Angiosperm Anatomy Essential Concepts

Angiosperm Anatomy – Essential Concepts

FLOWER ANATOMY

Obj 1: Identify and describe parts of a flower

The pedicel is the flower stalk that supports the receptacle, the stem tip that bears the flower parts. The flower parts consist of reproductive and auxiliary organs. The auxiliary organs, known as the perianth or floral envelope, include the sepals (collectively termed the calyx) and petals (collectively termed the corolla). Sepals are protective scales around the unopened bud and sometimes are petal-like in an open flower. On flowers where there is no clear distinction between petals and sepals, as in a tulip, the individual units are called tepals.

The sexual organs are the stamens and carpels. Stamens are the male reproductive organs that produce pollen and ultimately sperm. Stamens are made up of a usually tube-shaped anther, where pollen develops, and the filament, a stalk that lifts the anther to an effective position. Carpels are the female reproductive structures that bear and enclose ovules. A flower’s set of carpels is referred to as a pistil.

A carpel is made up of the ovary, where seeds develop and that ultimately becomes fruit; the stigma, the sticky receptive tip to which pollen adheres; and the style, an elongated “tube” connecting the stigma to the ovary.

The location of the ovary, with respect to the perianth, is an identification tool. An inferior ovary is embedded in the receptacle so other floral parts seem to arise from the top of the ovary. A superior ovary is borne above the points of origin of the sepals and petals from the receptacle.

In addition to classifying flowers based on the position of the ovary, flowers can be described by which flower parts are present. These flower types are complete (has sepals, petals, stamens, and carpels), perfect (has both stamens and carpels), staminate (has stamens but not carpels), and carpellate (has carpels but not stamens).

Some plants have both staminate and carpellate flowers on the same individual and are called monecious. When staminate and carpellate flowers are on different individuals, the plant is dioecious. Some plants have perfect and imperfect flowers on the same individual and are called polygamous.

LEAF ANATOMY

Obj 2: Identify parts of a leaf and ways of describing them.

Leaves are organs with a primary mission of collecting radiant energy (sunlight) to power photosynthesis. A leaf that is attached to a stem by a leaf stalk (petiole) is petiolate. A leaf that is attached directly to the stem is sessile. At the juncture of the petiole with the stem, small green appendages (stipules) are found in some plant species. The usually flattened part, the “leaf” is correctly referred to as the blade or lamina. Running through the petiole and the blade are bundles of vascular tissue that transport material into and out of the leaf. These bundles, which are often clearly visible, are called veins. Veins also provide some structural framework for the blades.

Leaves are often classified (and their plants identified) based on their venation (arrangement of veins). Leaves with veins that run the length of the leaf in parallel lines have parallel venation. Grasses, lilies have parallel venation. Oaks and geraniums, on the other hand, have net (or reticulate) venation with their veins running in various directions and sometimes crossing. Types of net venation include pinnate venation where the veins extend from a central rib, like pinna of a feather (e.g. elm); and palmate venation where the veins extend from a central point like the fingers from the palm of a hand (e.g. maple).

Leaf blades are further classified as being either simple (the blade is a single structure) or compound (the blade is divided into a number of leaflets). A sure way of determining if a “leaf” is a simple leaf or a leaflet on a compound leaf is to notice where the axillary bud is. All leaves have a bud at the point where they connect to the stem, while leaflets have no such bud.

Compound leaves are either pinnately compound (leaflets extend from a central stalk as in a rose) or palmately compound (leaflets extend from a central point as in a buckeye).

Leaf margins (edges) can be entire (smooth, lacking indentations, e.g. dogwood), toothed (e.g. elm), or lobed (indentations are deep, sometimes almost to the main veins, e.g. maple).

Leaf forms or outlines can be linear (e.g. grass), circular (e.g. nasturtium), lanceolate (e.g. willow), heart-shaped (e.g. redbud), ovate (e.g. apple) or many other shapes (see handout).

Leaf arrangement on a stem can be opposite (bearing two leaves or two buds at a node on opposite sides of a stem), alternate (a single leaf or bud occurs at a node) or whorled (having three or more leaves, or other parts, occurring at a node).

STEM ANATOMY of dicots (e.g. oak, rose)

Obj 3: Describe the structure of angiosperm stems.

The primary function of stems is support for above ground tissues (leaves and flowers) and transport of water and various substances through the plant. Stems can be distinguished from other parts of a plant because (1) they have well-marked nodes (where leaves and buds attach to stems), (2) they arise externally as branches from buds formed on the surface (root branches arise internally, and not from nodes), (3) they have leaves as their characteristic appendages, and (4) they mostly - but not always - grow up above the soil.

Obj 4: Distinguish monocot stems from dicot stems.

The tissues that make up stems of herbs, and the primary tissues of woody stems, are the epidermis, the vascular cambium, and vascular tissue. The epidermis is a single surface layer of cells and is usually cutinized and thus nearly waterproof. The vascular cambium is a single layer of meristematic (cell-producing) cells arranged in a circle around the stem that produces vascular tissue. There are two types of vascular tissue: xylem, which conducts water and dissolved substances and furnishes support to the stem (the major component of “wood”), and phloem (or inner bark) that transports various substances - particularly products of photosynthesis and hormones - from their source to where they are being used. Xylem and Phloem in some species is arranged in bundles around the stem (called vascular bundles) rather than being continuous. The cortex is composed of cells that add strength and store food and other materials. Pith is located in the center of the stem, functions as storage tissue, and is dead in older stems. Bark is a secondary tissue and is the outer layer of a stem, defined as all tissues outside the vascular cambium; includes phloem, cork, and other tissues. “Outer bark” refers to all tissues outside the conducting phloem while “inner bark” refers to conducting phloem tissue.

STEM ANATOMY of monocots (e.g. corn, lily)

Most monocots have no cambia in stems and thus have no secondary growth. Thus, all growth is from cells produced by terminal bud.

The xylem and phloem in monocots is always arranged in bundles scattered through the stem rather than arranged in a definite circle as in dicots.

STEMS WITH FUNCTIONS OTHER THAN TRANSPORT AND SUPPORT

Some stems do not rise above the ground and have functions other than transport and support. Stolons (runners) are horizontal stems that run above the soil surface, often developing new plants at nodes (e.g. strawberry and spider plants). Rhizomes are similar to stolons but run below the soil surface and are sometimes enlarged by food storage (and are then called tubers, e.g. potato). A true bulb (e.g. onion) is really a single large bud with a small stem at its basal end with numerous fleshy, scalelike leaves that grow from the upper surface; adventitious roots emerge from the bottom of the stem.

ROOT ANATOMY

Obj 5: Distinguish types of roots and their basic structures

Roots can be distinguished from other plant organs because (1) theygenerally grow downward into the soil, (2) they do not have nodes and internodes, (3) they have branches that arise internally, and (4) they bear absorptive root hairs close to the growing tips.

Diffuse root systems havenumerous slender roots, the main ones being nearly equal in size. Diffuse root systems can be FIBROUS (as in grass) or FLESHY (as in sweet potato where diffuse root becomes enlarged with stored food). Diffuse root systems can become woody with time.

Taproot systems (e.g. dandelion, carrot) are those systems where the primary root grows most rapidly and remains the largest root throughout the functional life of the root system. Taproots may be slender, fleshy or woody.

A root tip is bare but is usually slightly enlarged with a swelling called the rootcap that protects the meristem tissue that produces new cells. Behind the tip is region in which epidermal cells produce delicate, slender projections called root hairs that function to increase absorption capability and to anchor the growing tip.

Adventitious roots are those that arise from somewhere other than the root zone (usually a stem or leaf).