Families of Monocotyledons

CHAPTER VIII FAMILIES OF MONOCOTYLEDONS In number of families, genera, and species the monocotyledons are not nearly so numerous as the dicotyledons, but certain families, e.g., the Gramineae, are of outstanding importance. Origin. The origin of the monocotyledons has been the subject of much controversy. For some years they were thought to be more primitive than the dicotyledons and probably ancestral to them. This belief, however, is out of harmony with both morphological and paleontological evidence. It is now generally conceded that the dicotyledons are the older and gave rise to the monocotyledons. The prevailing belief today is that the mono- cotyledons were an offshoot of the primitive dicotyledons back in the early part of the Mesozoic era, and that they are mono- phyletic, i.e., of one origin, the first monocotyledons being A ancestral to all others. third and newer conception is that the monocotyledons are not a simple monophyletic group but are polyphyletic, different members having sprung at different times from distinctly separated representatives of the dicotyledons. The problem has been simplified by Coulter 1 and his associates, who have shown that cotyledons originate, not from a terminal cell but from a " peripheral cotyledonary zone" that may develop one, two, or several cotyledons, or, in a few species, none at all. In both subclasses of angiosperms two generally start, but if growth continues in only one, the embryo is monocotyledonous. The fact that the two subclasses can, in most cases, be dis- tinguished not only by their cotyledons but also by the venation of the leaves, the arrangements of bundles in the stem, floral structure, and size of the endosperm is evidence of a monophyletic group. With regard to these other characters, attention has been called to the fact that not all monocotyledons and dicotyledons are typical of their groups in all respects. The atypical struc- 1 COULTER, J. M., The Origin of Monocotyledony, Ann. Mo. Bot. Gard., 2: 175-183, 1915. 194 FAMILIES OF MONOCOTYLEDONS 195 lures are not, however, characteristic of large groups of mono- cotyledons or dicotyledons but are quite scattered. Examples are the netted veined leaves of Smilax, Trillium, and Arisaema and the somewhat scattered vascular bundles in the stem of Thalictrum, Peperomia, Podophyllum, and Nymphaea. Description. The monocotyledons are mostly herbaceous plants, but some (e.g., palms) are large trees. With rare excep- tions the leaves have unbranched parallel veins running either longitudinally or from the midrib outward. The embryo has one cotyledon that remains below ground with the endosperm during A germination. second may be present in more or less rudi- mentary condition. The stem is not divided into bark, wood, and pith, but the vascular bundles, collateral in type, are scattered throughout a soft pith-like parenchyma. The floral parts are usually in threes or multiples of three, i.e., 3-merous. The calyx in many species is colored and corolla-like. %/ 1, ALISMACEAE. Waterplantain Family These are mostly aquatic plants with simple morphology that places them among the lowest of the monocotyledons and close to FIG. 78. Alisma subcordatum (Alismaceae). (From Britton and Brown.) 196 A TEXTBOOK OF SYSTEMATIC BOTANY the ancestral Ranales. There are only about 14 genera and 60 species, widely distributed in warm and temperate waters. Familiar Examples. The best known are the waterplantains (Alisma spp.) and the common arrowheads (Sagittaria spp.). Stems and Roots. The plants are perennial, herbaceous and succulent, growing in marshy places or shallow ponds. Leaves. The leaves are simple, mostly entire and often sheathing the stem. Inflorescences and Flowers. The flowers are solitary or in whorls. They are trimerous, with three or six stamens and hypogynous insertion. The pistils are unicarpellate and few to many in each flower. The petals are usually white and showy. Fruits and Seeds. In most species the fruits are achenes. Economic Significance. The plants have some value as ornamentals, being grown in ponds and aquaria. Some species of Sagittaria have edible tubers of limited value. v 2. LILIACEAE. Lily Family CA3 Co3S6 <3>P The lilies and their relatives constitute a large family of splendid ornamental plants. There are about 200 genera and 2,500 species distributed all over the world and very plentiful, both wild and cultivated, throughout the North Temperate Zone. Some authorities would divide this family into three: Melanthaceae, Convallariaceae, and Liliaceae. Familiar Examples. Examples are the true lily (Lilium spp.), but not the callalily (Zantedeschia aethiopica Spreng.), adders- tongue or dogtooth violet (Erythronium spp.), hyacinth (Hyacinthus orientalisL.), tulip (Tulipa spp.), lilyofthevalley (Convallaria wajalis L.), onion (Allium spp.), asparagus (Asparagus officinalis L.), and Spanishbayonet (Yucca spp.). It must be