DESCRIPTION OF THE PHYLUM ANGIOSPERMOPHYTA

EUKARYA> ARCHAEPLASTIDA> VIRIDIPLANTAE> STREPTOBIONTA> EMBRYOPHYTA> TRACHEOPHYTA> SPERMOPHYTA> ANGIOSPERMOPHYTA

Angiospermophyta (an-ghe-o-sper-MA-fa-ta) is made of three Greek roots that mean a box (angos -ανγοσ); seed (sperma -σπέρμα); and plant (phyto -φυτό). The reference is to the enclosed seeds of the flowering plants. The earliest reference to this phylum name is by Berry (1915), cited in Berry (1917), who claims that angio is an is a latinized form of aggeon (αγγειον =receptacle).

INTRODUCTION TO THE ANGIOSPERMOPHYTA

The flowering plants include the most successful members of the Viridiplantae. Their success can be measured by their occurrence and dominance in almost all of the terrestrial and freshwater environments. Also, the flowering plants contain most of the species in the kingdom (more than 250,0000 species have been defined in this group). Although these plants vary in size from the almost microscopic watermeal (Wolffia) to oaks and other enormous arborescent forms, all flowering plants are united by the synapomorphies of double fertilization, the flower , and its derivative, the fruit .

THE FLOWER

The basic flower is a compound strobilus in which the floral parts are arranged in whorls that emerge from the receptacle (see Figure 1 below). Likely, all of the floral parts developed from modifications of sporophylls and sterile leaves on the outside. The whole organ almost certainly coevolved to entice and facilitate the movement of pollen from one flower to the next by animals, particularly insects. Many modern and very successful groups (e.g. the grasses, Poaceae) have reverted to wind pollination, a mechanism that seems to have evolved many times in the flowering plants.

The outermost whorl of the flower is the perianth . When all of the perianth segments are similar, they are called tepals . When they are differentiated into an outer whorl and an inner whorl, they are called the calyx ( sepals ) and corolla ( petals ), respectively. Within that whorl are the stamens , collectively called the androecium. The pollen -bearing portion of the stamen is the anther (microsporangia) that may or may not be borne by a differentiated filament . The gynoecium (the pistil ) is the center-most floral part, which typically has a pollen-receptive region called a stigma borne by the style from an ovulary which bears ovules .

The types and structures of flowers may vary considerably. Many groups have flowers that have lost one of the sexes and become unisexual ( imperfect ). In this situation angiosperm taxa may be monoecious (both staminate and pistilate flowers on the same plant) or dioecious (the sexes are separate). Any flower that has both stamens and pistils is said to be perfect . So, an angiosperm taxon may be one of three sexual types: perfect, monoecious, or dioecious.

Flowers that have all of the four components (calyx, corolla, androecium, and gynoecium) are complete and those that lack one or more floral whorls are incomplete . Thus, a plant may be incomplete and perfect. This is the case for many of the grasses like wheat in which their flowers have lost the corolla but retain both sexual whorls.

Floral symmetry is variable but mainly between actinomorphic (Figure 2) and zygomorphic (Figure 3) orientations. Likely, the actinomorphic flower is the ancestral condition and the zygomorphic flower is more derived. However, the zygomorphic condition has evolved independently many times, sometimes within the same family.

FIGURE 1. Diagram of a stylized flower highlighting components of the ovulary (left) and stamens (right). See the text for descriptions of the structures that are labeled.

Image produced by Ladyofhats and exists in the Public Domain.


FIGURE 2. Epiphyllum, an epiphytic night-blooming cactus, is an actinomorphic flower that is open only one night. Image from the Systematic Biology Diversity Archive	FIGURE 3. Streptocarpus, an African plant sometimes called a Cape Primrose, has a zygomorphic flower that clearly is derived from an actinomorphic floral form. Image from the Systematic Biology Diversity Archive

The receptacle is elongate in the center of the more primitive angiosperm flowers (e.g. Magnolia), but short and wide in the more derived taxa. The receptacle may also encase part or most of the flower. Generally, the perianth surrounds the center two whorls (as in the banana where the petals become the banana peel). In this case, the flower is said to be epigynous . If the flower sits atop the receptacle (as in Lilium), the flower is said to be hypogynous (see Figure 4 below).

