Plant Anatomy

Unlike mammals, plants have only a few basic organs:inflorescencesleavesstemsroots

Each has its own structure, but there are features (tissues or cell types) in common among a number of these ‘organs’.

(cork)

(related to cork cambium, but longer lived)

Each organ has an epidermis, the outermost cells. Generally, this is a single layer of flattened cells. Leaves and stems secrete a protective layer of cutin (a wax, forming the cuticle) on the outside surface of the epidermis.

Some plants have trichomes (hair-like projections) from the epidermis. Trichomes may aid in protection from insect herbivory, and/or may be glandular. Tomato leaves (shown below) are an example.

In woody plants, as they become woody, the epidermis is replaced by a surface tissue called periderm. The periderm consists of the phelloderm (a continuous source for cork cambium), the cork cambium (which produces the cork, cells that are dead and empty at maturity), and the cork layer. That is the surface structure depicted on the earlier slide.

There are 3 tissues in common among plant organs: parenchyma,

collenchyma, and sclerenchyma

Where do we find those tissue types and what are their functions?

Tissue type Characteristics Location Function

Parenchyma living cells, primary cortex and basic cell walls only, pith of metabolism usually spherical roots, stems; or only somewhat xylem, phloem elongated leaf mesophyll

Collenchyma living cells, leaf petioles, support elongated young stems, uneven cell walls petals

Sclerenchyma non-living at fibers: support maturity, xylem, phloem, elongate or various cortex; sclereids: thickened secondary cortex, pith, cell walls mesophyll

Parenchyma Collenchyma Sclerenchyma

Throughout the plant body there must be a means to move water, minerals, and photosynthate. The vascular system achieves that. It is comprised of two systems:

xylem – conducts water and minerals from the roots upward

phloem – transports organic materials synthesized by the plant

Vascular bundles ina dicot stem

phloem

xylem

There is important structure within the phloem and xylem bundles, and they’re different.

Xylem is comprised of tracheids, vessel elements, fibers, and some parenchyma.

Vessel elements indicate that this is an Angiosperm

Tracheids function in support. They have both a primary and a secondary cell wall. At maturity, these cells are dead. They function in support, but they are also the only conducting cell in vascular plants other than Angiosperms. They have numerous pits along their lateral cell walls, so that water and minerals can move between cells.

Vessel elements are shorter, wider, and have either many perforations in their end cell walls, or those end walls have virtually disappeared. They become what their name suggests – pipelike tubes. Remember, they are only present in the xylem of Angiosperms.

Phloem is basically composed of sieve tube members. There are also companion cells, fibers, and parenchyma.

Sieve tube members are (strange) living cells. They have only primary cell walls, and, when mature, the nucleus and most organelles have degenerated. Their end walls connect neighboring cells with sieve plates.

Sieve tube members are closely associated with companion cells (both developmentally and physiologically). There are numerous plasmodesmata between the cells; that permits the companion cell to control transfer of organic material into the sieve tube. The companion cells also have large nuclei, and metabolically regulate sieve tube members.

You’ve already seen one picture of the xylem and phloem in a stem. Is there more structure? Of course!

Each vascular bundle is (generally) surrounded by a bundle sheath, and how they are placed within a stem differs in differing types of plants. In monocots, the vascular bundles are scattered through the stem; in dicots they form a ring.

In a monocot stem:

xylem

phloem

bundle sheath

phloem

xylem

In a dicot stem:

parenchyma

(parenchyma and fibers)

Annual rings in the cross section of a woody plant represent the annual growth of xylem from the vascular cambium. The rings are visible because the cells of spring growth (called springwood) are larger (due to the wetter conditions) and apparently lighter in colour than those produced during summer (summerwood). The size (thickness) of annual rings can be used to estimate the climate during the year of formation. Climates covering a number of centuries can be evaluated using this method (called dendrochronology),.

This is an important tool in estimating climate change.

Roots

Roots have a meristem (growth region) protected by a root cap. Just behind that is a region of the root where cells elongate.

