Leaves
- Also called:
- flowering plant
Leaves initially arise from cell divisions in the shoot apical meristem. A slight bulge (a leaf buttress) is produced, which in eudicots continues to grow and elongate to form a leaf primordium. (Stipules, if present, appear as two small protuberances.) Marginal and submarginal meristems on opposite flanks of the primordium initiate leaf-blade formation. Differences in the local activity of marginal meristems cause the lobed shapes of simple leaves and the leaflets in compound leaves. An increase in width and in the number of cell layers is brought about by marginal meristems. Subsequent expansion and increase in length is achieved by cell division and the general enlargement of cells throughout the blade.
Leaf growth is determinate; the tip matures first, and maturation then progresses toward the base, after which the leaf cells cease to grow and divide. (Stem growth is generally indeterminate since the meristems are active indefinitely.) The petiole, when present, and the leaf base become thickened, and often the latter expands laterally and fully or partially encloses the stem. Soon after the cells of the marginal meristems begin to divide, procambial strands differentiate into the leaf from the stem bundles to form the midvein, or midrib. The smaller lateral veins of the leaf are initiated near the leaf tip; subsequent major lateral veins are initiated sequentially toward the base, following the overall pattern of leaf development. A major lateral vein may have one or more orders of smaller veins, which also are initiated in size from larger to smaller. This results in the netlike venation patterns characteristic of eudicotyledonous leaves.
The anatomy of a mature eudicot leaf generally reflects the habitat, especially the availability of water. Mesomorphic leaves are adapted to conditions of abundant water and relatively humid conditions; xeromorphic leaves are adapted to dry conditions with relatively low humidity; and hydromorphic leaves are adapted to aquatic situations, either submerged or in standing water. Mesomorphic leaves (the most common type) are characteristic of crop plants, such as tomatoes and soybeans. Their veins (vascular bundles) permeate the ground tissue of the dermal system—a single layer of epidermal cells with interspersed guard cells. The ground tissue system, the mesophyll, is divided into two regions: the palisade parenchyma, located beneath the upper epidermis and composed of columnar cells oriented perpendicular to the leaf surface, and spongy parenchyma, located in the lower part of the leaf and composed of irregularly shaped cells. The veins contain primary xylem and phloem and are enclosed by a layer of parenchyma called the bundle sheath. Only the midvein and some large lateral veins have any secondary growth.
The anatomy of mesomorphic leaves is designed to function optimally for water uptake and gas exchange in photosynthesis under mesic (moist) conditions. The spongy mesophyll with irregularly shaped cells provides increased surface area internally, while the elongate palisade cells provide optimal exposure of chloroplasts to light.
The anatomy of hydromorphic leaves is simplified: the cuticle is thin or lost; the guard cells are raised and are found only on the upper surface in floating leaves (they are lost in most submerged leaves); the mesophyll contains aerenchyma (an adaptation to promote water loss) and little or no collenchyma or sclerenchyma; and the vascular system (particularly the water-conducting element of the vascular system, the xylem) is only weakly developed since the water provides much of the mechanical support to the plant normally provided by the xylem. The abundance of water means that there is no need for mechanisms that prevent water loss and little need for additional supports. The leaves generally become large and thin, and the reduction or loss of cuticle, vascular tissue, and ground tissue (mesophyll) permits the rapid loss of water vapor (transpiration). The guard cells on the upper surface of floating leaves also monitor the rate of water loss through the central stomata. Such plants may wilt if the turgor (water) pressure is reduced. Water lilies (Nymphaeaceae) and rice crops contain hydromorphic leaves.
Xeromorphic adaptations to arid conditions are quite varied and tend to prevent water loss during periods when water is limited and must be conserved by the plant. There are many modifications limiting transpiration: two examples are a multilayered epidermis covered by thick layers of epicuticular wax or mucilages secreted into stomates; another is dense mats of trichomes on both surfaces of the leaf and guard cells and stomata sunken into the lower surface and often lined with numerous trichomes, which trap moisture, thereby inhibiting total water loss. Mesophyll modifications provide a means of storing water. Most xeromorphic leaves have a high volume-to-surface ratio—i.e., they are small and compact. In addition, many are fleshy and often oval to round in shape.