remembered that some dicotyledons with large, showy flowers are called lilies but do not belong in this family. Stems and Roots. The Liliaceae are nearly all perennial or biennial herbs, but a few shrubby species of Dracaena and Yucca are found in the subtropics. In most species the conspicuous underground portion is a bulb or a corm, rarely a rootstock. Many fibrous roots grow from these fleshy portions. In some cases the bulbs or corms serve merely to store food for seed pro- FAMILIES OF MONOCOTYLEDONS 197 duction the following year. In others they form in considerable numbers and serve for vegetative propagation. Leaves. The leaves of Liliaceae are all simple and parallelveined, but otherwise they are somewhat variable. In shape they grade from linear to broadly ovate. In some species they are all basal. . In others they are alternate, opposite, or whorled. Inflorescences and Flowers. Various forms of inflorescence are found racemes, panicles, and umbels and not a few species have solitary flowers. The flowers likewise are variable, but as a rule they are trimerous and bisexual and have petal-like sepals, equal petals, six stamens, and a tricarpellate pistil with a superior ovary, three seed chambers, and one style with a three- lobed stigma. In many species they are large and beautiful. Fruits and Seeds. The fruit is either a berry or a three-chambered capsule with loculicidal dehiscence, i.e., directly into the seed cavities (except in Colo- chortus). The number of seeds varies from few to many. They contain a small embryo within a large endosperm. Economic Significance. From the esthetic standpoint Fio. 79. Lilium philadelphicum (Liliaceae). A, longitudinal section of flower. J5, floral diagram. (After Bergen and Davis.) few families of plants compare with the Liliaceae. The great abundance, variety, and size of the flowers give them much promi- nence in the greenhouse, flower garden, dooryard, and field. The fact that many of them can so easily be grown from bulbs or corms brings them into the humblest homes and the stateliest mansions. Their use for human food is limited to asparagus and onions. Their forage value is low. They are not troublesome as 198 A TEXTBOOK OF SYSTEMATIC BOTANY Fio. SQ.Zygadenua venenosus (Melanthaceae) . Death camas. At least four species of Zygadenus found from Montana westward and southward are inown to be poisonous from the presence of the alkaloid zygademne. (One-half natural size.) FAMILIES OF MONOCOTYLEDONS 199 weeds, but the deathcamas (Zigadenus venenosus S. Wats.) should be mentioned as a poisonous weed. 3. JUNCACEAE. Rush Family CA3 Co3 S3-6P The rush family consists of 8 genera and about 300 species of herbaceous, aquatic, often grass-like or sedge-like plants. In reality they are more nearly related to the lilies than to the grasses and sedges, and they are included in the order Liliales. Familiar Examples. Familiar examples are common or bog rush (Juncus effusus L.) and woodrush (Luzula parviflora (Ehr.) Desv.) but not bulrushes, which belong to the Cyperaceae. Stems and Roots. The Juncaceae are all herbaceous and mostly perennial, growing in dense bunches often 1 to 3 feet high. The stems may be cylindrical or angled and contain a soft pith. The roots are fibrous. Leaves. The leaves are linear but variable as to thickness. In the larger genus, Juncus, the sheaths are split. In the other, Juncoides, they are closed. Inflorescences and Flowers. Unlike Fio. 81. Juncus lamprocarpus (Juncaceae). a, inflorescence. 6, single flower, c, pistil. the grasses and sedges, the inflorescences (After Strasburger.) are not made up of spikelets but of single flowers arranged in different ways racemes, panicles, corymbs, heads, or false umbels. The flowers in most species resemble in structure a reduced lily. There are three sepals and three petals of similar, broad, greenish-brown scales. The stamens are typically six in number but in some species have been reduced to three, four, or five. The pistil has three distinct carpels and usually three cavities. The general effect of the inflorescence is that of a sedge, olive-green or brownish in color; but close inspection shows a flower structure more like the lilies. Fruits and Seeds. The fruit is a tiny capsule containing three seed chambers, or by the omission of partitions only one. Sev- 200 A TEXTBOOK OF SYSTEMATIC BOTANY eral seeds are produced in each chamber. The embryo is small and straight in a fleshy endosperm. Economic Significance. Economically the Juncaceae are of little importance. Most species are wholly ignored by livestock, but a few, notably the woodrush, which grows in nonswampy land, are relished. 