Structure of (Flowering) Plants: Roots, Stems, Leaves
The basic angiosperm body has three parts: roots, stems, and leaves. These primary organs constitute the vegetative (nonreproductive) plant body. Together, the stem and its attached leaves constitute the shoot. Collectively, the roots of an individual plant make up the root system and the shoots the shoot system.
Root systems
The roots anchor a plant, absorb water and minerals, and provide a storage area for food. The two basic types of root systems are a primary root system and an adventitious root system. The most common type, the primary system, consists of a taproot (primary root) that grows vertically downward (positive geotropism). From the taproot are produced smaller lateral roots (secondary roots) that grow horizontally or diagonally. These secondary roots further produce their own smaller lateral roots (tertiary roots). Thus, many orders of roots of descending size are produced from a single prominent root, the taproot. Most eudicotyledons produce taproots—for example, the dandelion (Taraxacum officinale).
In some cases, the taproot system is modified into a fibrous, or diffuse, system, in which the initial secondary roots soon equal or exceed the primary root in size. The result is several large, positively geotropic roots that produce higher-order roots, which may also grow to the same size. Thus, in fibrous root systems there is no well-defined single taproot. In general, fibrous root systems are shallower than taproot systems.
The second type of root system, the adventitious root system, differs from the primary variety in that the primary root is often short-lived and is replaced or supplemented by many roots that form from the stem. Most monocotyledons have adventitious roots; examples include orchids (Orchidaceae), bromeliads (Bromeliaceae), and many other epiphytic plants in the tropics. Grasses (Poaceae) and many other monocotyledons produce fibrous root systems with the development of adventitious roots.
Adventitious roots, when modified for aerial support, are called prop roots, as in corn or some figs (Ficus; Moraceae). In many tropical rainforest trees, large woody prop roots develop from adventitious roots on horizontal branches and provide additional anchorage and support. Many bulbous plants have contractile adventitious roots that pull the bulb deeper into the ground as it grows. Climbing plants often grip their supports with specialized adventitious roots. Some lateral roots of mangroves become specialized as pneumatophores in saline mud flats; pneumatophores are lateral roots that grow upward (negative geotropism) for varying distances and function as the site of oxygen intake for the submerged primary root system. The plants mentioned above are only a few examples of root diversity in angiosperms, a condition that is unparalleled in any other vascular plant group.
Many primary root and adventitious root systems have become modified for special functions, the most common being the formation of tuberous (fleshy) roots for food storage. For example, carrots and beets are tuberous roots that are modified from taproots, and cassava (manioc) is a tuberous root that is modified from an adventitious root. (Tubers, on the other hand, are modified, fleshy, underground stems and will be discussed below.)
Stems
The stem is an aerial axis of the plant that bears leaves and flowers and conducts water and minerals from the roots and food from the site of synthesis to areas where it is to be used. The main stem of a plant is continuous with the root system through a transition region called the hypocotyl. In the developing embryo, the hypocotyl is the embryonic axis that bears the seedling leaves (cotyledons).
In a maturing stem, the area where a leaf attaches to the stem is called a node, and the region between successive nodes is called an internode. Stems bear leafy shoots (branches) at the nodes, which arise from buds (dormant shoots). Lateral branches develop either from axillary, or lateral, buds found in the angle between the leaf and the stem or from terminal buds at the end of the shoot. In temperate-climate plants these buds have extended periods of dormancy, whereas in tropical plants the period of dormancy is either very short or nonexistent.
The precise positional relationship of stem, leaf, and axillary bud is important to understanding the diversity of the shoot system in angiosperms. Understanding this relationship makes it possible to identify organs such as leaves that are so highly modified they no longer look like leaves, or stems that are so modified that they resemble leaves.
Branching in angiosperms may be dichotomous or axillary. In dichotomous branching, the branches form as a result of an equal division of a terminal bud (i.e., a bud formed at the apex of a stem) into two equal branches that are not derived from axillary buds, although axillary buds are present elsewhere on the plant body. The few examples of dichotomous branching among angiosperms are found only in some cacti, palms (Arecaceae), and bird-of-paradise plants (Strelitziaceae).
The two modes of axillary branching in angiosperms are monopodial and sympodial. Monopodial branching occurs when the terminal bud continues to grow as a central leader shoot and the lateral branches remain subordinate—e.g., beech trees (Fagus; Fagaceae). Sympodial branching occurs when the terminal bud ceases to grow (usually because a terminal flower has formed) and an axillary bud or buds become new leader shoots, called renewal shoots—e.g., the Joshua tree (Yucca brevifolia; Asparagaceae). Plants with monopodial growth are usually pyramidal in overall shape, while those with sympodial growth often resemble a candelabra.
By combining monopodial and sympodial branching in one plant, many different tree architectures have evolved. A simple example is found in dogwoods (Cornus; Cornaceae), where the main axis is monopodial and the lateral branches are sympodial.
