BOT 201 Coniferophyta Lab  

"If you've seen ONE, you've seen them ALL!!!!!!"

A Famous or Infamous Quote?????????

Conifers are much more complex than the plants we have studied so far. They exhibit many growth forms, which include sprawling shrubs, upright shrubs and trees of great stature and longevity. Some conifers are the largest and the oldest organisms on earth. I have seen the Sequoia and Redwood trees in Northern California several times. Each time I am amazed at their stature and beauty. No human structure, even the greatest cathedral, can match their fearful symmetry. They are one of the true Marvels of the World & even the cosmos as we know it!!!  I hope we can save more than one of each. We are starting to see how wholesale logging of tropical forests is leading to gross climactic disasters. Logging in North America is an Opera of Fiascoes and Environmental  Devastation. Even countries like Canada where the entire economy is based on Forestry, have plundered their Forests. The health of ecosystems can be gauged by the health of the native vegetation, especially the "climax" species. It is not enough to save a few trees. Its the ecosystems that need protection.

Conifers exhibit Secondary Growth. This is the first instance of this that we have encountered. Secondary Growth is that produced by Lateral Meristems. These divide parallel to the surface (Periclinal) to produce radial files of cells which increase the girth of stems and roots. The Vascular Cambium extends the meristematic activity of the Procambium. Procambium originates from the Apical Mersitems of the Shoot and Root. Most Procambium differentiates to form Primary Xylem & Phloem. However, some of it continues to divide and becomes Vascular Cambium which lies between the xylem & phloem.

Cambial activity spreads around the stem and unites the individual cambia of each bundle. This produces a ring of meristematic tissue which produces cells towards the outside and the inside. Cells displaced to the inside form Secondary Xylem. Cells displaced towards the outside form Secondary Phloem. This converts a Eustele into  an Ectophloic Siphonostele.

We will study the process of secondary growth formation in the next lab. It is sufficient that you recognize the secondary tissues for this lab.

The Xylem is composed entirely of Tracheids and Ray Parenchyma. Tracheids are highly elongated cells with overlapping end walls.They have thick, lignified walls that allows them to conduct water vertically, and provide structural support. The alternation of favorable and unfavorable environmental conditions causes changes in wall thickness and cell diameter. Narrow, thick-walled cells are produced during stressful periods like winter or dry seasons.

Tracheids have large pits in their cell walls which are used to conduct water horizontally  between cells and different regions of the plant. The pits are more concentrated on the end walls where tracheids overlap. Pits also occur on lateral walls but are less concentrated.  Tracheids are dead at maturity and their contents are governed by the laws of Physics.

Rays are composed of living Parenchyma cells. Rays are one cell wide and 5-10 cells high. They connect the living Phloem with the dead Xylem. The rays facilitate radial conduction of water from the xylem to the phloem and may contain a few tracheids.

The phloem is composed of Parenchyma and Sieve Cells. These are like tracheids because they are elongated with tapering, overlapping end walls. They have Sieve Pores which are large conduits between cells. These are  most concentrated on their end walls. This favors vertical translocation of carbohydrates but some lateral transport also occurs via lateral sieve pores. Sieve pores are lined with the carbohydrate Callose. Callose reversibly closes sieve pores during periods of stress. Sieve cells produce an enzyme which can hydrolyze callose so that the pores can reopen. Sieve Cells are alive but may be enucleate so that they require physiological assistance from adjacent parenchyma cells (albuminous cells). Sieve cells provide little structural support. Phloem may contain sclerenchyma fibers but these protect the phloem from being crushed and do not lend rigidity to the stem. Phloem is crushed by the accumulation of Secondary Xylem which increases the stems' diameter. However, the Vascular Cambium replaces dead cells like a dynamic equilibrium.

As the stems' diameter increases, pressure on the Cortex and Epidermis increases from the inside -> outside. The Epidermis does not resume meristematic activity. However, parenchyma cells in the outer cortex divide periclinally to produce the second lateral meristem, the Cork Cambium (Phellogen). This produces cells towards the outside in radial files. The walls of the derivative cells become impregnated with Suberin that makes them impervious to water and microbes. These are the Cork or Phellem cells which are dead at maturity. Air may be trapped within these cells, and this increases their insulating capacity. The phellogen may be one continuous cambial layer or it may occur in small lens-shaped sectors which overlap.

