Seed Plants – The Gymnosperms The seed plants evolved from fern-like non-seed ancestors. Several changes occurred to make this novelty possible. First, two types of spores, large megaspores and small microspores, appeared. This change is illustrated in Selaginella and some aquatic ferns. In both, the gametophytes are reduced in size, developing within the spore walls. The male gametophyte developing within the microspore wall became the pollen. The female gametophyte developed within the spore wall, and the spore was retained within the megasporangium. For fertilization to occur pollen was carried by wind to the megasporangium, the grains germinated as a tube and the male gametes moved to the egg cell. After fertilization, the embryo developed inside of the megasporangium, now called the ovule. The fertilized ovule then became a seed, with an embryo inside. This new life cycle had several advantages. First, the protected pollen grain was blown by wind to the site of germination, reducing the requirement for water and permitting these plants to sexually reproduce in much drier conditions. Secondly, the development of the seed provided a means of protecting the embryo against dessication and the storing of the embryo in dormancy until ideal conditions would trigger germination. Finally, modifications of the seed promoted dispersal by wind or animals. Plants with this life cycle are called gymnosperms because the ovule/seed is produced on a leaf-like structure and is unprotected, or naked. Gymno- = naked and -sperm = seed. We will see details of this modification and this new life cycle in two plants native to south Florida. There are four groups of living gymnosperms: the conifers, the cycads, Gingko, and Gnetum and its relatives. Ginkgo is represented by a single species: Gingko biloba. It does not grow in south Florida, but it is sold in health food stores as a tonic to improve cerebral circulation and memory in aging. The Conifers These trees are enormously important, as the source of softwood timber used in wood-based construction and the source of fiber in producing most of the world’s paper. They are dominant in certain forests at temperate and higher latitudes, as in northern latitudes and the northwest of the United States. Most of the coastal areas of south Florida, on the limestone ridge, were covered with a pine forest, called the pine rocklands. Little of this forest remains, having been replaced by agriculture and then commercial development. Conifers vary in their leaves and particularly in their female cones; some being reduced to look more superficially like a berry. They all share the same basic life cycle, that of a pine tree given as an example below. Three conifers native to south Florida are described in detail, and several exotic species are mentioned briefly.
Dade County Pine—Pinus elliottii var. densa. This extreme southern variety of the slash pine grows in a few remaining stands on the rock ridge of south Florida. Its wood is very resinous, extremely hard when dry, and very resistant of termite attack. It was used in the construction of homes and boats well into the last century. Dade County pine grows in the Ecosystem Preserve and the parking lot just to the east. A couple of trees also grow in the small conifer collection NE of the north parking lot. Dade County Pine, continued The fragile male cones and small purplish female cones develop in January-February. In a forest the air is yellow with the wind-carried pollen. After fertilization the female cone scales swell and close. Then the cones develop for the entire year, and open to release the winged seeds prior to the rainy season (May) the next year. The trees always have female cones in some stage of development, but the male cones soon fall off the tree after they shed their pollen. The Dade County Pine is a member of the pine family (Pinaceae) along with all pines and the firs, such as the Frazer’s fir from the Appalachian Mountains on sale before Christmas.
Procedure – examine pine twigs and leaves • Examine pine twigs having leaves (needles) and a terminal bud. Notice the number of needles; the length and number of leaves distinguishes many of the species Pinus. Questions • How are the needles arranged? • How many leaves are in a bundle? • How are pine leaves different from those of deciduous plants? • Why are pines called evergreens? • How do the structural features of pine leaves adapt the tree for life in cold, dry environment? Phylum Gingophyta: The Ginkiphyta consist of one species, Ginkgo biloba (Maindenhari plant), a large dioecious tree that does not bear cones. Ginkgo are hardy plants in urban environments and tolerate insects, fungi, and pollutants. Males are usually planted because females produce fleshy, smelly, and messy fruit that resembles cherries. Ginkgo has not been found in the wild and would probably be extinct but for its cultivation in ancient Chinese and Japanese gardens. Phylum Gnetophyta: This gnetophytes (71 species in 3 genera) include some of the most distinctive (if not bizarre) of all seed plants. They have many similarities with angiosperms, such as flowerlike compound strobili, vessels in the secondary xylem, loss of archegonia, and double fertilization.