Very different plant forms result from simply changing the lengths of the internodes. Extreme shortening of the internodes results in rosette plants, such as lettuce (Lactuca sativa; Asteraceae), in which the leaves develop but the internodes between them do not elongate until the plant bolts
when flowering. Extreme lengthening of the internodes often results in twining vines, as in the yam (Dioscorea esculenta; Dioscoreaceae).
Leaves
The basic angiosperm leaf is composed of a leaf base, two stipules, a petiole, and a blade (lamina). The leaf base is the slightly expanded area where the leaf attaches to the stem. The paired stipules, when present, are located on each side of the leaf base and may resemble scales, spines, glands, or leaflike structures. The petiole is a stalk that connects the blade with the leaf base. The blade is the major photosynthetic surface of the plant and appears green and flattened in a plane perpendicular to the stem.
When only a single blade is inserted directly on the petiole, the leaf is called simple. Simple leaves may be variously lobed along their margins. The margins of simple leaves may be entire and smooth or they may be lobed in various ways. The coarse teeth of dentate margins project at right angles, while those of serrate margins point toward the leaf apex. Crenulate margins have rounded teeth or scalloped margins. Leaf margins of simple leaves may be lobed in one of two patterns, pinnate or palmate. In pinnately lobed margins the leaf blade (lamina) is indented equally deep along each side of the midrib (as in the white oak, Quercus alba; Fagaceae), and in palmately lobed margins the lamina is indented along several major veins (as in the red maple, Acer rubrum; Sapindaceae). A great variety of base and apex shapes also are found.
(The willow leaf is simple. The walnut leaf is pinnately compound, or feather-shaped. The horse chestnut leaf is palmately compound, or hand-shaped.)
Many leaves contain only some of these leaf parts; for example, many leaves lack a petiole and so are attached directly to the stem (sessile), and others lack stipules (
The three patterns of leaf arrangement on stems in angiosperms are alternate, opposite (paired), and whorled. In alternate-leaved plants, the leaves are single at each node and borne along the stem alternately in an ascending spiral. In opposite-leaved plants, the leaves are paired at a node and borne opposite to each other. A plant has whorled leaves when there are three or more equally spaced leaves at a node.
Leaf modifications
Whole leaves or parts of leaves are often modified for special functions, such as for climbing and substrate attachment, storage, protection against predation or climatic conditions, or trapping and digesting insect prey. In temperate trees, leaves are simply protective bud scales; in the spring when shoot growth is resumed, they often exhibit a complete growth series from bud scales to fully developed leaves.
Stipules often develop before the rest of the leaf; they protect the young blade and then are often shed when the leaf matures. Spines are also modified leaves. In cacti, spines are wholly transformed leaves that protect the plant from herbivores, radiate heat from the stem during the day, and collect and drip condensed water vapor during the cooler night. In the many species of the spurge family (Euphorbiaceae), the stipules are modified into paired stipular spines and the blade develops fully. In ocotillo (Fouquieria splendens; Fouquieriaceae), the blade falls off and the petiole remains as a spine.
Many desert plants, such as stoneplants (Lithops; Aizoaceae) and aloe (Aloe; Asphodelaceae), develop succulent leaves for water storage. The most common form of storage leaves are the succulent leaf bases of underground bulbs (e.g., tulip and Crocus) that serve as either water- or food-storage organs or both. Many nonparasitic plants that grow on the surfaces of other plants (epiphytes), such as some of the bromeliads, absorb water through specialized hairs on the surfaces of their leaves. In the water hyacinth (Eichhornia crassipes), swollen petioles keep the plant afloat.
Leaves or leaf parts may be modified to provide support. Tendrils and hooks are the most common of these modifications. In the flame lily (Gloriosa superba; Colchicaceae), the leaf tip of the blade elongates into a tendril and twines around other plants for support. In the garden pea (Pisum sativum; Fabaceae), the terminal leaflet of the compound leaf develops as a tendril. In nasturtium (Tropaeolum majus; Tropaeolaceae) and Clematis (Ranunculaceae), the petioles coil around other plants for support. In catbrier (Smilax; Smilacaceae), the stipules function as tendrils. In certain vining angiosperms with compound leaves, some of the leaflets have modified into grapnel-like hooks—e.g., Tecoma radicans. Many monocotyledons have sheathing leaf bases that are concentrically arranged and form a pseudotrunk, as in banana (Musa). In many epiphytic bromeliads, the pseudotrunk also functions as a water reservoir.
Carnivorous plants use their leaves to attract and trap insects. Glands in the leaves secrete enzymes that digest the captured insects, and the leaves then absorb the nitrogenous compounds (amino acids) and other products of digestion. Plants that use insects as a nitrogen source tend to grow in nitrogen-deficient soils.
