spores were formed on the fronds or in place of them. Some of these were produced in sporangia much like those of ferns and functioned as pollen grains that seem, in some cases at least, to have developed motile antherozoids. Others were formed within scaly coverings and produced ovules that, when fertilized, became seeds. These seeds became dormant shortly following fertilization, before an embryo was produced. ORDER II. BENNETTITALES This order probably branched from the Cycadofilicales in the Carboniferous period, although no fossils have been found earlier than the Permian. The maximum development occurred in the Jurassic. Their end seems to have come in the Cretaceous period, and all forms are now extinct. Probably this order ran the shortest course, geologically, of any of the gymnosperms. They were abundantly distributed throughout the United States and Mexico, and some specimens have been found in Europe. Vegetative Characters. The commonest form of the Bennet- titales was a stocky, unbranched trunk 2 or 3 feet high, sur- mounted by a crown of large, pinnately compound leaves. However, some of the trunks were scarcely taller than broad, and in one genus the stem was rather slender and branched. Reproduction. The most characteristic structures of the Ben- nettitales were the strobili. They were very numerous in the A axils of the leaves and were bisexual. typical strobilus had the superficial appearance of a flower. Above a rosette of leaves there formed numerous compound sporophylls bearing sporangia filled with pollen grains. These surrounded a central cone bear- ing numerous short-stalked ovules, each having a characteristic micropyle. The presence of pollen tubes or antherozoids has not been established. Within the seed a dicotyledonous embryo was formed. The flower-like structure of the bisexual strobilus and the dicotyledonous seed have led some taxonomists to believe that the Bennettitales were the direct ancestors of angiosperms of the Magnolia type, but this idea is strenuously opposed by Chamber- lain. 1 1 Loc. tit., p. 59. 4 FAMILIES OF GYMNOSPERMS 81 ORDER III. CYCADALES From the fossil remains we find evidence that the Cycadales first appeared in the late Carboniferous period, became more abundant during the early Mesozoic era, and then declined, leaving at the present time only one family with nine genera and less than a hundred species. They were formerly widely distributed and abundant. The living forms are mostly tropical and subtropical, being especially abundant in Mexico and the West Indies, South Africa, and Australia. In a few places they form a conspicuous part of the dense vegetation, but generally they are scattered. It is believed that their ancestors were Cycadofilicales. FIG. 23. A B Sperrnatozoids of Zamia floridana. A, side view, showing spiral band of cilia. B, apical view. (Redrawn after Webber.) Vegetative Structures. The vegetative structure of the living Cycadales is fern-like, with large, pinnately compound fronds and usually short, stout stems, although the stems of some fossil forms were very slender. They are seldom more than a few feet high. Reproduction. Among the fossil Cycadales the spores of some species were borne on the fronds, and in others they were in strobili. In present-day Cycadales all species are dioecious. The female strobili of some species are of huge size, 3 feet or longer. The pollen grains germinate and form tubes that require several months to penetrate the nucellus, from which they absorb food and thus function as haustoria. In the basal portion of the pollen tube are produced motile antherozoids that are of unusual size and complexity. Some are actually visible to the unaided eye and bear hundreds of cilia. Following fertilization an embryo is formed that becomes dormant as the seed ripens an advance over the Cycadofilicales. 82 A TEXTBOOK OF SYSTEMATIC BOTANY ORDER IV. CORDAITALES Of the four orders of the coniferophytes, the Cordaitales is the only one that is extinct, although two others are nearly so. Like the Cycadofilicales, the Cordaitales appeared in the Devonian period or earlier, but whether the latter evolved from primitive members of the former, making the gymnosperms monophyletic, or whether the two evolved separately, making the class diphyl- etic, is still uncertain. The Cordaitales reached their maximum profusion in the Carboniferous period, when they were world-wide in distribution, but seem to have disappeared in the Permian, making them the first of the gymnosperms to become extinct. Vegetative Characters. In their prime the Cordaitales formed great forests, some of the trees being 100 feet high with trunks up to 3 feet in diameter. The leaves were simple and in some species very large, as much as 3 feet in length, but more commonly only a few inches. Usually the trunks were bare for a consider- able distance, the upper half being much branched. Reproduction. The micro- and macrosporangia were borne in separate strobili; some species were monoecious while others were dioecious. Both kinds of strobili were borne on stalks, the male sporangia being more numerous than the