Herbaceous plants

The term herbaceous, in its botanical sense, is applied to plants that usually have no woody parts. In general, the leaves and stems of these plants die back at the end of the growing season. Perennial herbaceous plants overwinter by means of underground storage organs; annual herbaceous plants usually survive winter by means of seeds.

Herbaceous plants do, however, show extremely varied forms. One of the smallest is the aquatic species of duckweed Wolffia ar-rhiza, which is a minute green plant that floats on the surface of fresh water, it has no true roots or stem and is about 0.5 to 1.0 millimeter across. Conversely, herbaceous plants can grow to a large size; the taro (Colocasia escu-lentaJ produces leaves 3 feet (91 centimeters) or more in length. Other herbaceous forms include prostrate creepers, such as the creeping jenny (Lysimachia nummularia), and matlike plants, such as the pearlwort (Sagina sp.).

Herbaceous borders are a common feature in cultivated gardens. All garden flowers originated in the wild, but many have been selectively bred to form hybrids. The longer-lasting perennials are generally planted at the back of a herbaceous border where they are allowed to grow tall. Biennials and annuals are usually smaller plants and are planted at the front of the border where they can be easily replaced.

Perennials

There are three basic types of herbaceous plant, categorized according to their life cycles: perennials, biennials, and annuals. Herbaceous perennials live for a number of years, surviving from one growing season to another. In temperate climates, perennials must survive a winter between two summers, and in tropical areas they may have to live through a dry season between two wet seasons. This survival is generally achieved by means of a surface-dwelling or underground organ (which is also by a means of vegetative reproduction). These organs lie dormant during the cold season and produce new aerial parts each year, which die back at the end of the growing season.

The life span of perennials varies from species to species. Some plants are generally short-lived and others, such as the peonies (Paeonia spp.), may persist for many years. Many familiar garden flowers are herbaceous perennials and include such plants as Michaelmas daisies (Aster spp.), pampas grass (Cortaderia selloana), dahlias (Dahlia spp.), chrysanthemums (Chrysanthemum spp.), and numerous plants that have bulbs, corms, and tubers.

Many terrestrial and epiphytic tropical plants—the bromeliads (Bromeliaceae), for example—are also herbaceous perennials although their aerial parts persist from one growing season to another.

A weed is any plant that grows where it is not wanted. Dandelions are successful weeds because they are fast growers and can survive even when most of their leaves have been removed.

Biennials

As their name suggests, biennials live for two years. In the first year, the seedling grows into a rosette of leaves attached to an underground storage organ. This storage organ becomes swollen with food at the end of the growing season and, in the following year, produces a flowering stem with a few sparse leaves. After the seeds have been dispersed, the whole plant—including the underground storage organ—dies away.

Occasionally an axillary bud at the base of the stem remains; this will develop as another basal rosette for the following year, with the result that seeds are produced every year rather than every two years. The common foxglove (Digitalispurpurea) has axillary buds that persist regularly, as does the garden hollyhock (Althaea rosea).

Many crop species are biennials and include carrots (Daucus carota), parsnips (Pas-tinaca sativa), and some species of cabbage.

Culinary herbs, as their name suggests, are mostly herbaceous perennials and annuals used in cooking throughout the world. Sweet basil, which is native to India and Iran, is often used in tomato-based recipes, whereas dill is a European herb used in sauces and salads.

Annuals

Each year, a new crop of annual plants grows from the previous season’s seeds. The germinating seedlings mature, flower, scatter their seeds, and die within one growing season. Some of the seeds may survive in a dormant state for several years, until conditions are right for germination. Many culinary herbs are herbaceous annuals and include basil (Oci-mum basilicum), caraway (Carum carvi), coriander (Coriandrum sativum), and dill (Anethum graveolens). Annuals grown as crops include peas (Pisum sativum), lentils (Lens culinaris), chick peas (Cicer arietinum), and all the grain crops.

Similar in character to the above annuals are shorter-lived annuals. These plants have an even shorter life cycle. They manage to compress the period from germination through seed dispersal to death of the parent plant into a few weeks. If conditions are favorable, several life cycles may take place in the space of one growing season. This group of plants includes many weeds and desert plants. Groundsel (Senecio vulgaris) and, in Europe, shepherd’s purse (Capsella bursapastoris) are typical ephemerals and, as a result, are successful weeds.

Passion flowers (Passiflora spp.) are herbaceous vines that climb with the help of tendrils. Many species are cultivated for their brightly colored flowers and their edible fruits. This plant grows wild in the Bolivian Andes at an altitude of about 6,000 feet (1,800 meters).

Stem structure

All flowering plants, both monocotyledons and dicotyledons, can be separated into two groups on the basis of their stem support structure. Plants such as trees and shrubs have persistent woody aerial parts that survive for a number of years. Nonwoody or herbaceous plants do not have persistent aerial parts although the stems of some species, such as goldenrod (Solidago sp.) and sunflower (Heli-anthus sp.), may show some secondary thickening.

The stems of small and short-lived herbaceous plants depend mostly for their support on living tissues with thickened walls (collen-chyma) found on the outer areas of the stem and on the veins of the leaf. All herbaceous plants, however, have some strengthening tissues associated with the vascular bundles and, in most species, there is some production of secondary tissue within the bundles. Other herbaceous plants, such as the garden hollyhock, have dead, thick-walled tissue (scleren-chyma), which often contains lignin and helps to support the stem. The distribution of scle-renchyma is often related to the areas of the plant that are subjected to mechanical stress.