As the stem or root increase in diameter, new Cork Cambia arise in the Cortex at deeper and deeper layers. Everything to the outside of the Phellogen eventually dies and becomes the "outer bark". Secondary Phloem becomes the inner bark which consists of living cells. After the Cortical parenchyma cells have been exhausted, Phellogen(s) arise in the Secondary Phloem which is replenished by the Vascular Cambium. An equilibrium is established between the Phellogen which is cutting into the Secondary Phloem & the Vascular Cambium which is producing new Secondary Phloem.

Imagine a fresh container of Ice Cream. Your scoop is the Phellogen. You remove all of the top layer by making a series of shallow scoops. This is the outer Cortex being conquered by new Cork Cambia. The next person takes the next layer and so on until the tub is empty. The next person is out of luck.

Now imagine that the ice cream is infinitely deep and that a piston is attached so that the top is replenished after each person cuts off a layer. This is the equilibrium which exists between the Phellogen which chews off living cells and the VC which replenishes them.

Another analogy would be like trimming away dough from edge of a culture that was continuously enlarging because ingredients (yeast & flower) were being added to the center. If you added too much yeast and flower, like the Sorcerers' Apprentice, you couldn't' keep up and the dough would spread out across the table. Old periderm strips would be the trimmings from the edge. The Vascular Cambium would be the center of dough propagation.

Secondary xylem accumulates over time and results in a continuous increase in stem or root diameter. In conifer stems this results in a conical growth form due to the highly coordinated growth of lateral branches. The roots also produce a heirerarchial growth pattern, at least during early growth.

Conifers produce cones. Cones are composed of sporophylls that bear sporangia. Conifers are Heterosporous and unlike Selaginella, there are separate cones from mega and microsporangia. Cones are produced by lateral branches which   terminate in Strobili.

Each Microsporophyll produces two Microsporangia. These are Eusporangia and contain many Sporogenous Cells. These undergo Meiosis to produce tetrads of Microspores. Each of these separates and undergoes
several mitotic divisions to produce the Microgametophyte
known as Pollen. Each Pollen Grain has a thick weather-proof wall. Some have wing-like wall extensions which aid in wind disposal.

The Megasporangiate cones are composed of sporophyll-like units. Each bears two Ovules. The ovules have a vascular connection to the sporophyll but are on the surface of the sporophyll and thus exposed to the atmosphere. The Ovule is diploid and consists of an Integument which surrounds the Nucellus. There is a gap in the Integument called the Micropyle. One cell (Megasporocyte) of the Nucellus , enlarges dramatically and undergoes Meiosis to produce four Megaspores. Three of these die. The fourth enters a protracted phase of mitotic divisions and enlargement.  It is the Megagametophyte. The surrounding Nucellus is degraded by the enlarging Megagametophyte. The megagametophyte occupies most of the volume in the Ovule, but some Nucellus remains,
especially at the micropyle.

Several Archegonia with massive eggs develop at the micropylar end of the megagametophyte. When the Megagametophyte is receptive it secretes a Pollination Droplet which collects airborne particles, including Pollen. The Pollen Grain germinates and it grows through the Nucellus towards the Archegonia. This can take a long time with some species. Eventually the Pollen Tube reaches  the Archegonia and grows into the Neck. The two Sperm cells (sans Flagella) are released and one fertilizes the Egg. Due to the presence of several Archegonia and several pollen tubes, multiple fertilizations are the rule.

The embryos produce an exceptionally long Suspensor
which thrusts the Embryo proper to the opposite end of the Megagametophyte. The Embryo literally grows backwards. The Cotyledons and Shoot Apex develop at the Distal (far) end of the Gametophyte, a Hypocotyl and Root Apex organize Proximal (towards) the Micropyle in that order. Confiers are Polycotyledonous (more than two).

