Objectives:

At the end of this lab session students should have:

Named and identified the major vegetative and reproductive structures of cycads and pines

Examined the internal structure of the female cone

Observed and examined a spectrum of floral specimens in the lab

Identified features of plants including flower symmetry, monocot/dicot, floral types (bisexual/unisexual) etc.

Introduction: Vascular plants with seeds.

The first seed plants emerged some 300 million years ago but they did not really dominate the landscape until some hundred million years later.

The Origin of the Seed

The progymnosperms, which existed during the late Devonian Period (about 360 million years ago), are thought to have evolved into the

gymnosperms. The progymnosperms were large woody plants with fern-like leaves and heterosporous reproduction with large megaspores and

small microspores.

Aside from the number of cotyledons, there are several other differences between monocots and dicots which are summarized in the following

table.

Characteristic Dicotyledons Monocotyledons

Number of

Cotyledons Two One

Arrangement of

major vascular

bundles in leaves

Network Parallel

Root system type Taproot Fibrous

Arrangement of

vascular bundles in

stem cross-section

Ring-like Scattered

Origin of secondary

vascular tissues Vascular cambium

Cambium-like

(rare)

Numerical plan of

flowers Multiples of 4 or 5 Multiples of 3

Observations

A. CYCADOPHYTA - Cycads and Zamia

Sporophyte- morphological characters (vegetative)

Examine the specimens provided (Zamia and Cycas) and note the vegetative characters typical of the group:

a. Survey the available living cycads for the following macroscopic characters: a) short, vertical stems that bear numerous roots; and b) spirally arranged

leaves which are fern-like in appearance.

b. Note the large pinnately-compound leaves which are strongly xeromorphic;

c. Short stems with abundant pith and cortex and scant wood.

Draw a gross morphological drawing of the megaphylls of one of the specimens provided

Sporophyte – morphological characters (reproductive)

1. All gymnosperms, including the cycads, exhibit heterospory, i.e. they produce two different kinds of spores.

The microspores develop into the male gametophytes (microgametophytes), whereas the megaspores produce female gametophytes

(megagametophytes). However, the heterospory associated with the seed habit is considerably different from the simple heterospory seen

in Selaginella.

Re-examine the specimen of Zamia pollen cone. The cone consists of a central axis bearing numerous microsporophylls. Each microsporophyll

produces abundant microsporangia on its lower surface.

Obtain a prepared slide marked Zamia male (pollen) cone. Identify the central axis, microsporophylls and microsporangia with dense masses

of small microspores. Each microspore will develop into a 3-celled immature male gametophyte (called a pollen grain) which is

released into the wind currents. Only a vanishingly small percentage of pollen grains manage to pollinate the ovule surface.

Megasporangia and microsporangia borne on separate cones.

(b) The megasporophylls of Cycas are leaf-like and not aggregated into cones.

(c) The reduction in size and number of ovules borne by sporophylls that are aggregated into cones.

Note the pollen (male) or ovules (female) cones which arise on short lateral branches. These cones consist of modified sporophylls (spore-

bearing leaves) that bear either microsporangia in the pollen cones or megasporangia in the ovulate cones. Examine the presented specimens

of Zamia pollen and ovulate cones.

Examine the megasporophyll of Zamia.

a. it is reduced to a peltate head attached to a stalk.

b. the position of the ovules.

c. What is the significance of the position of the ovules?

d. Where do you expect to find the female gametophyte?

What evolutionary advantage does this pollination process confer to the cycads and other seed plants?

2.Re-examine the specimen of a Zamia ovulate cone. This cone is composed of a central axis bearing peltate megasporophylls. Each

megasporophyll develops two large ovules on its lower surface. In essence, an ovule is defined as an integumented megasporangium. The

outer portion of the ovule consists of an integument which surrounds the megasporangium (termed the nucellus in seed plants) except for a

minute channel called the micropyle. The megasporangium produces a single megaspore-producing cell which undergoes meiosis to form

four haploid megaspores. Only one of these megaspores survives to develop into the female gametophyte.

3.Identify the integument, megasporangium (known as the nucellus in seed plants), female gametophyte, and archegonia with

large egg cells.

How does a sperm reach the egg in an archegonium?

Fertilization takes place when the sperm fuses with the egg to create the diploid zygote. Be certain you can distinguish between pollination and

fertilization.