Pine life cycle In seed plants, the gametophyte gneration is greatly reduced. A germinating pollen grain is the mature microgametophyte (male cones) of a pine. Pine microsporangia are borne in pairs on the scales of the delicate pollen-bearing cones. Megagametophytes (female cones), in contrast, develop within the ovule. The familiar seed-bearing cones of pines are much heavier than the pollen-bearing cones. Tow ovules, and ultimately two seeds, are borne on the upper surface of each scale of a cone. In the spring, when the seed-bearing cones are small and young, their scales are slightly separated. Drops of sticky fluid, to which the airborne pollen grains adhere, form between these scales. These pollen grains geminate, and slender polled tubes grow towards the egg. When a pollen tube grows to the vicinity of the megagametophyte, sperm are released, fertilizing the egg and producing a zygote there. The development of the zygote into an embryo occurs within t the ovule, which mature into a seed. Eventually, the seed falls from the cone and germinates, the embryo resuming growth and becoming a new pine tree. Pine Life Cycle diagram
Procedure – examine pine cones Examine young living or preserved ovulate cones. These cones will develop and enlarge considerable before they are mature. Examine a prepared slide of a young ovulate cone ready for pollination. Each ovuliferous scale of the female cone bears two megasproangia, each of which produces a diploid megaspore mother cell. Each megaspore mother cell undergoes meiosis to produce a megaspore that develops into a megagametophyte. A megagametophyte and its surrounding tissues constitute an ovule and contains at least one archegonium with an egg cell. Examine a prepared slide of an ovulate cone that has been sectioned through an ovule. An ovule develops into a seed. Examine a mature ovulate cone and notice its spirally arranged ovuliferous scales. These scales are analogous to microsporophylls of staminate cones, but ovuliferous scales are modified branches rather than modified leaves. At the base of each scale you’ll find two naked seeds. Notice that the seeds are exposed to the environment and supported (but not covered) by an ovuliferous scale. Procedure – examine a pine seed Examine a prepared slide of a pine seed. Locate the embryo, seed coat, and food supply. Seeds are released when the cone dries and the scales separate. This usually occurs 13-15 months after pollination. Examine some mature pine seeds, noting the winglike extensions of the seed coat. Questions On which surface of the scale are the seeds located? How large in a staminate cone compared to a newly pollinated ovulate cone? A mature ovulate cone? What is the make gametophyte? What is the female gametophyte? What is the function of the winglike extensions of a pine seed? How are other gymnosperms similar to pines? How are they different? Procedures and questions about conifer reproduction
Bald Cypress—Taxodium distichum. This is a conifer in another family, the Taxodiaceae. Its female cones are much smaller and the individual scales are rounded to produce a round cone. It is a swamp tree, growing in stands throughout the southeast. It was once common in a strip of swamp forest down the southeast coast of Florida, and more common along the west coast, as in the Big Cypress National Preserve. Bald Cypress trees were planted on pond margins at FIU soon after it opened. We now have some bald cypress “domelets”, with cypress knees (the pneumatophores that assist in oxygen uptake to the roots) and Everglades wading birds sitting on branches. The bald cypress is unusual among conifers in that it loses its short needle foliage during the winter months. Few of the original cypress domes remain; the majority of these swamp forests were logged before and during the Second World War, partly for the construction of PT boats.
Phylum Cycadophyta: the cycads These gymnosperms are no longer widely distributed, only found in mostly dry tropical regions, but they were once dominant plants. These were the primary food of the large herbivorous dinosaurs. Most cycads are extremely tough, thorny, and often very toxic. Fairchild Tropical Garden, and the adjacent Montgomery Botanical Center, have the largest cycad collection in the world. Cycads have life cycles similar to the conifers, but certain details (as the flagellate male gametes) are different. Cycad plants are female (producing long-lived female cones) or male (producing ephemeral male cones). We illustrate the cycad life cycle with the example of the coontie, Zamia pumila, and describe a few cycads commonly encountered in south Florida (and on campus). Zamia Life Cycle
Coontie—Zamia pumila. The coontie is the only cycad native to the United States, growing in south Florida Pinelands. Its rhizomes are full of starch, which was the source of the first manufacturing industry in south Florida. The "trunks" were ground up to release the starch, the starch was then washed to remove the toxic cycasin, and the product dried and ground. Florida “arrowroot” was then shipped up the east coast for cooking and stiffening the collars of Victorian shirts. The coontie is a small plant, less than half a meter high. It grows on campus in the Ecosystem Preserve, the Campus Security Compound, and by the Conservatory. Recently, the remarkable discovery was made that the coontie is pollinated by beetles, that feed on both the male and female cones.