The Integument matures to become the Seed Coat. This is usually hard and dry. Some Species develop winged extensions which assist wind dispersal. Animals also eat the seeds which are very nutritious and rich in oil and carbohydrates. Indigenous people in the Western states valued pine seeds and made them a regular part of their diet. Some seed coats are particularly durable and their seeds can survive in the seed bank for years.

The hypocotyl elongates and the root protrudes from the seed. The cotyledons enlarge and remain inside the megagametophyte for some time during germination. Experiments have shown that seedling growth is directly related to the period of time the cotyledons are in contact with the Megagametophyte. Eventually the Shoot Apex (Epicotyl) starts to grow.

Most conifers produce needle or scale-like leaves. Some have small flat laminas. These are regarded as Megaphylls. The Vascular Tissue (xylem & phloem) is centralized and surrounded by Transfusion Tissue. This contains Tracheids and is thought to provide lateral transport. An Endodermis surrounds the Vascular Tissues. Photosynthetic Mesophyll occupies the space between the Endodermis and the Epidermis. In some species the mesophyll resembles pieces of a jigsaw puzzle. In others a Palisade/Spongy organization is present. Resin ducts are usually encountered in the Mesophyll. A lignified Hypodermis is frequently present. Stomata are usually sunken. The Epidermis has a thick waxy cuticle.

Waxy Buds are produced in the Temperate-Arctic Zones. These protect delicate Leaf Primordia and the Shoot Apex. These flush dramatically and elongate to mature length in a matter of days. Some internodal elongation occurs but this is small compared to Angiosperms.

Conifers produce a Tap Root System. Both the root and shoot apices are multicellular and closely resemble those seen in Angiosperms. The root has secondary growth which is identical to that in shoots. Live Long and Prosper.

Observe branches of Podocarpus, Pinus or Araucaria (Norfolk Island Pine).

Note the following:

Origin of Branches

Leaf Origin

Leaf Shape and Organization (Macroscopic)

Evidence of Internodal Elongation

Presence of Bark (Periderm)

Stain Cross sections of Pinus or Podocarpus Twigs.with Phloroglucinol to locate Lignified & Suberized Cells.

Stain with Toluidine Blue and locate the Phloem AND the Vascular Cambium.

Locate the Periderm? What type of cell divisions occur in the Phellogen (Cork Cambium)????

What is the consequence of this for the appearance of the Cork Cells (Phellem)?????

Compare with a DEMO slide of a Podocarpus Twig.

Examine Commercial Slides of Young ("Meristematic Stems") and Old Twigs of Pinus.

Find the Vascular Bundles in "Meristematic Stems". These are extremely young and have prominent leaf bases.

Dissect the Microsporangiate cones of Araucaria or Podocarpus.

What is the relationship of the Sporophylls to the Sporangia?

Does this seem similar to what we saw with Selaginella Microsporangia?

 

Mount Pollen in water & observe with the compound microscope.

Compare the Pollen Grains with Commercial Slides of Pinus Pollen.

Examine Demo of Pine Pollen Tubes

Commercial Slide:

Examine a longitudinal section through a Podocarpus Pollen Cone and relate this to the intact Pollen cones that you dissected above.

Examine whole Ovules of Podocarpus & a Commercial Slide.

Locate the Micropyle.

What happens here?

What part of the Ovule produces the Micropyle?

What is its Ploidy?

Where are the Archegonia likely found?

What part of the Ovule produces the Archegonia?

What is its Ploidy?

Examine Longitudinal Sections of a Podocarpus Ovule and

Identify the Integument and the Megagametophyte.

Compare this with the intact Ovule above.

Examine Longitudinal section of Pinus Megagametophytes and locate the following

Micropyle

Integument

Megagametophyte

Archegonia

 

Examine DEMO of a Pinus Ovulate Cone which shows Pollen Grains trapped inside the Micropyle

Examine Old Megasporangiate Cones of Pinus and Araucaria and relate them to the Microsporangiate cones.

What is the relationship of the Sporophylls to the Sporangia?

Does this seem similar to what we saw with Selaginella Megasporangia?

Observe Commercial Slides showing Megagametophytes which have Archegonia Present. The large cells are   ___________ ?

Observe Demo of a Sectioned Gymnosperm Seed and locate the Embryo and the Megagametophyte.