Many flowers occur singly, but most emerge in groups called inflorescences (see Figure 5 below). In some cases, the inflorescence type defines the family (e.g. umbels in the Apiaceae, formerly called the Umbelliferae). The two major types of inflorescences are determinate and indeterminate and can be distinguished by the development or age of the flowers on the peduncle, the main stem of the inflorescence. A cyme is a determinate inflorescence, but most inflorescence types are indeterminate (e.g. spike or raceme).

The bract is a specialized leaf that subtends a flower. That is, the flower grows from the axil of the bract. In some cases, a specialized bract can subtend a whole inflorescence. Consider the inflorescence of Jack-in-the-Pulpit (Figure 6). The spike has staminate flowers at the top and pistilate flowers at the bottom, and the bisexual spike is subtended by a bract called a spathe. Some “flowers” like those of poinsettia (Figure 7) are inflorescences of tiny green imperfect flowers surrounded by showy bracts. Thus, the petal-like structures are not petals at all.

FIGURE 4. PLACEMENT OF FLORAL OVULARIES. This is from Figure 10-16 of Jones and Luchsinger (1986)

FIGURE 5. FLORAL INFLORESCENCE TYPES. Cymes are determinate inflorescences meaning that the terminal bud that makes the flower stops growing and a new axis develops from the axil of a subtending leaf. The other inflorescence types are determinate in which the terminal bud continues to produce new flowers. The raceme is the simplest type. A panicle is a compound raceme. A spike is a raceme without pedicels. Specialized spikes include catkins, pendent unisexual spikes. A spadix is a spike of unisexual flowers (often with the staminate flowers at the tip and the ovulate flowers at the base), all subtended by a large bract or spathe. A corymb (simple or compound) is a raceme with pedicels of different lengths to form a flattened top. The axis of the umbel is very short and the pedicels, which are of equal lengths, appear to emerge from the tip of the supporting axis. A head has flowers without pedicels all attached to a common receptacle. Thyrse is a compound inflorescence of racemes with terminal cymes. Verticil has florets attached in whorls on a stem, each whorl associated with a bract.
Image from Figure 10-14 of Jones and Luchsinger (1986).


FIGURE 6. Arisaema, commonly called Jack-in-the-Pulpit, produces a bisexual spike that is subtended by a specialized leaf called a spathe. The entire inflorescence functions as a flower, which is pollinated by flies. They are attracted by the odor of rotting flesh and the streaks of red-brown on the underside of the spathe. Image from the Systematic Biology Diversity Archive	FIGURE 7. Euphorbia pulcherrima, Poinsettia, has multiple small inflorescences of tiny unisexual flowers. The clustered inflorescences are surrounded by bracts that turn bright red when the flowers mature. Image from the Systematic Biology Diversity Archive

THE ANGIOSPERM LIFE HISTORY

The fundamental life history of flowering plants (see Figure 8 below) differs from those of the gymnosperms in several important ways. First, the microspores become pollen only with the division of the microspore nucleus. Angiosperm pollen does not have prothallial cells . Also, pollination occurs when the pollen lands on a stigma; pollen cannot enter the micropyle because the ovule is completely encased in the pistil. So, the pollen tube must grow down the style and enter the micropyle, grow through the nucellus and enter the megagametophyte (called the embryo sac). One sperm nucleus fertilizes the egg to form the zygote, and the other sperm nucleus fertilizes the double fusion nucleus in the middle of the megagametophyte, which matures into the endosperm (see the description below).

The development of the ovule (called embryo sac development) can occur in many different ways. However, there are two major developmental types: the Polygonum type and the Lilium type (see Figure 9 below). The Polygonum megagaspore mother cell produces a linear tetrad of megaspores, three of which are aborted, a process that is similar to most of the gymnosperms. The single megaspore nucleus divides three times to make eight nuclei which arrange themselves such that there are three at the at the base of the embryo sac (called the antipodals), two in the middle (called the double fusion nuclei) and three under the micropyle, two are the synergids and one is the egg. One sperm nucleus fertilizes the egg to form the zygote and, ultimately, the embryo. The second sperm nucleus fertilizes the double fusion nucleus to make a triploid endosperm nucleus. It is this tissue into which the parent plant first stores food to nurture the developing embryo.