Seed Plants – the Angiosperms – Flowering Plants The angiosperms are seed plants, similar to gymnosperms, but with some important evolutionary modifications. Flowers are reproductive organs derived from leaf-like appendages. The relationship of the accessory flower organs, petals and sepals, is obvious. The stamens and pistils can also be seen in development to originate from leaf-like structures. In the flowering plant life cycle, the male gametophyte which develops within the microspore wall into a pollen grain are even more reduced than in the gymnosperms. Its movement to the ovule is often aided by appearance and scent, attracting pollinators. The female gametophyte develops as the embryo sac, within an ovule, and within a new structure: the ovary. In pollination the pollen grain germinates on the stigma of the pistil and grows down the length of the style to the opening of the ovule. After fertilization, the embryo sac and ovule develop into the seed. A second fertilization produces a nutritive tissue, the endosperm, that surrounds the embryo. At maturity, the ovules, or seeds, are protected within the ripened ovary wall to become a fruit. The fruit, fleshy or dry, aids in dispersal. Peduncle – flower stalk Sepals – the lowermost or outermost whorls of structures, which are usually leaflike and protect the developing flower; the sepals collectively constitute the calyx. Petals – whorls of structures located inside and usually above the sepals; the petals collectively constitute the corolla. Androecium – the male portion of the plant; consists of stamens, each of which consist of a filament atop which is located an anther; inside the anthers are pollen grains which produce the male gametes Gynoecium – the females portion of the plant; consist of one or more carpels, each made up of an ovary, style, and stigma; the ovary contains ovules that contain the female gametes. The term pistil is sometimes used to refer to an individual carpel or a group of fused carpels.
More information about Angiosperms Flower symmetry The sepals and petals are usually the most conspicuous parts of a flower, and a variety of flower types are described by the characteristics of the perianth (combined calyx and corolla). In regular (actinomorphic) flowers such as tulips, the members of the different whorls of the flower consist of similarly shaped parts that radiate from the center of the flower and are equidistant from each other. The flowers are radially symmetrical. In other flowers such as orchids, one or more part of at least one whorl are different from other parts of the same whorl. These flowers are generally bilaterally symmetrical and are said to be irregular (zygomorphic). Two classes of angiosperms Monocots • One cotyledon per embryo • Flower parts in sets of three • Parallel venation in leaves • Multiple rings of vascular bundles in stem • Lack a true vascular cambium (lateral meristem) Dicots • Two cotyledons per embryo • Flower parts in sets of 4 or 5 • Reticulate (i.e., netted) venation in leaves • One ring or vascular bundles in stem • Have a true vascular cambium (lateral meristem) A radially symmetrical flower Photo by Gita Ramsay A bilaterally symmetrical (irregular) flower Photo by Gita Ramsay
Angiosperm life cycle Eggs from within the embryo sac inside the ovules, which, in turn, are enclosed in the carpels. The pollen grains, meanwhile, form within the sporangia of the anthers and are shed. Fertilization is a double process. A sperm and egg come together, producing a zygote; at the same time, another sperm fuses with the polar nuclei to produce the endosperm. The endosperm is the tissue, unique to angiosperms, that nourishes the embryo and young plant.
Basic Leaf Information Leaves differ from stems in not having an apical meristem, so leaves are determinate (i.e., limited in their growth), while stems are indeterminate (theoretically capable of growing forever). In the root apical meristem, the differentiating cells produce the root cap, a structure that protects the root apical meristem as it pushes its way through the soil, and the root body, which is the part of the root that we see. Thus, the apical meristems of the root and shoot differ in their structure—the root apical meristem is internal, surrounded by cells on all sides, whereas the shoot apical meristem is external and not covered by cells. You usually need to look at sections of plants under the compound microscope to see these differences, but on some plants, such as the screw pine or Pandanus, next to the OE pond on campus, you can clearly see the root cap of the prop roots before they enter the ground. Examine plants on campus, identifying roots, stems, leaves, apical meristems and axillary buds. Both roots and shoots can branch. The branches form more roots, if they are root branches, and more shoots, if they are shoot branches. Root branches are produced inside the root itself, breaking out through the root, while shoot branches form from axillary buds. Axillary buds are produced in the upper angle between the leaf and the stem, which is called the axil of the leaf (Figure 1). Leaves are produced in a very organized manner at the shoot apex. This results in a predictable arrangement of the mature leaves on the stem. This arrangement is called the phyllotaxis of the leaf. Common patterns are for the plant to produce 1 leaf at a time at the apex, resulting in an alternate phyllotaxis. Sometimes twp leaves are produced at a time at the apex, with successive leaf pairs at 90o from each other. This is an opposite phyllotaxis. If more than twp leaves are produced at a time, the phyllotaxis is whorled, but this is a much more rare occurrence. See the examples in Figure 4.
