The Plant Body

THE ANATOMY OF LEAVES

Most leaves consist of a stalk, or petiole, and a flattened blade. Veins run from the petiole into the blade. Some leaves, particularly those of the grass-like plants, lack petioles and the base of the blade is attached directly to the stem. The large veins of dicot leaves usually diverge into the blade from the midrib; this is the pinnate (feather-like) pattern. In some dicot species the major veins diverge from the base of the leaf like fingers from a palm (palmate venation). In all dicots, the smaller veins (the smallest of which can barely be seen with the naked eye) have a reticulate (net-like) pattern, so dicots are said to have net venation. The veins of monocot leaves run parallel to one another; such leaves are said to have parallel venation.

Leaves of dicots may be either simple (a single blade attached to the stem by a petiole) or compound (a single blade divided into many distinct leaflets). The leaves are arranged on the twig in a manner that is constant for a given species (see figure below). Each leaf is some distance removed from the one immediately below it. If only one leaf is attached at a node, the leaves are alternate. If two leaves are formed at the same node, on opposite sides of the twig, the leaves are termed opposite. In a relatively small number of plants more than two leaves arise from the same node in a whorled arrangement. These arrangements are found on herbaceous stems as well as on twigs.

Fig. 1. Arrangement of leaves on a shoot: (A) Alternate; (B) Opposite; (C) Whorled

Almost all legumes have compound leaves; indeed, in some species the leaves are doubly-compound (each leaflet divided again). The soybean has a pinnately compound leaf but in some plant species the leaflets are arranged in a palmate pattern, and others have a whorled arrangement. What is the adaptive significance of the different leaf types, if any? Lots of debate - no conclusions.
How can you tell the difference between a simple leaf and a compound leaf? Leaflets are similar in appearance to simple leaves so it is sometimes difficult to determine whether the structure is a leaflet or a leaf. There are three criteria that you can use to distinguish leaflets from leaves: (1) Axillary buds are found in the axils of leaves, both simple and compound, but not in the axils of leaflets; (2) Leaves extend from the stem in various planes, whereas the leaflets of a given leaf all lie in the same plane; and (3) Dicot leaves, whether simple or compound, have a petiole that "grasps" the stem where it attaches. Leaflets do not have such an attachment.

Leaf Modifications:

Bud scales are modified leaves that protect dormant buds in perennial plants.

In some plants leaves are modified to form spines, which are had, dry, and nonphotosynthetic. The term “spine” and “thorn” are frequently used interchangeably, but technically thorns are modified branches that arise in the axil of leaves. Another term used incorrectly is prickle. A prickle is neither a stem nor a leave but a small, slender, sharp outgrowth from the cortex and epidermis of a stem. The so-called thorns on rose stems are prickles.

Tendrils are modified leaves that grow indefinitely, unlike normal leaves, and coil around objects they touch. This coiling is a growth response: when the tendril touches an object, the side facing the object stops growing while the other side continues to grow.

Insect traps have evolved from leaves in several families. Passive traps like the pitcher-leaves of several species secrete a watery digestive fluid. Numerous trichomes (hairs) point downward and lead insects to their doom. The sundew is an active trap: when an insect comes in contact with the sticky digestive fluid secreted by a glandular trichome, adjacent trichomes bend toward it and the entire leaf blade curls around the victim. The Venus flytrap is very active: when an insect trips two hairs in quick succession, or one hair twice, motor cells in the midrib quickly lose water causing the two halves of the modified leaf blade to move together. Glands secrete digestive fluid and the digested material is absorbed. Eventually, the motor cells regain water and the trap opens again, ready for a new victim.

Stems

The basic body plan of a plant consists of a root-shoot axis. The shoot consists of the stem and the leaves. Leaves are inserted on the stem at the nodes, and the stem regions between nodes are the internodes. The node is not just a surface feature of the stem: at this point the internal vascular tissues diverge into the leaf and bud. Later or axillary buds are formed during primary growth in the axils of the leaves (the axil is the upper angle between the leaf and the stem). Sometimes buds are too small to be easily seen, since they usually stop growth at a very early stage. If and when the bud grows out it produces a branch, i.e., a lateral shoot. Some plants have a single shoot because the axillary buds are suppressed in their growth by the apical meristem (terminal or apical bud). They can usually be forced to branch by removing the apical bud, a common gardening practice called pinching. Other plants may be normally bushy because this apical dominance is naturally limited.

Tallness in plants is determined largely by degree of expansion of internodes. For plants growing in a dense population, for example a forest, an obvious strategy is to increase internode length and therefore height so that the leaves will be exposed to the light. Some grasses, on the other hand, have extremely short internodes so that the all-important apical meristem hugs the ground and is not damaged by grazing buffalo (or lawn mowers); if the leaves are clipped off they just produce more.

Stems are variously and sometimes greatly modified in their structure and function in many plant species. Some stems are aerial; these modified stems can function in asexual (vegetative) reproduction and in food storage. The strawberry begonia and the strawberry itself can reproduce by stolons (runners) that produce new shoots periodically. Some stems are underground stems. A rhizome, such as found in the ginger plant, is a more or less horizontal underground stem. Rhizomes are used for storage and vegetative reproduction. The rings around the ginger rhizome are nodes. Tubers are fleshy underground stems used for storage. Sprouts arise from the 'eyes' (nodes). Bulbs (e.g., onions) are large buds consisting of a small conical stem with attached leaves. Food is stored in the thickened bases of the leaves.

TYPES OF STEM AND LEAF MODIFICATIONS

(click on image for larger version)

(A & B) stem tendrils; (C, F, and G) thorns; (D & E) leaf-like stems

External Features of Woody Stems

Note the buds on the twigs. Buds occur at the tips-the terminal buds-and in the axils of leaves-the axillary, or lateral, buds of the twigs.

After the leaves fall, leaf scars, with their bundle scars, can be seen beneath the axillary buds. The bundle scars are the severed ends of vascular bundles that extended into the petiole of the leaf prior to abscission.

Groups of terminal-bid-scale-scars reveal the location of previous terminal buds, and until they are obscured by secondary growth, these groups of scars may be used to determine the age of portions of the stem. The portion of the stem between two groups of such scars represents one year’s growth.

The lenticels appear as slightly raised areas on the stem.

External Features of Woody Stems

Root Systems

The first root of a plant originates in the embryo and is called the primary root. New cells are constantly being produced in the apical meristem and the root grows downward through the soil. The apical meristem of the root also replaces cells of the root cap that have been sloughed off. In dicots and conifers, the primary root becomes a taproot; it grows directly downward, giving rise to one large vertical root with smaller branch roots or lateral roots produced along the way. The older lateral roots are found near the neck of the root and the youngest near the root tip. This type of root system, one that develops from a taproot and its branches, is called a taproot system. In addition to anchorage and absorption, taproots store food for the plant and many taproots are used as food (e.g., carrots, turnips, radishes).

In monocots, the primary root is usually short-lived, and the root system develops from multiple adventitious roots that arise from the stem. Adventitious roots are roots that arise from anywhere other than from the bottom of the stem, i.e., from the stem above ground or even from leaves. Note the adventitious roots on the ivy. In the corn plant, a monocot, special adventitious (prop) roots grow out of the stem and help stabilize the plant against strong winds. The adventitious roots of monocots and their lateral roots give rise to an extensive fibrous root system, in which no one root is more prominent than the others. The root system of monocots is a fibrous system

Examine the roots on demonstration in the Study Center and determine whether they are fibrous or taproot systems.