4. ARACEAE. Arum Family X The family Araceae contains about 100 genera antf 1,500 species, most of which are subtropical or tropical. They tend to be aquatic but some are epiphytic. Familiar Examples. The common calla or callalily of green- houses (Zantedeschia aethiopica Spreng ), Jackinthepulpit or Indian turnip (Arisaema triphyllum (L.) Schott.), sweetflag (Acorus calamus L.), and skunkcabbage (Symplocarpus foetidus (L.) Nutt.) are familiar examples. Leaves. The leaves are mostly large, simple or compound, usually parallel -veined, but netted in some species, as Arisaema triphyllum. In the western skunkcabbage (Lysichiton kamtschatcensis (L.) Schott.), they are of enormous size more than 3 feet long and half as wide. In Monstera dehciosa Liebm. of the tropics, sometimes grown in greenhouses, the huge leaf blades are perforated with many natural openings, 1 to 3 inches long by J/<2 to 1 inch wide. Stems and Roots. The Araceae are herbaceous, or the larger forms slightly woody. Some are vine-like and climb by aerial roots. Epiphytic forms are occasionally found in the tropics. Underground stems are usually developed in the form of rootstocks, tubers, or corms, which often have a very pungent taste. Inflorescences and Flowers. The striking feature of the Araceae is the inflorescence. This consists of a spadix subtended by a spathe. This entire structure is sometimes of astonishing appearance, being nearly 3 feet long in devilstongue (Amorphophallus rivieri Dur.) and colored deep red. In callalily and most other members of the family the inflorescence commonly passes for a flower. The true flowers are unisexual or rarely bisexual. Generally the staminate flowers are at the top of the spadix and FAMILIES OF MONOCOTYLEDONS 201 he pistillate below, but some species are dioecious. The peri- nth is reduced to tiny scales or ridges or is entirely lacking, ^here are commonly ten stamens or fewer. The single pistil has ne to several carpels. Fruits and Seeds. The fruit is a berry or utricle. In some pecies the seeds contain endosperm; in others there is none. FIG. 82. Arisaema triphyllum and Arum (Araceae). A, entire plant. B, padix with portion of spathe cut away C, pistillate flower. D, staminate lower. E. inflorescence. S, staminate flowers. P, pistillate flowers. (After Curtis.) Economic Significance. General interest in this family is centered in the beauty or oddity of the inflorescence. The corms >r tubers are edible in a few species, but they are generally too icrid to be palatable. A few of the members are reputed to be 3oisonous. The calamus of the drug trade is obtained from the ootstocks of Acorus Calamus. 202 A TEXTBOOK OF SYSTEMATIC BOTANY J 5. PALMACEAE. Palm Family CA3 Co3S3~6P This is a family of tropical and semitropical trees and shrubs of some 140 genera and 1,200 species. They are naturally of wide distribution, and because of their beauty they have been extended as far into the southern United States as the climate will permit. The smaller species are grown extensively in greenhouses. Familiar Examples. The ornamental species are found in several genera, and also we have large interest in the datepalm (Phoenix dactylifcra L.), the coconutpalm (Cocos nucifcra L.), raffia (Raphia ruffid), and palmetto (Sabal palmetto Lood.). Stems and Roots. The palms are mostly trees, some of them of great height, but there are a considerable number of shrubs and a few woody vines. Leaves. The leaves are persistent, very large in size except on the vines, and either compound or, if simple, deeply cleft and plicate. On the larger trees they form a dense tuft at the top of the unbranched trunk, the basal ones dying as new ones are formed in a huge bud at the top. Inflorescences and Flowers. The prevailing inflorescence is a spadix. Some of these are simple, others are in great panicu- late clusters with a large spathe covering the cluster during development. The flowers are small and greenish, trimerous, the three carpels usually united and each containing one ovule in most species. Fruits and Seeds. The fruit is usually a drupe that develops from one of the three carpels, the other two degenerating. The endosperm is large, especially in the coconut, where it forms the bulk of the edible portion. Economic Significance. Most species in use are grown for ornamental purposes, often giving a tropical appearance to the landscape. The value of coconuts and dates is well known, 6. CYPERACEAE. Sedge Family The sedges, often called sloughgrasses, resemble the true grasses so much that they are commonly mistaken for them. There are about 75 genera and 3,200 species, widely distributed over all parts of the world, but especially abundant in wet soils, FAMILIES OF MONOCOTYLEDONS 203 Only about 10 per cent of the species are found in the United States. The sedges are distinguished from the grasses by the* following characters, to some of which there are occasional exceptions: (1) angular stems without joints, (2) three-ranked leaves, (3) closed leaf sheaths, (4) only one glume at the base of each flower, (5) perianth composed of bristles or lacking, and (6) seed free from wall of fruit (achene). Familiar Examples. Sloughgrass (Carex spp.), bulrushes (Scirpus spp.), and