One way to begin to analyze what’s what on a plant is to consider where different parts fit into the overall ground plan of the plant. For example, a thorn that is lateral to another structure (the stem) and has a third structure in its axil (the axillary bud) is in the right position to be equivalent to a leaf. Leaf Identification Figure 4. A = palmately compound leaf, opposite leaf arrangement; B = pinnately compound leaf, alternate leaf arrangement; C = simple, lobed, petiolate leaves, alternate leaf arrangement; D = simple leaves, opposite leaf arrangement; E = simple, lobed and toothed, petiolate leaf, opposite leaf arrangement; F = simple leaves, alternate leaf arrangement; G = simple lobed leaf, alternate leaf arrangement; H = simple linear leaf with sheathing leaf base, alternate leaf arrangement; I = simple leaves, whorled leaf arrangement; J = simple needlelike leaves, alternate leaf arrangement; K = simple bilobed leaf, alternate leaf arrangement.
Plants: Reproduction Flowers and Inflorescences Flowers are short shoots (rosettes) specialized for sexual reproduction. The stem is called the receptacle and bears leaf homologues. Although the number of parts can vary, flowers can have up to 4 whorls of “leaves”. The first 2 whorls, the sepals and petals, are sterile and are often modified for protection of the developing flower and/or for attraction of pollinators (Figure 1). The term for all of the sepals is calyx, while the term for all of the petals is corolla. The last two whorls, the stamens and carpels, are the fertile parts. The stamens are usually differentiated into the filament and anther (Figure 1). The anthers are the site of meiosis and produce the pollen or male gametophyte. The carpels are usually differentiated into the stigma, which receives the pollen, the style that supports the stigma, and the ovary (Figure 1). The ovules are inside the ovary. Meiosis also occurs in the ovules, producing the female gametophyte, which, after double fertilization, makes the embryo and endosperm. The ovules mature into the seeds, while the ovary, sometimes with additional parts, matures into the fruit. Figure 1. Flowers, thus, have a number of functions. They provide plants with the opportunity to spread genes, since both the pollen and seeds can leave the parent plant. Because they enable the plant to reproduce sexually, flowers mix male and female genes and contribute to genetic diversity. Through the production of fruits they help to disperse the next generation, and through provisioning of the seeds, they help that generation to begin to grow. There is enormous variation in flower structure among species. They can lack sepals and/or petals, or these whorls can resemble each other, as in many monocots, such as lilies. The parts of a whorl can fuse to each other, as in the tubular corollas of sunflowers, or to adjacent whorls, as when stamens are attached to the corolla. A fundamental difference is in the position of the carpels in relation to other parts of the flower. If the sepals, petals, and
stamens are inserted on the top of the ovary, the ovary is said to be inferior and the flower is epigynous (Figure 2). The individual flowers of the sunflower provide an example. If the sepals, petals, and stamens are inserted below the ovary, the ovary is superior and the flower is hypogynous (Figure 2). Bean flowers are hypogenous, as are those of Brassica. Sometimes the other floral parts are fused halfway to the ovary, or fuse to themselves, forming a cup that comes up partway around the ovary. These flowers are perigynous. The number of parts per whorl also varies. In general, monocots have parts in 3s or multiples of 3, while dicots have parts in 4s or 5s or multiples of these numbers. The overall symmetry of a flower can be radial (actinomorphic), with the whorls distributed evenly around the receptacle, as in strawberry flowers or the flowers of Brassica (Figure 3). Alternatively, the flower can have bilateral symmetry (be zygomorphic), in which case it has a distinct top and bottom, as in orchid flowersor bean flowers (Figure 3). Figure 2.