The Lilium type of embryo sac development is more complex. The megaspore mother cell nucleus divides by meiosis without cytokinesis and the four nuclei arrange themselves so that three are at the antipodal end and one is at the micropylar end. During the following mitosis, the micropylar cell divides to form two haploid cells, but the three haploid antipodals align themselves such that all three divide together and make two triploid daughter nuclei. After a second division and the migration of the polar cells to the center, the embryo sac has two haploid synergids, a haploid egg, a pair of polar nuclei (one haploid and one triploid), and three triploid antipodal cells. After double fertilization, the embryo is diploid, but the endosperm is pentaploid.

The seed usually matures as the embryo matures. It may have two cotyledons or one (a synapomorphy of the monocots). The food stored in the endosperm may remain there until germination begins (e.g. coconuts), or all of the food can be transferred to the embryo where it usually is stored in the cotyledons (e.g. peanuts).

FIGURE 8. Life History of an Angiosperm. The flower shown is a Magnolia (A), but the overall lifecycle is typical of flowering plants.

Pollen Development (b-e): The anther (b) and its cross-section (c) contain sporogenous tissue. The microspore mother cell undergoes meiosis forming a tetrad of microspores. The microspore divides once within the spore wall to form a mature pollen grain (d) with a generative cell and a tube cell. The pollen germinates (e) after it lands on a receptive stigma. The tube cell directs the growth of the pollen tube and the generative cell divides to form the two sperm.

Polygonum-Type Ovule Development (B-H): Within the ovule the megaspore mother cell (B) undergoes meiosis (C) to form four linear megaspores. The terminal three abort and the remaining megaspore begins to develop by three mitotic events (D) to produce a mature embryo sac (E). The egg cell leies beneath the micropyle and between the synergid cells. The the other end of the embryo sac are three antipodals. The center of the embryo sac has two polar nuclei, each contributed by the mitotic propagules from each pole. The pollen tube grows down through the style and through the micropyle into the embryo sac. One sperm fuses with the egg to form a zygote (2N) and the other fuses with the polar nuclei to form the endosperm (3N; F). The endosperm grows and form a food storage tissue (G). The embryo develops from the zygote (H) and the mature seed is formed when the embryo plus endosperm is surrounded by matured integuments to form a seed coat. In other groups, the food stored in the endosperm is transferred to the cotyledons (embryonic leaves) by the time the seed has matured.

In flowering plants, a Lilium-type ovule development occurs (see Figure 3).

Figure 17-3 of Norstog and Long (1976)

FIGURE 9. Comparison Between Polygonum-type and Lilium-type Embryo Sac Development. The top set of illustrations (the Polygonum-type) mirrors B-E in Figure 2. In the Lilium-type, however, separate megaspores are not formed. Instead, all four megaspore nuclei are formed and three move to the antipodal end. When they divide, the haploid nuclei at the micropylar end divide as usual, but the three nuclei at the antipodal end behave as one large nucleus and form 2 triploid nuclei. All cells divide again to form 8 nuclei, 4 haploid and four triploid. As in the Polygonum-type, the middle cell at the micropylar end is the functional egg, and each of the groups of four contributes a nucleus to the polar nuclei (also called double fusion nuclei). This time, the resulting endosperm is 5N (1N + 1N +3N).
Image by Marco Schmidt and placed in the Public Domain

THE FRUIT

The fruit is a matured ovulary, and, therefore, a derivative of the flower, and thus contains seeds. The most primitive fruits are derived from simple pistils with unfused ovularies (e.g. a pea legume or a magnolia follicle). Fruits may be fleshy or dry. The dry fruits may be indehiscent (e.g. a sunflower “seed”, really an achene) or dehiscent (e.g. a cotton capsule). All fruits, particularly fleshy fruits, of epigynous flowers have an outer component that is derived from the receptacle or other floral parts (I have already mentioned the banana in this regard). An apple is another fruit that grows from an epigynous flower such that the fleshy component is the receptacle and the core is the derivative of the ovulary (see Figure 10 below).