umbrella plant (Cyperus altcrnifolius L.) are the commonest examples. Stems and Roots. The Cyperaceae are practically all herbaceous, annuals or perennials, rarely biennials. The stems are usually solid and without nodes between inflorescence and crown. In the majority of species they are triangular, but in some they are quadrangular, flattened, or cylindrical. The roots are fibrous and fascicled, many of them on perennial species being adventitious from underground rootstocks. Leaves. The leaves are mostly linear and grass-like, but the leaf sheath in nearly all species is closed. They have a threeranked arrangement on the stem. The li gules are absent or very much reduced. Inflorescences and Flowers. As in the Gramineae, the spikelet is the unit of structure in the inflorescence. The flower structure differs chiefly in the further reduction of the perianth to a few bristles, and even these may be absent. The flowers have a single glume at the base instead of two as in the Gramineae. They may be bisexual or unisexual. There are typically three stamens but sometimes more or fewer. The carpels are united in the pistil, which has two or three styles or style branches. Fruits and Seeds. The fruit is an achene, usually three- angled, containing one seed. This is free from the ovary wall and has a very small embryo and a large soft endosperm. Economic Significance. Compared with the Gramineae the Cyperaceae are of limited importance. The grains as a rule are small, few in number, and insignificant. However, many species grow extensively in swampy regions and make good pasturage. Others are too succulent in the spring and too hard and wiry in the fall. This harsh texture is due in part to serrations on the leaves and in part to deposits of silicates in the leaves and stems. The papyrus, early used for papermaking, belongs to this 204 A TEXTBOOK OF SYSTEMATIC BOTANY SC FIG. 83. Carex rostrata (Cyperaceae) . A, apex of flowering stem with one pistillate and two staminate spikes (Xl). B, pistillate scale (X8) C, perigynium (X8). D, achene, continuous with persistent, twisted style and three stigmas (X8) E, floral diagram. FAMILIES OF MONOCOTYLEDONS 205 family, but it has been largely superseded by straw, linen, and wood pulp. None of the Cyperaceae is poisonous, and they are unimportant as weeds except where swampy land is cultivated. J 7. GRAMINEAE (POACEAE). Grass Family The Gramineae constitute the second largest family of monocotyledons, containing about 400 genera and 4,500 species. Of the species found in the United States more than three-fourths are native. The grasses are the most universally distributed, and the most numerous in individuals, of any family of spermatophytes. The great adaptability of the different species has enabled them to thrive under the most varied conditions: in standing water and in deserts, on sun-baked hillsides and in all but the most densely shaded forests, in sand and in gumbo clay, far above the timber line on lofty mountains, and well into the arctic regions. Fossil remains of grasses have been found in rocks of the Lower Cretaceous period, but in all probability the great diversity of form now found within the family is a relatively recent development. Their simplicity is due to degeneration or specialization rather than to primitiveness. It is probable that the order Graminales should be looked upon as a specialized side branch of the main genetic line of monocotyledons, and as one that has given rise to no other orders. Familiar Examples. The well-known members of the Gramineae are too numerous to mention. Representative examples are wheat (Triticum aestivum L.), corn (Zea Mays L.), oats (Avena saliva L.), rice (Oryza saliva L.), sorghum (Holchus Sorghum L.), sugarcane (Saccharum officinarum L.), bamboo (Bambusa spp.), timothy (Phleum pralense L.), and Kentucky bluegrass (Poa pralensis L.). Stems and Roots. The grasses are nearly all herbaceous. The prevailing type of stem is a cylindrical culm with conspicuous nodes and hollow internodes, although solid stems are not unusual among the larger members such as corn, sorghum, and sugarcane. Annual, biennial, and perennial species are all numerous. The largest woody species are the bamboos, which may grow to more than 100 feet in height and several inches in 206 A TEXTBOOK OF SYSTEMATIC BOTANY diameter. Among the perennial grasses there is a marked tendency to the formation of rhizomes, underground rootstocks that in some species, such as quackgrass, extend for a considerable distance and serve the function of vegetative propagation. The roots are usually fibrous and fascicled. Small tubers or corms are found in a few species. Leaves. Almost without exception the blade (lamina) of the leaf is narrow and ribbon-shaped. The arrangement on the stem is alternate and two-ranked. The leaf is attached to the node by a long sheath, which clasps the stem. This sheath in most species is split throughout