Ferns

• Have roots, stems and leaves (fronds)
  
• Live in damp, shady areas.
  
• Have vascular systems comprised of a series of tubes that run from roots to stems to leaves
  
• There about 12,000 species in existence today. Although ferns are of little economic value today, the large species that existed 300 mya lived in swamps in abiotic atmospheres. Their slow decomposition produced the fossil fuels we use today.

Ginkos

• Only one species exists today; called a "living fossil"
  
• More resistant to air pollution than many trees therefore see it in urban surroundings (look for ginko trees along MacArthur Avenue here on campus)
  
• Separate male and female plants. (Usually only male plants are planted because the female plant produces seeds with fleshy coverings that can irritate skin. The seeds also quickly rot and give off a putrid smell)
  
• The herbal compound, ginkoba, comes from ginko trees.

Gymnosperms

• Name means "naked seed" --- seeds have no outer shell and are hidden among scales of cones.
  
• Most gymnosperms are evergreens.
  
• Some plants have male and female structures on the same plant; others have separate plants for each sex.
  
• Most can withstand cold conditions, as sap within them contains an antifreeze-like substance that allows transport of nutrients in sub-freezing weather
  
• Fewer than 1000 species exist today.

Angiosperms

• Classified according to differences in leaves, flowers and fruits.
  
• Leaves are usually broad, giving more surface area for collection of sunlight.
  
• Broad surface of leaves also allows for more water loss; systems in place to counteract this. (see later)
  
• Most angiosperms lose their leaves in the fall, due to a loss of chlorophyll
  
• More than 350,000 species of angiosperms are living today.

Additionally, angiosperms can be further divided into 2 classes, monocotyledons (monocots) and dicotyledons (dicots). Fossil records show that dicots developed first, followed by the monocots. The following table lists the differentiating characteristics of the two classes:

PARTS OF A PLANT

The example for this study is the division of angiosperms.

A. Roots

Roots (or, more specifically, the root system – the main root and all its branches) are important to a plant for three main reasons:

• To anchor the plant into the ground and give it support
  
• To absorb water and nutrients
  
• To produce growth hormones for the plant

(You can find diagrams in your textbook on pages 114-15.)

Dicots

1. Epidermis – forms the outer layer of the root. It is only a single layer of cells. These cells form the root hairs.
2. Cortex – Next layer of cells, going inward. It is composed of irregular shaped cells called parenchyma. These cells are loosely packed, so that water and minerals can moves through the cortex area without entering into the cells. The function of cortex area: food storage
3. Endodermis – The next innermost layer is a single-cell layer that forms the boundary between the cortex and the inner vascular bundle. Cells here fit snugly, and contain substances that make the layer impermeable to water and mineral ions. (Only access to the vascular bundle is from the endodermal cells.)
4. Vascular cylinder or bundle – Cells here retain ability to divide and can give rise to secondary roots here. This area contains the vascular tissues:

• xylem – cells that are arranged like a star within the bundle. These cells transport water and mineral solutes in plants.
  
• phloem --- cells fill in the areas between the star-like radiating arms of xylem cells in the bundles. These cells conduct transport of organic solutes within plants.

Monocots

Roots in monocots differ from those in dicots by:

1. Not having as much secondary growth
2. Have a central tissue called pith, which is surrounded by alternating bundles of xylem and phloem throughout.

Specialized Roots

Some roots take on specific additional functions. These specialized tasks include:

1. Food storage roots -- sweet potatoes, yams. Parenchymal cells store large amounts of carbohydrates.
2. Water storage roots – Pumpkins and other plants that grow in arid regions.
3. Propagative roots – Produce adventitious buds (buds appearing in unusual places) along the root. Buds then develop into aerial stems. Ex – As seen in our own ecosystem: Canadian thistle, Tree of Heaven

B. Stems

Purposes for the stems of plants include:

• Give support to leaves, allowing them to be exposed to as much light as possible.
  
• Support vascular bundles
  
• Storage of water and nutrients in some plants (cacti)

Growth of stems is comparable to that of roots.

Primary growth comes from the apical meristem at the stem tip. This tissue is called shoot apical meristem. This tissue is within a terminal bud that is protected by a leaf primordia(an envelope of immature leaves. The terminal bud becomes dormant in cold weather in temperate areas, protected by bud scales. In the spring, these scales fall of and leave a scar.

Leaf primordia are produced by apical meristem at regular intervals. The area of a stem between these nodes is called internode area. When a stem grows, all growth takes place in the internode areas.

A young stem is called a shoot. In addition to the specialized leaf primordia found here, three other tissues arise from the primary meristematic tissue:

• protoderm – outermost part of tissue; gives rise to the epidermis
  
• ground meristem – gives rise to parenchyma cells, which are further differentiated into pith cells and cortex cells
  
• procambrium – produces first xylem cells (primary xylem) and first phloem cells (primary phloem)

Stems are identified as:

1. Herbaceous (non-woody)

These stems exhibit only primary growth. The epidermis contains a waxy cuticle layer which prevents water loss from the plant. Phloem and xylem are arranged within bundles, with xylem located the most inward and phloem in the outer area. The cortex may be green and may carry out some photosynthesis, much like in leaves.