How are the pollen tubes and male gametes of the cycads different from those of the other seed plants?

4.Examine the specimens of Zamia seeds. In essence, seeds are modified ovules in which a fertilized egg has developed into an embryonic

sporophyte. Thus, the various parts of the seed can be understood by reference to the ovular structures from which they developed.

The integument has been transformed into he seed coat. Just interior to the seed coat lie the shrunken remains of the megasporangium.

Most of the central portion consists of the female gametophyte which serves to provide nutrition to the developing embryo.

5.Locate the embryo which has differentiated into the radicle (embryonic root), plumule (embryonic shoot), and two cotyledons.

Numerous starch grains are packed in the cells of thefemale gametophyte around the embryo. What is the function of these starch

grains? (At this point, make sure you review the life cycle of the cycad to ensure the new vocabulary makes sense – if not ask your

demonstrator!)

B. CONIFEROPHYTA - Pinus

Sporophyte - morphological -vegetative characters

a. Record the vegetative structures of the Pinus sp.

b. Note the presence of long shoots, short shoots, and needle- shaped xeromorphic leaves.

c. Note also the fact the main stem (trunk) in cross section contains a large amount of wood (xylem tissue).

Sporophyte – morphological - reproductive characters

a. Draw the structure of the male and female cones.

a. Examine and draw the structure of the microsporophyll.

The microsporophyll are grouped into male cones

Male gametophyte (pollen grain)

Place a few male gametophytes on a slide and examine under the microscope and draw.

1.Survey the specimen branches of various Pinus species.The typical pine tree exhibits an excurrent growth form, i.e., strong apical

dominance, with a tiered arrangement of lateral branches. All species of Pinus and most conifers are considered evergreen in the sense that

their foliage leaves function for two or more seasons. If present, note the morphological differences between the pollen and ovulate cones.

Can you explain the differences in apparent durability in terms of the separate roles these cones play in the pine life cycle ?

2.Obtain a prepared slide of a cross-section of pine needles. Note the single prominent midvein. This leaflet exhibits several unusual

features: 1) the thick cuticle, 2) the sunkenstomates whose guard cells are positioned toward the leaf interior, and 3) the thick-

walled hypodermis beneath the epidermis. Why might this leaf that persists through the winter exhibit the same characteristics as the

leaves that develop on desert plants?

3.First scan the slide to obtain a general concept of the relative abundance of vascular tissue and parenchyma, i.e. the

central pith and the peripheral cortex, in this slide.

Locate the several layers of compact cork cells of the periderm at the stem exterior. Identify the cortex, secondary phloem, vascular

cambium, secondary xylem, and pith. Notice the readily distinguishable zones of annual rings in the secondary xylem. You can use these

growth rings to determine the age of your specimen.

4.All species of Pinus are monoecious, i.e. the ovulate and pollen cones develop on the same plant. The essential features of the Pinus life

cycle are virtually identical to those already described for the cycads. Review the life cycle diagram of a pine in Atlas Fig. 8.38 (p. 112)

before you proceed with this section.

Examine a Pinus staminate (male) cone. The microsporangia are borne on sporophylls. Examine the pollen grains under high power. What

function might the translucent lateral wings serve in the pollination process? The pollen is shed at the tree-celled stage, which includes a

large tube cell and two inconspicuous prothallial cells. The gametophyte will undergo several more cell divisions to reach its mature 6-celled

condition called the pollen tube wich contains 2 non-flagellated sperm. The tube grows through the nucellus to reach an archegonium of the

female gametophyte where it delivers the sperm to the egg. How do pollen tubes and male gametes in the pine differ from the same structures in

the cycads?

Obtain a Pinus young ovulate cone. Locate the developing ovules on prominent structures called ovuliferous scales which are reduced lateral

shoots in evolutionary terms.

Study a prepared slide of Pinus ovule with archegonia which shows both female and male gametophytes in their mature condition. Locate

the integument and nucellus for reference. The megaspore has matured into the female gametophyte with several archegonia, each of which

produces an enormous egg. The pollen tubes are visible in most of our slides as vacuolated regions in the upper part of the nucellus.

5.Proceed to the demonstration slide at a dissecting scope of a Pinus embryo. Recall that the fertilized ovule becomes transformed into the

seed. Locate the embryo in the center of the slide. Identify the cotyledons, plumule (embryonic shoot), and radicle (embryonic root).