Shoot system modifications
Entire shoot systems are often modified for such special functions as climbing, protection, adaptation to arid habitats, and water or food storage. The modifications generally involve structural and shape changes to the stem and the reduction of the leaves to small scales. Many of the modifications parallel those previously described for leaves. In the passion flower (Passiflora; Passifloraceae) and grape (Vitis vinifera; Vitaceae), axillary buds develop as tendrils with reduced leaves and suppressed axillary buds. In the grape these axillary tendrils are actually modified and reduced inflorescences. In the plant from which strychnine is obtained (Strychnos nux-vomica), the axillary buds develop into hooks for climbing. The tendrils of English ivy (Hedera helix; Araliaceae) produce enlarged cuplike holdfasts.
Thorns represent the modification of an axillary shoot system in which the leaves are reduced and die quickly and the stems are heavily sclerified and grow for only a limited time (determinate growth). Thorns appear to protect the plant against herbivores. Examples are found in the Bougainvillea (Nyctaginaceae), where the thorn is a modified inflorescence, the honey locust (Gleditsia triacanthos; Fabaceae), the anchor plant (Colletia paradoxa; Rhamnaceae), and Citrus (Rutaceae).
Cladodes (also called cladophylls or phylloclades) are shoot systems in which leaves do not develop; rather, the stems become flattened and assume the photosynthetic functions of the plant. In asparagus (Asparagus officinalis; Asparagaceae), the scales found on the asparagus spears are the true leaves. If the thick, fleshy asparagus spears continue to grow, flat, green, leaflike structures called cladodes develop in the axils of the scale leaves. The presence of cladodes in unrelated desert angiosperm families is an excellent example of convergent evolution, or the independent development of the same characteristic in unrelated taxa.
All cacti (Cactaceae) have cladodes, and many desert members of the spurge (Euphorbiaceae) and milkweed (Apocynaceae) families have similar vegetative morphologies that are derived by modifying different parts to look and function in the same way. Each of these plant groups has columnar, water-storing green stems, reduced leaves, and protective spines or thorns. They are often called stem succulents. In the cacti, the leaves on the main stems last for a very short time (they do not even develop as scale leaves) and the leaves of the axillary buds (the round cushion areas, or areoles, on the trunks) develop as spines. In many members of the Euphorbiaceae, the leaves on the main stems are green but short-lived, and the stipules develop as spines. In a number of plants of the Apocynaceae family, the leaves are also small and ephemeral, and the axillary buds develop as thorns. The cacti are almost exclusively New World plants adapted to dry or arid habitats, and the Euphorbiaceae and Apocynaceae occur in similar habitats in Asia and Africa. The reduction of leaves is so extreme in the Cactaceae that the epiphytic cacti (e.g., Epiphyllum) of the Neotropics can no longer produce leaves; rather, they produce thin, flat cladodes that superficially resemble leaves.
Many shoot systems have been modified into organs of food storage, reproduction, or both, called rhizomes, tubers, and corms. Rhizomes are distinguished from roots in having nodes with reduced leaves and internodes. Rhizomes are horizontal, usually subterranean shoots with scale leaves and adventitious roots on the underside. Their chief functions are vegetative reproduction and food storage; food stored in the rhizomes allows these plants to survive drought and extended winters. Most rhizomes are perennial, sending up new shoots from the nodes and spreading the colony. Often the terminal bud of a rhizome becomes upright and then flowers, with a rhizome axillary bud becoming a renewal shoot. Many economically important plants, such as banana, and almost all grasses, including bamboo, and sugarcane, have rhizomes. Such plants are propagated primarily by fragmentation of the rhizome. In some plants, the growing tips of rhizomes become much enlarged food storage organs called tubers. The common potato (Solanum tuberosum; Solanaceae) forms such tubers. The much-reduced scale leaves and their associated axillary buds form the eyes of the potato. Tubers should not be confused with tuberous roots. Tubers are modified shoots, whereas tuberous roots are modified roots. The common feature, and hence the similar names, derives from the fleshy nature of both organs. Tubers and tuberous roots function in water and food storage, but only tubers are involved in vegetative (nonsexual) reproduction. Tuberous roots develop from taproots in carrots and form adventitious roots in dahlias (Dahlia; Asteraceae).
Another distinctive modification for food storage is the corm, a short, upright shoot system with a thick, hard stem covered with thin membranous scale leaves as in jack-in-the-pulpit (Arisaema triphyllum; Araceae) and gladiolus (Gladiolus; Iridaceae). Corms are usually hard and fibrous and function for overwintering and drought resistance.
Slender creeping stems that grow above the soil surface are called stolons, or runners. Stolons have scale leaves and can develop roots and, therefore, new plants, either terminally or at a node. In the strawberry (Fragaria; Rosaceae), the stolons are used for propagation: buds appear at nodes along the stolons and develop into new strawberry plants.