2. Woody

These stems have primary and secondary tissues. Primary tissues are formed each year from primary meristem. Secondary tissues develop during the second and subsequent years of growth from the lateral meristem. After secondary growth has continued for several years, it is no longer possible to make out individual vascular bundles. Instead, a woody stem has three separate areas -- bark, wood and pith – that have formed.

Cork, formed by cork cambrium, replaces the epidermis. Phloem can exist in this soft layer. In older trees, the heartwood no longer functions in water or nutrient transport, as its xylem cells have become clogged with substances like lignin, tannins and resins. These substances give the tree support and also act as antibacterial agents, prevents the growth of bacteria in these "dead" areas of the tree. Woody plants grow in girth due the presence of vascular cambrium and cork cambrium.

The growing/living layer of a woody stem is the sapwood. This tissue is weaker in its support of the tree, and makes up most of the stem thickness in younger trees. A layer of new sapwood is added to the outer edge of heartwood each year of growth, giving rise to the familiar tree rings.

Just as some roots perform specialized function, so do some stems. Some of these are:

1. Rhizomes – Horizontal stems that grow below the ground, usually near the surface of soil. EX - Irises, some grasses, some ferns
2. Runners and stolons -- Generally grow along the soil surface, above ground. EX – strawberries
3. Tubers – Produced by the accumulation of food at the tips of stolons. EX - Irish potatoes
4. Bulbs – Are actually large buds with a small stem at the lower end surrounded by fleshy leaves. EX – Onions, lilies, tulips
5. Corms – Similar to bulbs, with fewer fleshy leaves. EX – Crocus, gladiolus

C. Leaves

The main function for leaves on a plant is to carry out photosynthesis, an energy producing process in plants. Leaves receive water from roots via the stems.

Exterior Parts of a Leaf

• petiole – stalk that attaches leaf to the stem
  
• blade – wide portion of a leaf
  
• leaf axil – upper acute angle between the petiole and the blade

Leaves may also protect flower buds (calyx); attach to objects (tendrils); and capture insects.

Leaf Arrangements on the Stem

(See page 115 in textbook)

• simple – individual blade
  
• compound leaf – divided into leaflets in various ways
  
• pinnately compound – leaves have leaflets in pairs along a single stalk
  
• palmately compound – all leaflets are attached at the same point at the end of the petiole
  
• opposite – a pair of leaves are opposite each other on a stem
  
• alternate – leaf placement in staggered on the stem, alternating sides of the stem
  
• whorled – several leaves circle the stem at the same point

Interior Structure of a Leaf

If we cut a leaf transversely and look at it under a microscope (and we will in a future lab session!) three distinct areas can be seen:

1. Epidermis : Single layer of cells that covers the entire leaf.

Epidermis on the "upper" side of a leaf – the side that faces the sun – has a waxy cuticle on it to prevent the leaf from drying out. This cuticle also prevents gas exchange from plant to atmosphere and vice versa.

However, the "lower" epidermis contains tiny pores, called stomata, through which gases can and do move in and out of the leaf. (Some plants have stomata on both sides, such as corn and alfalfa; some have stomata only on the upper surface, as water lilies. Some plants, like submerged underwater plants, lack stomata altogether.) These stomata range from 1,000 to 1.2 million per square centimeter of leaf surface. An average sunflower, such as we have in our ecosystem, can have 2 million stomata per leaf!

Each stomata is bordered by 2 small cells called guard cells. The guard cells regulate the opening and closing of stomata. When the guard cells are expanded by water, the swelling opens the stomata. When the amount of water decreases, the cells contract and close off the stomata, thus preventing any further loss of water.

2. Mesophyll

Most photosynthesis takes place here. Mesophyll has two regions:

• palisade mesophyll – found on upper part of the leaf. These cells are compactly stacked, like fence posts. They contain 80+% of the chloroplasts (chlorophyll containing structures) of plants
  
• spongy mesophyll – loosely arranged parenchyma cells. This arrangement gives room for gas exchanges.

3. Veins

Veins can be seen externally in leaves in a net-like pattern in dicots and in a parallel arrangement in monocots. Veins house the vascular bundles in leaves.

Specialized Leaves

1. Tendrils – climbing or clinging leaves. Seen in cucumbers, potatoes, squash, pumpkin

2. Spines, thorns and prickles – Reduces water loss in plants and protects from animals. Examples include mesquite and black locust trees

3. Storage leaves – These are modified leaves which can store water. Usually their parenchyma cells lack chlorophyll. Seen in succulent plants such as cacti.

4. Floral bracts – These are the "colored petals" of such flowers as the poinsettia and dogwood. The true "flower" is the small center portion of these arrangements.

5. Insect traps – Examples of these plants are the pitcher plant and Venus flycatcher.

REFERENCES

1. ----, The Way Nature Works, Macmillan. 1998.
2. Capon, Brian, Botany for Gardeners. Timber Press. 1990.
3. Mader, Sylvia S., Biology, 6th Edition McGraw-Hill. 1998.
4. Stern, Kingsley R., Introductory Plant Biology, 7th Edition. Wm. C. Brown Publishers. 1997

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