its length on the side opposite the attachment of the blade. The union of blade and sheath is usually not a smooth [floret one. Often it appears as if the blade attached slightly below the top of the sheath, and this apparent projection of the sheath above the attachment is called a ligule. It is thinner and ume more membranous than the sheath .first glume. .pedicel proper and is sometimes fringed. The base of the blade at its two margins often projects to form two small ear- FIG. 84. Spikelet of Bro- mus marginatua showing the glumes and several florets typical of the Gramineae. (One-half natural size ) like portions called auricles. Inflorescences and Flowers. The inflorescences and flowers of the grasses are highly specialized, and until their structure has been mastered, they seem difficult to understand. The inflorescences are com- posed of several to many spikelets, which are combined in various ways on a main axis called the rachis. Some are in compound spikes, as wheat, others are in racemes, as certain species of Festuca and Paspalum, while still others are in panicles, as oats. It should be understood, however, that the panicles of Avena, Pamcum, etc., are not quite comparable with the panicles of Yucca, Heuchera, and Thalictrum, for example, since in the former each slender stalk bears a spikelet of flowers while in the latter it bears a single flower. FAMILIES OF MONOCOTYLEDONS 207 Each spikelet bears one to several florets attached to a central stalk or rachilla. The usual structure of a spikelet is as follows: At the base is a pair of glumes, the lower, outer one called the first, and the upper, inner one called the second. Above the glumes, and partly enclosed by them, is a series of florets. Each SECOND GLUME FIG. 85.- Representative spikelet of grass. The rachilla here is elongated to show the parts more clearly. floret has at its base a lemma and a palea. The lemma is the lower, outer scale of the floret, which in many species bears a long slender awn or beard as an extension of the midrib at the tip or back. In its axil the floral parts are borne. The palea, often with two longitudinal ridges (keels or nerves), stands between them and the rachilla. The floral parts consist of a perianth 208 A TEXTBOOK OF SYSTEMATIC BOTANY (sometimes absent) that is reduced to two or three minute scalelike lodicules, three stamens (one, two, or six in a few species), and a central pistil of three united carpels, only one of which is functional. The superior ovary bears two (rarely three) feathery stigmas with short styles or none. Many species have other florets on the rachilla, the upper or lower of some being sterile. ovary lodiculc Floret FIG. 86. Representative floret of grass, much enlarged. Flower study in the grasses is complicated by the fact that there has been much suppression and modification of floral parts, and this has taken place in varying degrees in different species. The perianth condition of the grass ancestor is unknown, but it is not unlikely that it consisted of two whorls of three parts each. Three lodicules can still be found in some species. There probably were six stamens, this number still being found in bamboo and in rice. The pistil doubtless had three carpels; in fact, studies in the early development of the flowers have shown that this number still persists, although they FAMILIES OF MONOCOTYLEDONS 209 are so intimately fused that they appear as one. Three styles, suggestive of as many carpels, are not unusual. In most species the flowers are bisexual, but in corn, for example, the stamens and pistils are in separate flowers. Even here bisexual flowers in the staminate tassels are fairly common. This specialized floral structure furnishes ground for the belief that the grasses are not primitive plants but derivatives of lily-like ancestors. Fruits and Seeds. The one-ovulate pistil develops into a hard, dry, seed-like fruit, similar to an achene, but strictly speaking a caryopsis, since the seed adheres closely to the surrounding ovary wall. In some species, such as oats and hulled barley, the palea and lemma continue to enclose the grain after threshing. The FIG. 87. pi, pistil. pa, palea. Floral diagram of grass. 8, stamen, lo, lodicule. le, lemma, r, rachilla. Fiu. 88. Poition of leaf of Elymus at the level where the culm emerges from the sheath. (Three times natu- ral size.) seed is composed largely of starchy endosperm with a relatively small straight embryo near the point of attachment. Economic Significance. Undoubtedly the Gramineae excel all other families in their importance to mankind. Without Pinaceae we might live in earthen houses; without cotton we might clothe ourselves in other fabrics; but without Gramineae most of us would starve. Notwithstanding the prevalence of hundreds of other families of plants the earth would support far less animal life were it not for the grasses and grains. The explanation is not difficult. In