The development of monocotyledonous leaves after initiation of cell division on the shoot apical meristem is different from that of eudicotyledons and results in leaves with different morphologies from those of eudicotyledons. Leaves in monocotyledons have either a radial leaf tip or are expanded in the same plane as the stems instead of at a right angle to the stem, as in eudicotyledons. The leaf buttress begins as a ring that encloses the stem. The upper portion of the buttress develops a meristem on the side facing the stem (adaxial meristem). Growth at this adaxial meristem forms the flattened leaf with the radial (cylindrical) leaf tip typical of the monocotyledons. If the adaxial meristem is long-lived (), long flat leaves in the same plane as the stem are formed (Iris; Iridaceae); if short-lived (), flat leaves with short cylindrical tips develop (snake plant, Sansevieria trifasciata; Asparagaceae). When the radial (topmost aspect of the leaf) is short, the base becomes flattened because the marginal meristems (those on either side of the midvein) continue to expand outward. A monocot leaf grows either radially or along the margins, but not both in the same region. The monocot leaf grows in length from a meristem at its base, which is why it is possible to mow grass and have the leaf blades continue to grow.
The developmental pattern from a basal intercalary meristem has placed constraints on the anatomy of monocot leaves, particularly with respect to venation and the position of stomates. This has produced a leaf anatomy characteristic of the monocots. There is no midvein, and veins are longitudinally parallel. The stomates are in rows between the veins, and the mesophyll is often poorly developed and mostly parenchymatous with scattered bundles of fibers. Thus, most monocot leaves are uniform in appearance and texture. Most of the hydromorphic and xeromorphic modifications found among eudicot leaves, however, also occur in monocot leaves in similar environments.
Reproductive structures
General features
The broad range of variation in the morphology and structure of nonreproductive (vegetative) organs within the angiosperms has been outlined above. There is a similarly broad range in the morphology and structure of the reproductive organs of the plant.
Many vegetative buds sooner or later become flower buds. Flower buds are modified leaves borne on a short axis with very short internodes and no axillary buds. The floral axis has determinate growth, in that at some point it ceases to grow.
Flowers, the reproductive tissues of the plant, contain the male and/or female organs. They may terminate short lateral branches or the main axis or both. Flowers may be borne singly (as in the daffodil and Magnolia) or in clusters called inflorescences (e.g., bromeliads, snapdragons, and sunflowers). Fruits are derived from the floral parts of the angiospermous plant.
A complete flower is composed of four organs attached to the floral stalk by a receptacle (). From the base of the receptacle upward these four organs are the sepals, petals, stamens, and carpels. In eudicots the organs are generally grouped in multiples of four or five (rarely in threes), and in monocots they are grouped in multiples of three.
The sepals, the outermost layer, are usually green, enclose the flower bud, and collectively are called the calyx. Petals are the next layer of floral appendages internal to the calyx; they are generally brightly colored and collectively are called the corolla. The calyx and corolla together compose the perianth. The sepals and petals are accessory parts or sterile appendages; though they protect the flower buds and attract pollinators, they are not directly involved with sexual reproduction. When the color and appearance of sepals and petals are similar, as in the tulip tree (Liriodendron tulipifera) and Easter lily (Lilium longiflorum), the perianth is said to be composed of tepals.
Internal to the corolla are the stamens, spore-producing structures (microsporophylls) that are collectively called the androecium. In most angiosperms, the stamens consist of a slender stalk (the filament) that bears the anther (and pollen sacs), within which the pollen is formed. Small secretory structures called nectaries are often found at the base of the stamens and provide food rewards for pollinators. In some cases the nectaries coalesce into a nectary or staminal disc. In many cases the staminal disc forms when a whorl of stamens is reduced into a nectiferous disc, and in other cases the staminal disc is actually derived from nectary-producing tissue of the receptacle.
At the centre of the flower are the carpels, collectively called the gynoecium. Carpels are megasporophylls that enclose one or more ovules, each with an egg. After fertilization, the ovule matures into a seed, and the carpel matures into a fruit. Carpels, and thus fruit, are unique to angiosperms.