Figure 3. Because one of the functions of flowers is to enhance pollination (the transfer of pollen from the anthers to a stigma), the structure of flowers varies with the type of pollinator. Wind pollinated flowers are generally not colorful (the wind can’t see), very small, have no or reduced sepals and petals, and may separate the anthers and stigmas into different flowers. They also produce huge amounts of pollen. Animal-pollinated flowers are often more colorful, have sepals and petals, and vary in size, color, and symmetry depending on the type of pollinator. Because hummingbirds see red, hummingbird-pollinated flowers are often red, whereas bee-pollinated flowers tend to be yellow or blue, because bees see these colors. Moth-pollinated flowers are often white, but have strong scents that are emitted at night, as moths are sensitive to odor and are active at night. Flowers have to both attract pollinators and provide them with a reward, so that they will visit other flowers of the same species. Common rewards are pollen itself, which is often rich in proteins and lipids, and nectar, which may be secreted by glands in the flower.
Meiosis in Anthers Stamens produce the male gametophytes of flowering plants. This is an important stage in the life cycle because pollen often leaves the parent plant, providing one of the few times plants can move genes around. The stamens are subdivided into the filaments and anthers. The anthers bear 4 microsporangia internally. The microsporangia produce microspore mother cells that undergo meiosis, producing 4 pollen grains per microsporocyte. These microspores are initally held together in groups of 4 by the original mother cell wall. This wall enentually breaks down, however, and the microspores are released. Each microspore will divide once to make the pollen vegetative cell and generative cell. The generative cell will divide to produce the two sperm that fertilize the egg cell and polar nuclei in double fertilization. This second division happens late in the life of a pollen grain, often occurring after pollination! Because these different parts of the life of a pollen grain look different, you can assess the developmental stage of the pollen by squashing the anthers and seeing whether the pollen is in groups of 4 (tetrads, which occur immediately post-meiosis), or is single with a heavy wall, which is older pollen that will soon be dispersed (Figure 5). Remember the difference between pollination and fertilization. In pollination pollen is transferred from anthers to the stigma. The pollen germinates on the stigma, grows down the style, and passes into the micropyle of the ovule. It grows through the nucellus, releasing two sperm into the embryo sac. Fertilization comes at this point: one sperm fertilizes the egg and thus forms the first cell of the daughter embryo; the other sperm fuses with the polar nuclei, producing the triploid endosperm. Figure 5.
A seed is a mature ovule that includes a seed coat, a food supply, and an embryo. The stages of embryo development in the seed of Capsella (a dicot) is show to the right/blow. The developing embryo grows, absorbs the endosperm, and stores those nutrients in “seed leaves” called cotyledons. Development includes the following stages: • Proembryo stage –. Initially the embryo consists of a basal cell, suspensor, and a two celled proembryo. The suspensor is the column of cells that pushes the embryo into the endosperm. Note that the endosperm is extensive but is being digested. • Globular stage – A stage that is radially symmetrical and has little internal cellular organization. • Heart-shaped stage – Differential division produces bilateral symmetry and two ctyledons forming the hear-shaped embryo. The enlarging cotyledons store digested food from the endosperm. Tissue differentiation begins, and root and shoot meristems soon appear. • Torpedo stage – the cotyledons and root axis soon elongate to produce an elongated torpedo-stage embryo. Procambial tissue appears and will later develop into vascular tissue. • Mature embryo – has large, bent cotyledons on either side of the stem apical meristem. The radicle, later to form the root, is differentiated toward the suspensor. The radicle has a root apical meristem and root cap. The hypocotyl is the region between the apical meristem and the radicle. The endosperm is depleted and food is stored in the cotyledons. The epicotyl is the region between the attachment of cotyledons and stem apical meristem; it has not elongated in the mature embryo. (a) A garden bean (dicot seed); will absorb the endosperm before germination; (b) a corn seed (monocot); the single cotyledon is an endosperm-absorbing structure called a scutellum.