If the flowers are borne in clusters, the fruits also occur in clustered form. Some simple fruits like strawberries are formed from a fleshy receptacle covered by many small indehiscent dry fruits (achenes). Such fruits are called aggregate fruits . Some fruits are derived from an inflorescence and thus develop from many flowers into a structure that looks like a single fruits (e.g. pineapple which is derived from a spike). Such inflorescence-derived fruits are called multiple fruits ).

FIGURE 10. FRUIT TYPES IN THE FLOWERING PLANTS.

Indehiscent Dry Fruits: Achene has a single seed that does not fuse with the fruit wall. A nut has a single seed (usually) that fuses with a stony pericarp. Urticle is similar to an achene with several seeds. A schizocarp is a fruit from two carpels which split apart (each mericarp may be dehiscent or indehiscent). Samara is like an achene with an elongate wing.

Dehiscent Dry Fruits: Capsules develop from flowers with two or more carpels. Typically, they split according to the structures of the carpels. Follicle is a fruit from a flower with a single carpel, and it splits along one side to release the seed or seeds. A legume is similar to a follicle, but it splits open on both sides of the fruit. A loment is a kind of legume that breaks apart in segments. A silique is a simple capsule derived from two elongate carpels, which after opening leave a persistent wall. Silicle is the same as a silique but the carpels are not elongate.

Fleshy Fruits: A berry is a fleshy fruit which has a single carpel. A drupe is a special berry in which the internal part of the pericarp becomes stony. A pepo is a modified berry in which the outer pericarp becomes hard. Pome is a fruit in which the berry is enveloped by fleshy receptacle (accessory tissue). Hips are aggregates of drupes surrounded by fleshy accessory tissue. Aggregates are many simple fruits from a single flower (examples strawberry and raspberry). A multiple fruit is a single fruit like a pineapple that forms from an inflorescence. In this case a spike of flowers fuse their ovularies to form a multiple infructescence.

Images Figures 10-23A & B from Jones and Luchsinger (1986)

SYSTEMATICS OF THE ANGIOSPERMOPHYTA

Traditional taxonomic systems divide the flowering plants into monocots and dicots, based on anatomical details of their vegetative and reproductive structures (go to monocot and dicot for classical descriptions of the groups based on the system of Cronquist, 1981). The classical system and the phylogeny (see Pearson, 1995) that supports it have come under fire during the last decade. Bremer et al. (1998), a large group known as the Angiosperm Phylogeny Group, generated a taxonomic system based on cladistic analyses of rbcL, atpB, and 18S rDNA (see APG phylogeny). Their consensus view was that the monocots and eudicots were monophyletic groups, but arose from a paraphyletic collection of basal taxa. In addition, they identified the commelinoids as a natural group within the monocots and the rosids, eurosids I, eurosids II, asterids, euasterids I, and euasterids II as natural groups within the eudicots. Then, Qiu et al. (1999 & 2000), Barkman et al. (2000), Soltis et al. (2000), and Zanis et al. (2002) produced analyses that established the basal nature of the Nymphiales (water lilies and their relatives). These analyses also questioned the primitive nature of the Magnoliales (see my summary of the Tree of Life Project for a synopsis of this view).

APG II (2003) strengthened the relationships between the major taxa of flowering plants and established the basal nature of the Amborellaceae, Nymphaeaceae, and the Austrobaileyales. It also supported the monophyly of the magnoliids + monocots. In addition, the eudicots form a monophyletic group with the Ceratophyllales basal to the eudicots. Otherwise, the APG II (2003) phylogeny for the eudicots is almost unchanged from that of APG (1998).