ovules. Antherozoids have not been found, but it is believed that they existed. Study of the fossilized seeds has been rather meager, and embryos have not been found in them. ORDER V. GINKGOALES This order was never so abundant as those just described, either in number of species or wealth of individuals. It appears to have originated from the Cordaitales in the Carboniferous period and to have reached its greatest abundance in the Permian. At the present time only one living species remains, Ginkgo biloba, the maidenhair tree of China and Japan. It is not even certain that this tree is still found growing wild, although a few specimens have been reported in western China by travelers. Some authorities believe that this tree would have become extinct but for its culture as an ornamental tree. It is an interesting fact that this species has undergone no detectable change for more than 150 million years, nor is it known to have given origin to any other species, although some of the FAMILIES, OF GYMNOSPERMS 83 fossils may represent other species of the same genus. Ginkgo biloba has been very thoroughly studied and will be used as the basis for further description of this order. Vegetative Structures. Ginkgo biloba is a large, beautiful tree, grown extensively in the Orient and introduced into many parts of the United States and Europe where the climate is not too severe. The leaves are deciduous and very characteristic in appearance. They are broad, rather thick, glossy, and fanshaped, with conspicuous, dichotomously branched veins radiat- ing from the end of the petiole to the margin. Many of the leaf petioles are branched into two lobes. The wood shows definite annual rings and a small pith. Reproduction. The trees are dioecious. The male and female strobili, on separate trees, are borne on short stalks. The male strobilus is compact and consists of a large number of sporophylls, each bearing, as a rule, two microsporangia. The female strobilus is a loosely branching structure bearing a few ovules, which are generally in pairs. The pollen grains form germ tubes, which, like those of the cycads, function as haustoria in the nucellus, and two large antherozoids are produced in the base of each. In each of the ovules there forms an oval, many-celled game- tophyte containing two archegonia. After fertilization, a dicotyledonous embryo develops, and as the seed ripens, its outer portion becomes soft and fleshy like a fruit. It has a disagreeable odor, and for this reason staminate trees are sometimes preferred to pistillate for ornamental planting. ORDER VI. CON1FERALES The best known of the gymnosperms arc the Coniferales, which form almost the exclusive forest type in many cold, semiarid regions and are widely distributed, especially in the North Temperate Zone and extending to the arctics. The Coniferales 1 appear to have come from the Cordaitales in the Permian period and to have reached their climax in number of genera and species in the Cretaceous, but they are still very abundant perhaps more in number of individuals now than in prehistoric times. 1 The common name "conifer" should not be used as a svnonym of "gymnosperm" but restricted to the order Coniferales. 84 A TEXTBOOK OF SYSTEMATIC BOTANY A Vegetative Characters. few of the Coniferales are low shrubs, but most of them are trees, Some of gigantic size, and usually they are evergreen. The stem structure is essentially like that of the dicotyledons, but the wood lacks the tracheae of angiosperms. Reproduction. There is considerable variation in the strobili of the Coniferales. Some are dioecious but more are monoecious, and a few have bisexual strobili. The pollen grains produce pollen tubes but no antherozoids. The ovules produce arche- gonia in which embryos are developed following fertilization. Since this order of gymnosperms is now more important than all the others combined, it will be described in greater detail. 1. PINACEAE. Pine Family A very important family of cone-bearing, mostly evergreen trees, containing about 8 genera and 150 species. They are of world-wide distribution, extending even to lofty mountain sides and far into northern regions. The Pinaceae were more abun- dant during the later Cenozoic era (see page 78) than now, but they have held their own better than most of the gymnosperms, probably because of their greater adaptability to the cold, dry climates that in most parts of the temperate zones replaced the subtropical conditions which prevailed before the Glacial epoch. The Pinaceae and other conifers may be looked upon as the highest evolutionary development of the gymnosperms. Even so, they doubtless are not the direct ancestors of the angiosperms but rather a somewhat specialized side branch of the main genetic line. Familiar Examples. The best known examples of the pine family are pine (Pinus spp.), Douglasfir (Pseudotsuga spp.), hemlock (Tsuga spp.), fir (Abies spp.), spruce (Picea spp.), and larch or tamarack (Larix spp.). Stems and Roots. The Pinaceae are all woody and are, for the most part, stately trees with straight, slightly tapering trunks, sometimes more than 250 feet high and 