Fruits Simply stated, fruits are ripened ovaries. Once fertilization occurs the ovules develop into seeds, and the ovary wall develops into the fruit wall. The wall develops from leaf-like structures, called carpels. A fruit may develop from a single, or many, carpels. How the carpels fuse together determines the numbers of chambers in the fruit, from one to many, and each of these may contain one to many seeds. Under exceptional circumstances the fruit may develop in the absence of seeds (as a seedless grape or naval orange), a process called parthenocarpy. It is possible to examine a fruit to determine the ovary’s position in the flower. If scars or parts of old petal and sepals are at the tip of the fruit, the flower was inferior (as an apple). If at the base then superior (as an orange). If the ovary wall is fleshy, the fruit is a berry, if dry at maturity and breaks open, the fruit is a capsule. Sometimes the ovary wall develops into a fruit of different layers, including an inner one that is stony—a drupe (like a peach). Sometimes accessory parts form part of the flesh of the fruit, an accessory fruit or pome (like an apple). Sometimes the flower forms multiple pistils, and the ovaries fuse together to form an aggregate fruit (like a raspberry). Sometimes the ovaries of separate flowers fuse together to form a compound or multiple fruit, such as a pineapple. You can quickly find a great diversity of types of fruits by examining the produce in a supermarket, looking at the fresh fruits and nuts. • Fleshy fruits A. Simple fruits (i.e., from a single ovary) 1. Flesh mostly of ovary tissue a) endocarp hard and stony; ovary superior and single-seeded (cherry, olive, coconut): drupe b) endocarp fleshy or slimy; ovary usually many seeded (tomato, grape, green pepper): berry 2. Flesh mostly of receptacle tissue (apple, pear, quince): pome B. Complex fruits (from more that 1 ovary) 1. Fruit from many carpel son a singlr flower (strawberry, raspberry);: aggregate fruit 2. Fruit from carpels of many flowers fused together (pineapple): multiple fruit II Dry fruits A. Fruits that split open at maturity (usually more than one seed) 1. Split occurs along two seems in the ovary. Seeds borne on one of the halves of the split ovary (pea and bean pods, peanuts): legume 2.Seeds released through pores or multiple seams (poppies, irises, lilies): capsule B. Fruits that do not split open at maturity (usually one seed) 1. Pericarps hard and thick, with a cup at its base (acorn, chestnut): nut 2. Pericarp thin and winged (maple, ash, elm): samara 3. Pericarp this and not winged (sunflower, buttercup): achene (cereal grains): caryopsis Dichotomous Key to Major Types of Fruits
Features of Mature Woody Stems Examine the features of a dormant twig. A terminal bud containing the apical meristem is at the stem tip surrounded by bud scales. Leaf scars from shed leaves occur at regularly spaced nodes along the length of the stem. The portion between the stem and nodes are called internodes. Vascular bundlesscars may be visible within the leaf scars. Axillary buds protrude from the stem just distal to each leaf scar. Search for clusters of bus scale scars. The distance between clusters or from a cluster to the terminal bud indicates the length of yearly growth. The shoot apex – Examine a living coleus plant and not the arrangement of leaves on the stem. Examine a prepared slide of a longitudinal section of the root tip of Coleus (above). Note that the dome-shaped shoot apical meristem is not covered by a cap as the room apical meristem would be. The shoot apical meristem produced young leaves (leaf primordia) that attach to the node. An auxiliary bud between the young leaf and the stem for a branch or flower. This is a cross-section of a sunflower stem. An epidermis covers the stem. The epidermis is coated with a waxy, waterproof coating called the cutin. Below the epidermis is the cortex, which stores food. The pith in the center of the stem also stores food. Also note the vascular bundle composed of phloem and xylem. Xylem transports water and minerals; phloem transports most organic compounds in the plants.
Internal Anatomy of Leaves Examine the diagram above of the internal anatomy of a leaf. Note that the leaf is only10-15 cells thick – pretty thin for a solar collector! The epidermis contains pores called stomata, each surrounded by two guard cells. Just below the upper epidermis are closely packed cells called palisade mesophyll cells; these cells contain about 50 chloroplasts per cell. Below the palisade layers are spongy mesophyll cells with numerous intercellular spaces. Questions: • What is the function of the stomata? • Do epidermal cells of leaves have a cuticle? Why is this important? • What is the significance of chloroplasts being concentrated near the upper surface of the leaf? • Based on the arrangement of vascular tissues, how could you distinguish the upper versus lower surfaces of a leaf? A stoma. Unlike the other epidermal cells, the guard cells flanking this stoma contain chloroplasts. Water passes out through the stomata, and carbon dioxide enters by the same portals.