A complete flower contains all four organs, while an incomplete flower is missing at least one. A bisexual (or “perfect”) flower has both stamens and carpels, and a unisexual (or “imperfect”) flower either lacks stamens (and is called carpellate) or lacks carpels (and is called staminate). Species with both staminate flowers and carpellate flowers on the same plant (e.g., corn) are monoecious, from the Greek for “one house.” Species in which the staminate flowers are on one plant and the carpellate flowers are on another are dioecious, from the Greek for “two houses.”
Floral organs are often united or fused: connation is the fusion of similar organs—e.g., the fused petals in the morning glory; adnation is the fusion of different organs—for example, the stamens fused to petals in the mint family (Lamiaceae). The basic floral pattern consists of alternating whorls of organs positioned concentrically: from outside inward, sepals, petals, stamens, and carpels (). It is possible in most cases to interpret the flower with respect to missing parts and/or the modification of parts to function as missing parts simply by positional relationships. In a complete five-merous flower (starting from the outside) there would be a whorl of five sepals, followed by an alternating whorl of five petals, followed by an alternating set of five stamens. In the floral diagram (), the midline of each petal is midway between the midlines of two adjacent sepals. Because the whorls alternate, the midline of each stamen of the stamen whorl is between the midlines of two adjacent petals and on the midline of each sepal. When the petals are missing and bracts appear colored and petaloid as in the Bougainvillea, one of the three whorls is missing: there are only two whorls of five organs instead of the three whorls of five organs described above. Because one whorl of the flower is obviously composed of stamens that bear functional pollen and the other whorl is composed of a brightly colored set of organs that resemble petals one might conclude that the sepals are missing. But examination of positional relationships between the whorls reveals that the midline of each stamen is on the same line as the midline of the organs of the brightly colored set. Thus, position tells us that the brightly colored whorl represents a sepal whorl and that the sepals have assumed the function of the missing petals.
The receptacle
The receptacle is the axis (stem) to which the floral organs are attached. Floral organs are attached either in a low continuous spiral, as is common among primitive angiosperms, or in alternating successive whorls, as is found among most angiosperms.
The peduncle is the stalk of a flower or an inflorescence. When a flower is borne singly, the internode between the receptacle and the bract (the last leaf, often modified and usually smaller than the other leaves) is the peduncle. When the flowers are borne in an inflorescence, the peduncle is the internode between the bract and the inflorescence; the internode between the receptacle of each flower and its underlying bracteole is called a pedicel. Thus, in inflorescences, bracteole is the equivalent of bract, and pedicel is the equivalent of peduncle.
Often the bract subtending an inflorescence is brightly colored, as in the poinsettia (Euphorbia pulcherrima; Euphorbiaceae), or provides protection, as in the woody, boat-shaped bracts in many palms. Bracteoles in the inflorescence of Bougainvillea also are brightly colored to attract pollinators. In some angiosperms, the receptacle becomes fleshy; in the strawberry, for example, the receptacle is the fleshy edible part of the strawberry and, when eaten by small mammals and birds, aids in seed dispersal. In others, the peduncle or pedicel becomes fleshy; in the cashew (Anacardium occidentale; Anacardiaceae), for example, the pedicel is made into a drink in the Neotropics, and it also aids in fruit dispersal of the much smaller cashew nut. In cacti (e.g., prickly pear), the fleshy part of the edible fruit forms from the receptacle and peduncle, and several internodes below that grow up and surround the carpels; this is why there are axillary buds in cacti (areoles) with spines on the fruit surface.
The calyx
The sepals (collectively called the calyx) most resemble leaves because of their generally green color. From their base and along most of their length, sepals remain either separate (aposepalous, or polysepalous) or marginally fused (synsepalous), forming a tube with terminal lobes or teeth. The number of calyx lobes equals the number of fused (connate) sepals.
The sepals enclose and protect the unopened flower bud. The calyx is commonly persistent and evident when the fruit matures (e.g., persimmon, Diospyros virginiana; Ebenaceae), in contrast to the more short-lived petals and stamens. Sepals may be brightly colored and function as petals when true petals are missing—for example, the virgin’s bower (Clematis; Ranunculaceae) and the Bougainvillea. Petaloid sepals in this case differ from tepals because the first group of stamens are on the same radii as the sepals, indicating the absence of the petals, which would normally be positioned on alternating radii in the next floral whorl.