10 feet in diameter. The root system is generally wide-spread but not deep. The wood of the Pinaceae, like that of most gymnosperms (except Gnetales), differs from that of angiosperms in lacking tracheal vessels in the secondary xylem, their place being taken by tracheids. All parts FAMILIES OF GYMNOSPERMS 85 of the tree contain intercellular resin ducts, which become non- functional in the old heartwood. Leaves. The Pinaceae all have rather thick, linear, alternate leaves. In the pines they are lj^ to 12 inches long, but in the other genera they are shorter. This reduced leaf surface com- bined with a sunken condition of the stomata results in low trans- piration, and the trees are thus able to survive in dry or frozen soils. With the exception of the larches or tamaracks the Pina- ceae are evergreens, i.e., they are clothed with foliage the year round. Each leaf persists on the tree from 3 to 10 years, depending on the species, but ultimately falls off. In addition to the regular foliage leaves the pines have scales, sometimes called primary leaves, on the buds and young shoots. These are relatively broad and thin, and deciduous. In the pines the leaf bases are enclosed in sheaths one, two, three, four, or five leaves (depending on the species) growing from each sheath. In the other genera the leaves are single. The evergreen species have leaves of a very dark green, giving a somber appearance to the forests that they compose. Strobili. All members of this family are heterosporous, pro- ducing microspores in male strobili and macrospores in female strobili, both borne on the same tree. In the male strobili pollen sacs are borne on the underside of each scale, and in the female strobili two ovules are borne on top of each scale. % Seeds. The seeds mature in to 3J^ years after fertiliza- tion. They are produced in large woody cones that open to release them. In most species each seed bears a broad wing that causes it to spin around in falling and so delays its descent that even a light wind will carry it to a considerable distance. The seed contains a relatively large endosperm and a straight embryo, usually with several cotyledons. Economic Significance. The Pinaceae are of great importance to mankind. But for this family of trees our familiar frame buildings could hardly exist. Pine, fir, spruce, and hemlock are not only more abundant than other kinds of lumber, but they are easy to work and do not warp badly when exposed to the weather. The turpentine that is an essential ingredient of ordinary paint and the rosin of varnish are extracted chiefly from pine stumps. The wood of this family is not of the highest grade for fuel, but because of its abundance in many regions where the hardwoods 80 A TEXTBOOK OF SYSTEMATIC BOTANY are lacking it is a great asset and is extensively used. Enormous quantities are utilized for making certain grades of paper. Lastly, the forests of pine and other conifers cover many dry, rugged mountain sides that would be desolate without them and thus conserve the moisture and beautify the landscape. Where mixed with the light-green, broad-leaved forest trees the darker colors of this family give a most pleasing effect. 2. TAXODIACEAE. Taxodium Family This family, once one of the most conspicuous on earth, is now reduced to eight genera, nearly extinct in the United States and FIG. 24. -Taxodium (Taxodiaceae) showing natural habitat in swamp. The "knees" are formed by angles of the roots projecting above the mud and water. (After Bergen and Caldwell.) widely scattered over the rest of the world. They are adapted to warm, moist climates, and throughout most of the Cenozoic era, especially the Miocene epoch, in the balmy climate that generally prevailed, their gigantic forms covered almost the entire Northern Hemisphere with a few extensions into the Southern. Familiar Examples. The Taxodiaceae are rare in the United States, being confined to two species of Sequoia, the big trees (S. gigantea (Lindl.) Decne.) and the redwood (S. sempervirens (Lamb.) Endl.) of California and Oregon, and two species of Taxodium, baldcypress 7 (2 . distichum (L.) L. C. Rich.) and pondcypress FAMILIES OF GYMNOSPERMS 87 (T. ascendens Brongn.) of the southeastern states (not to be confused with the cypresses of the family Cupressaceae). Stems and Roots. The few remaining species of this family are trees, the best known of which are the sequoias of California. Some of these are 350 to 400 feet high and 20 to 30 feet in diam- eter the largest forms of life the earth has ever known. The largest sequoias are estimated to be nearly 4,000 years old, having spent half their lifetime before the birth of Christ. A wondrous record of alternating prosperity and famine, as drought succeeded favorable climatic conditions, can be read in the varying widths of their annual rings. In Taxodium the growth of anchor roots, going on for centuries under so great a weight, has resulted in huge gnarled buttresses at the surface of the ground. Leaves. The leaves are linear or scale-like and alternate. Strobili. Both male and female strobili are borne on the same tree. There are several