The zenith of the premolecular systematic treatments of the flowering plants can be seen in Cronquist (1981) and Takhtajian (1997). Molecular-based systems of the flowering plants (as given by APG (1998), APG II (2003), APG III (2009), and the Tree of Life Project) are not compatible. Also, they are hard to use because the molecular differences are not manifest in clear anatomical synapomorphies. Judd et al. (2002) have produced the best systematic treatment to date in which they have attempted to place the taxa in the traditional hierarchical form and have given morphological synapomorphies whenever possible. Figure 11 is our interpretation of the system of Judd et al. (2002) in light of APG II (2003). In this system the angiosperms make up a phylum of plants with four classes and an additional monotypic class (Ceratophyllidopsida) which is basal to the eudicots. The difference between this system and the traditional system of Cronquist (1981) may appear to be subtle, but is profound in its juxtaposition of many different taxa. We use the cladistic hierarchical classification of Judd et al. (2002) with some modification by APG II (2003) and APG III (2009).

The origin of the flowering plants is one of the great questions of biology, and almost every group of seed-bearing plants has been considered as their sister group (See an expansion on this in the Nymphaeopsida page). Even Darwin (1879 in a letter to J.D. Hooker, cited in Davies et al. 2004) said the the origin of the flowering plants was an abominable mystery, though much less so now. Converging evidence from molecular and paleontological studies (so-called clocks and rocks) places the origin of the flowering plants in the Triassic Period (e.g. Wang, et al., 2007, and Davies et al., 2004) followed by repeated radiation events.

MAJOR CLADES OF THE FLOWERING PLANTS

1. The Flowering Plants

2. The Basal Families

3. Plicate Flowering Plants

4. Magnoliids + Monocots

5. Magnoliids

6. Monocots

7. Dicots

8. Basal Dicots

9. Eudicots

FIGURE 11. MAJOR CLADES OF THE FLOWERING PLANTS. Clade 1 is all flowering plants defined by the flower and its derivative, the fruit. Clade 2 is a paraphyletic group of basal taxa mostly with ascidiate (forming by rolling) carpels. Clade 3 includes all the other flowering plants characterized by plicate (forming by folding) carpels. Clade 4 is defined on the basis of molecular synapomorphies (e.g. Soltis et al. 2000, APG II 2003, and APG III 2009). Clade 5 is the Magnoliid clade characterized mostly by plesiomorphic morphological characters (e.g. numerous tepals, elongate receptacle, and floral parts arranged in a spiral) but they are defined by molecular synapomorphies (e.g. APG 1998 and APG II 2003). Clade 6 is the Monocot clade, which is well defined by both structural (e.g. parallel veins in the leaves, atactosteles, floral parts in threes) and molecular synapomorphies. Clade 7 is the Dicot clade in a narrow sense; flowering plants in which the anther is well-differentiated from the filament (Soltis et al. 2000). Clade 8 is the Basal dicot clade, represented mainly by Ceratophyllales. Clade 9 is the Eudicot clade in which all plants produce tricolpate pollen.

LITERATURE CITED

APG I, K. Bremer, M. W. Chase, P. F. Stevens, A. A. Anderberg, A. Backlund, B. Bremer, B. G. Briggs, P. K. Endress, M. F. Fay, P. Goldblatt, M. H. G. Gustafsson, S. B. Hoot, W. S. Judd, M. Kallersjo, E. A. Kellogg, K. A. Kron, D. H. Les, C. A. Morton, D. L. Nickrent, R. G. Olmstead, R. A. Price, C. J. Quinn, J. E. Rodman, P. J. Rudall, V. Savolainen, D. E. Soltis, P. S. Soltis, K. J. Sytsma, and M. Thulin (Angiosperm Phylogeny Group). 1998. An Ordinal Classification for the Families of Flowering Plants. Annals of the Missouri Botanical Garden. 85:531-553.

APG II, B. Bremer, K. Bremer, M. W. Chase, J. L. Reveal, D. E. Soltis, P. S. Soltis, P. F. Stevens, A. A. Anderberg, M. F. Fay, P. Goldblatt, W. S. Judd, M. Källersjö, J. Kårehed, K. A. Kron, J. Lundberg, D. L. Nickrent, R. G. Olmstead, B. Oxelmann, J. C. Pires, J. R. Rodman, P. J. Rudall, V. Savolainen, K. J. Sytsma, M. van der Bank, K. Wurdack, J Q.-Y. Xiang, and S. Zmartzy. 2003. The update of the angiosperm phylogeny group classification for the orders and families of flowering plants: APG II. Botanical Journal of the Linnean Society. 141:399-436.