In cross section, the center of the root contains the vascular bundles (the vascular cylinder). It is called the stele.

In the center of the stele is the xylem, usually star-shaped (i.e. having projections outward) in dicots. Between the arms of the star is the phloem. In monocots there is a ring of vascular bundles, alternating xylem and phloem.

In both root forms the outer layer of the stele is called the pericycle. This tissue is meristematic, that is it can give rise to new growth in the form of root branches.

From the surface epidermis project root hairs. They are key to maximizing absorption of water and minerals, enormously increasing the effective surface area of the roots.

Roots have differing patterns of growth in different habitat conditions and among species. The basic difference is between a taproot design – a thick principal root from which branches develop, and a branched fibrous root system – many essentially equal diameter roots with branching.

fibrous roots of barley

tap root of a dandelion

By having differing types of roots and extension to different depths, plants can reduce the intensity of competition for water and nutrients. On the prairies of central North America, some plants have roots that go down < 1m, while others, e.g. Andropogon gerardii, the characteristic grass of tall grass prairie, extend down at least 4m, and Rosa suffulta, the prairie rose, has roots that extend> 7m.

In addition to totally below ground roots, some species have adventitious roots. These roots originate on leaves and stems above ground. Corn has prop roots that originate from the stem just above ground. Banyan trees from Australia have extensive aerial roots, reaching down to the soil from far up in the branches of the tree. Mangroves have extensive, spreading adventitious roots above the surface of the water.

corn – prop roots banyan aerial roots

mangrove mangle

Finally, roots act as storage organs of various types in many species:

Leaves

Leaves differ in shape, edge pattern, and organization on the stem. First, leaf shape:

Then, leaf edges:

Leaves differ in the pattern of veination…in monocots the usual pattern is parallel veins; in dicots the pattern is more frequently net veins.

Whether dicot or monocot, the basic organization of leaf parts and their attachment to the stem have many similarities…

And, finally, arrangement on the stem:

Leaves are the source of many adaptations plants have made to drought…

Flowers

There are 4 basic parts of a flower organized into what are usually called four whorls.

1. The whorl of sepals, collectively called the calyx

2. The whorl of petals, collectively called the corolla

3. The whorl of male structures, the androecium, made up of pollen-producing anthers, each supported by a filament

4. The whorl of female structures, called the gynoecium, consisting of the carpel(s). A single carpel may consist of a simple pistil, or a fused compound pistil. Each carpel, of whatever type, is made up of a stigma (the receptive surface), a style (the supporting column), and an ovary.

An inflorescence – the set of flowers on a plant – can be organized in different ways…

Flowers can be ‘perfect’, having both male and female parts, or can be ‘imperfect’, lacking either male or female parts.

If the male parts are lacking, then the flowers are called pistillate. If the female parts are lacking, they are called staminate.

If male and female flowers are separately present on a single plant, the plant is described as monoecious. If individual plants have flowers of only one sex, the species is described as dioecious.

The arctic birch I study is an example of a monoecious species. Sugar maples, though they can switch sexes over the course of their lives, are a dioecious species, since each tree has only one sex of flowers in any year.

In arctic birch, male flowers appear before female flowers each year. This picture is of female flowers. Each inflorescence takes the form of a catkin (like a cone) and the receptive surfaces of individual flowers are visible as red-purple, sticking out from the catkins.

In plant taxonomy, one important characteristic used in determining species is the position of the ovary in relation to the remaining whorls of the flower.

If the sepals, petals and androecium are inserted (connected) below the ovary, it (the ovary) is described as superior and the flower is described as hypogynous.

If the other whorls are inserted above the ovary, the ovary is described as inferior and the flower as epigynous.

Finally, if the other whorls are inserted around the gynoecium at the same level, the flower is called perigynous.

Here are diagrammatic images of the three organizational patterns:

Now let’s consider how the whole plant goes together, comparing monocot to dicot morphology…