A transverse section of a leaf shows a central vein (vascular bundle) with supporting fiber cells with thick walls below and bundle sheath cells at the sides and top. Leaves are kept rigid by their veins and the palisade and spongy mesophyll cells that are full of water (turgid).

Roots and storage parts

Herbaceous plants have many different types of root system and underground storage parts. Fibrous root systems are common in herbaceous perennials and grasses, whereas taproots are found in plants such as carrot and dandelion (Taraxacum spp.).

In some parasitic species, the seed germinates and the root grows underground without producing an aerial shoot, except at intervals when the plant is ready to reproduce. The curious-looking bird’s nest orchid (Neottia nidus-avis) and Indian pipe orchid (Monotropa uniflora) produce an aerial stem only when the plant is flowering, with simple yellow or pink leaves and flowers that hang from the end of a branch. Other species of orchid grow in a similar way, such as the coral root (Corallorrhiza sp.) and ghost orchids (Epipogium spp.).

Certain species have no roots and produce only shoots. In the case of such species, the plant’s stem takes over the water- and mineralabsorbing function of the roots. Typical of these species are the aquatic and terrestrial bladderworts (Utricularia spp.), which have modified leaves—known as rhizophylls—that behave like roots.

Rhizomes make up the permanent axis of many herbaceous perennials and are swollen stems (usually partially or totally underground). Whereas aerial shoots may die down at the end of each growing season, rhizomes survive to produce new shoots the following year. Similarly, bulbs are the swollen extensions of perennial stems and are formed from a mass of modified leaf bases. Some perennials rely on underground storage parts called corms, which are also swollen stem bases.

The “root” vegetables that we eat are not always true roots in the botanical sense. Potatoes are the swollen extensions of underground stems (tubers) of herbaceous plants, whereas onions are modified storage leaves (bulbs) of herbaceous monocotyledons. Carrots and radishes, however, are the swollen roots of herbaceous plants.

Types of leaves and tendrils

Herbaceous leaves show a great variety of shapes. The most common leaf form is swordshaped or linear, as produced by the irises (iris spp.), grasses (Craminaceae), and other monocotyledons. Leaf edges can be smooth or have simple serrations or they may have deep regular or irregular incisions—for example, those of the dandelion. A further development of this form is the pinnate leaf, which is a characteristic of poppies (Papaver spp.) and many umbellifers (Umbelliferae).

Other sorts of simple leaf form include palmate (as in geraniums, Geranium spp.), spear-shaped (as in some arums, Arum spp.), heart-shaped (as in the lesser celandine, Ranunculus ficaria), and circular (as in penny-wort, Hydro-cotyie spp.). Leaves like those of the lupine (Lupinus spp.) are said to be digitate, and those of hellebores (Helleborus spp.) are pedate.

Carnivorous herbaceous plants have a variety of unusual leaf forms that are usually adaptations to their capturing insects. Pitcher plants (Sarracenia purpurea), as their name suggests, have funnel-like leaves that trap insects that are subsequently digested by enzymes. The leaves of sundews (Drosera spp.) are covered with sticky glandular hairs that trap insects.

The Venus’s-flytrap (Dionaea muscipula), another carnivorous plant, has a spring mechanism that allows the leaves to snap shut when the trigger hairs on their surface are touched, thus trapping any insect that may be on the leaves.

Some herbaceous plants have tendrils (modified leaves) for climbing—for example, the legumes (Papilionaceae). In some climbing herbaceous plants, tendrils are entirely separate parts of the plant. The cucumber family (Cu-curbitaceae) has tendrils that grow from the leaf base, and in white bryony (Bryonia dioical half of each tendril twists in one direction and half twists in the other direction so as to pull the plant closer to its support. In certain species, it is the plant’s branches that are the means of support; in others, it is the stem itself that twines, such as bindweed (Convolvulus sp.).

Some species are scramblers, supporting themselves on other, taller vegetation by means of hooks and stiff hairs—for example, bedstraw (Galium aparine). Species such as dodder (Cuscuta sp.) are parasitic climbers that obtain their nutrients from the host’s stem. Others are parasitic on the roots of plants, such as the broomrape (Lathraea ciandestina). These plants may have no aerial shoots, and their flowers are produced at the soil surface.

Cartwheel flower (Hera-deum mantegazzianum) is a fast-growing perennial that can reach up to 13 feet (4 meters) in height in one growing season. It is native to the Caucasus but is grown elsewhere as an ornamental plant.

Growth rate

Herbaceous plants can produce a phenomenal amount of growth each year. Plants such as rhubarb (Rheum rhaponticum) and butterbur (Petasites hybridus) produce leaves that often measure 3 feet (91 centimeters) or more in diameter, and some temperate species, such as gunnera (Gunnera manicata), can grow leaves that are twice this size.

Perennial climbers such as bindweed (Convolvulus sp.) produce many feet of growth each year and are unpopular with gardeners, as is morning glory (Ipomoea sp.). Many biennials and perennials produce stems up to 6 feet (1.8 meters) tall with a similar-sized spread of side-shoots—for example, the teasel and cartwheel flower (Heradeum mantegazzi-anum). These species have perennating organs that allow them to start growing early in spring. Himalayan balsam Umpatiensglandulif-era), which is an annual plant, produces as much growth starting from seed each year as some perennial species. Its stem may measure up to 2 inches (5 centimeters) at the base and is produced in about 5 months.

Annuals are plants that may condense their growing season into a few weeks. In Central Australia, where rain is scarce, plants take advantage of a summer shower to germinate, flower, and scatter their seeds.