APG III, B. Bremer, K. Bremer, M. W. Chase, M. F. Fay, J. L. Reveal, D. E. Soltis, P. S. Soltis, P. F. Stevens, A. A. Anderberg, M. J. Moore, R. G. Olmstead, P. J. Rudall, K. J. Sytsma, D. C. Tank, K. Wurdack, J Q.-Y. Xiang, and S. Zmartzy. 2009. An update of the angiosperm phylogeny group classification for the orders and families of flowering plants: APG I II. Botanical Journal of the Linnean Society. 161: 105-121.

Barkman, T. J., G. Chenery, J. R. McNeal, J. Lyons-Weiler, W. J. Ellisens, G. Moore, A. D. Wolfe, and C. W. dePamphilis. 2000. Independent and combined analyses of sequences from all three genomic compartments converge on the root of flowering plant phylogeny. Proceedings of the National Academy of Sciences U.S.A. 97:13166-13171.

Cronquist, A. 1981. An Integrated System of Classification of Flowering Plants. New York. Columbia Univ. Press. New York.

Dahlgren, R. M. T. and H. T. Clifford. 1982. The Monocotyledons – A Comparative Study. Academic Press, New York.

Davies, T. J., T. G. Barraclough, M. W. Chase, P. S. Soltis, D. E. Soltis, and V. Savolainen. 2004. Darwin’s abominable mystery: insights from a supertree of the angiosperms. Proceedings of The National Academy of Sciences 101: 1904-1909.

Jones, S. B. and A. E. Luchsinger. 1986. Plant Systematics. 2nd edition. McGraw-Hill Book Co. New York.

Judd, W. S., C. S. Campbell, E. A. Kellogg, P. F. Stevens, and M. J. Donoghue. 2002. Plant Systematics: A Phylogenetic Approach. Second Edition. Sinauer Associates, Inc. Sunderland, MA.

Qiu, Y. L., J. H. Lee, F. Bernasconi-Quadroni, D. E. Soltis, P. S. Soltis, M. Zanis, E. A. Zimmer, Z. D. Chen, V. Savolainen, and M. W. Chase. 1999. The earliest angiosperms: evidence from mitochondrial, plastid and nuclear genomes. Nature. 402:404-407.

Qiu, Y. L., J. H. Lee, F. Bernasconi-Quadroni, D. E. Soltis, P. S. Soltis, M. Zanis, E. A. Zimmer, Z. D. Chen, V. Savolainen, and M. W. Chase. 2000. Phylogeny of basal angiosperms: Analyses of five genes from three genomes. International Journal of Plant Sciences. 161:S3-S27.

Soltis, D. E., P. S. Soltis, M. W. Chase, M. E. Mort, D. C. Albach, M. Zanis, V. Savolainen, W. H. Hahn, S. B. Hoot, M. F. Fay, M. Axtell, S. M. Swensen, L. M. Prince, W. J. Kress, K. C. Nixon, and J. S. Farris. 2000. Angiosperm phylogeny inferred from 18S rDNA, rbcL, and atpB sequences. Botanical Journal of the Linnean Society 133:381-461.

Takhtajan, A. 1997. Diversity and Classification of Flowering Plants. Columbia University Press. New York.

Wang, X., S. Duan, B. Geng, J. Cui, and Y. Yang. 2007. Schmeissneria: a missing link to angiosperms? BioMedCentral Evolutionary Biology. 7-14: (13 pages).

Zanis, M. J., D. E. Soltis, P. S. Soltis, S. Mathews, and M. J. Donoghue. 2002. The root of the angiosperms revisited. Proceedings of the National Academy of Sciences. U.S.A. 99:6848-6853.

By Jack R. Holt. Last revised: 04/08/2013

Diversity of Life

DESCRIPTION OF THE PHYLUM ANGIOSPERMOPHYTA

DESCRIPTION OF THE PHYLUM ANGIOSPERMOPHYTA (BERRY 1915)

DESCRIPTION OF THE PHYLUM ANGIOSPERMOPHYTA (BERRY 1915)

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