pollen sacs on the under side of each staminate scale and several ovules on the upper side of each ovuliferous scale. The female strobili develop into woody seed- bearing cones. Seeds. The seeds mature either the same season or the second season following fertilization. There are three to seven on each A scale. narrow wing extends as a margin all around each seed in Sequoia but is absent from the triangular seeds of Taxodium. Economic Significance. Were the trees of this family as abundant as those of Pinaceae, their value would be almost incalculable. Unfortunately, they are restricted to a small area. The redwood of California, 8. sempervirens, furnishes lumber of unusual quality. It is reddish in color (except the sapwood, which is nearly white), evenly grained, light in weight, but rather strong, though brittle. It is rich in tannic acid and resists decay better than the woods of the Pinaceae. The lumber of 8. gigantea is not of such good quality. There is, however, well-grounded sentiment against cutting these trees, for they are the last of the noblest race of trees the earth has produced. Furthermore, these great trees when cut shatter almost like glass and the waste is enormous sometimes 60 to 90 per cent by the older methods of lumbering. The baldcypress (Taxodium) is less restricted in its range than the sequoias, being found in the swampy regions of a dozen 88 A TEXTBOOK OF SYSTEMATIC BOTANY of the southeastern states. Its haBitat protects it somewhat from the woodman's axe, but it is slowly disappearing. The wood is of unusual quality, being heavier, harder, and stronger than that of the redwood and resisting decay even better. It is greatly sought for structural work in contact with the soil, where decay is most destructive, for greenhouse work, for staves, and indeed for a wide range of uses. In comparison with the sequoias of the Pacific coast, little effort is being made to conserve it. 3. CUPRESSACEAE. Cypress Family The family Cupressaceae is rather small, containing but ten genera, widely distributed over both hemispheres. Familiar Examples. The commonest examples of the Cupressaceae are the cedars (Thuja and Juniperus spp.), junipers (Juniperus spp.), arborvitae (Thuja spp.), and cypresses (Cupressus sp.), but not the baldcypress (Taxodium distichum (L.) L. C. Rich.), which belongs to the Taxodiaceae. Stems and Roots. Many of the Cupressaceae are handsome shrubs, and some are large forest trees, the giant cedar reaching a height of 200 feet and a diameter of 15 feet. The tissues are resinous and in most species aromatic. Leaves. The leaves are small and linear, or, more often, scalelike and closely covering the twigs. They are evergreen, persisting usually from 3 to 5 years. They have an opposite or whorled arrangement on the stem, in which respect they differ from those of the Pinaceae and Taxodiaceae. Strobili. Some members are monoecious and others dioecious. The scales are few in number, each bearing either several pollen sacs or one to many ovules. In most genera the ovuliferous strobili develop into small woody cones, but in Juniperus the scales fuse into round blue or reddish fleshy bodies commonly called berries. Seeds. In the dry, woody cones the seeds generally produce narrow marginal wings. In the fleshy " berries " there are one to six seeds, which are wingless and usually angled. Economic Significance. The largest member of the family, the giant cedar (Thuja plicata D. Don), is a splendid forest tree growing extensively on the Pacific coast from Alaska to Cali- fornia. The others are smaller trees or shrubs. In general the wood is light in weight, soft and easily worked, but very durable. FAMILIES OF GYMNOSPERMS 89 It is, therefore, extensively used for shingles, fence posts, poles, ties, bridge work, and boats. Special uses are for cedar chests, where the aromatic wood acts as a repellent to moths, and for lead pencils, which require a soft, smooth-grained wood. Cedarwood oil is extracted for technical purposes. Several of the smaller trees and shrubs are grown for ornamental purposes, but the red cedar (Juniperus virginiana L.), found extensively in the eastern United States, is an alternate host for the apple-rust fungus and is being replaced by other species that do not have this objectionable feature. ORDER VII. GNETALES The order Gnetales is very small. The living representatives belong to only three genera. Apparently originating as late as the Upper Cretaceous from unknown ancestors, the Gnetales have never been abundant or cosmopolitan. While they show some characters resembling angiosperms, their origin seems too late to permit their being ancestral to that class, and their naked ovules mark them definitely as gymnosperms. Vegetative Structures. The general appearance and habi- tat of the three genera are so different as not to suggest any relationship. Ephedra with twenty-five species is a low, profusely branching shrub with small, opposite leaves and xerophytic characters. It is found mostly in southwestern United States and Mexico, but there are scattered patches in South America, Europe, and Africa. Wehvitschia, having