Surrounding the embryo is the female gametophyte. The seed coat has been removed from this embryo. What part of the ovule gives rise

to the seed coat? Review the development of the seed in order to determine the ploidy level (haploid vs. diploid) of the various tissues in the

mature seed.

Reproductive Organs of the Angiosperms (Magnoliophyta)

Here we are going to restrict our attention to their reproductive organs, namely the flowers

1.Examine the available flowers of some representative angiosperms dicot and the monocot with the unaided eye. Typical monocots produce

their floral plants in multiples of 3. Count the number of sepals and petals in the flower. In contrast, the flower parts of typical dicot flowers

occur in multiples of 4 or 5. Count the number of green sepals and petals in the flower. What roles do sepals and petals play in flowering

plant reproduction?

2.Using the dissecting scope, study the stamen of a mature flower. Identify the parts of the stamen: the stalk-like filament and the anther

which consists of four microsporangia fused together. The microsporangia in the anther produce microspores which develop into two- or three-

celled microgametophytes called pollen grains. If the anthers on your flower have reached maturity, you should be able to pick up some

yellow pollen grains by touching the anther with a dissecting needle. Botanists believe that the stamen represents a reduced microsporophyll,

because some primitive angiosperms bear their microsporangia on leaf-like structures.

3.Using the dissecting scope, study the pistil of this flower. Locate the stigma, the style, and the ovary of the pistil. Just pull back the petals

to better observe the ovary. What role does the stigma serve in the pollination process?

Determine the relative position of this ovary with respect to the other floral organs. If the other floral organs appear to arise from the top of the

ovary, then we designate the ovary as an inferior ovary. If the other floral organs originate beneath the ovary, then the ovary is considered

a superior ovary. Presumably, the most primitive angiosperms (i.e. buttercups) have superior ovaries, whereas the advanced species (i.e.

orchids and composites) exhibit inferior ovaries.

Review: According to the classical concepts of floral evolution, the simple pistil represents a highly modified megasporophyll that has folded

around its ovules. Thus, while a simple pistil has evolved from one megasporophyll, the compound pistil is the fusion product of more than one

megasporophyll. You can determine whether a pistil is simple or compound by the arrangement of vascular bundles, thenumber of locules (i.e.,

the chambers within the ovary, and/or the number of stigma lobes).

Now cut several free-hand sections of the dicot ovary to confirm the presence of 2 or 3 locules which disclose the compound origin of this pistil.

4.Briefly survey the other flowers available in this lab, such as Lilium (lily) and Hibiscus. Identify the sepals, petals, stamens, and pistil(s) of

these flowers. Which flowers would be calledcomplete flowers (with sepals, petals, stamens, and pistil(s)), perfect flowers (with stamens

and pistil(s)), or imperfect flowers (with either stamens or pistil(s))? What roles do the various floral parts play in flowering plant

reproduction?

5.An inflorescence is defined as a group of flowers attached to a common axis. The flowers can be arranged in many different patterns

within the inflorescence. One common inflorescence is the head (or capitulum) of the composite family which includes such plants as daisy,

aster, sunflower, and dandelion. Use dissecting needles to remove the ray (outer) flowers and thedisc (central) flowers of the daisy or

another composite made available for this lab.

6.Obtain a slide marked germinated pollen to visualize the mature male gametophyte in the angiosperms. First, use high power to find the

ungerminated three-celled pollen grain which consists of two small deeply stained sperm cells and the larger tube cell with a more diffuse

nucleus. Then find a germinated grain with the emerging pollen tube. The two sperm cells are typically situated in front of the tube nucleus.

To be completed before the end of the lab:

1. Use the characteristics mentioned in the text to design a character table to classify all of the Angiosperm samples seen in the lab.

2. Use this information to prepare a dichotomous key to aid in the identification of these samples.

1. 10 points) Describe IN YOUR OWN WORDS the following terms, being sure to describe their internal structure and their role in the fertilization

process of seed plants. Use images to aid your explanations.

i.Megasporangium

ii.Ovule

iii.Seed

iv.Microsporangium

v.Pollen tube

Georgette Briggs

Sun @ 08:13 AM PST

2. (5 points) Diagram the life cycle of a gymnosperm, being certain to present labeled pictures of each important stage.

edit

3. (5 points) Diagram the life cycle of an angiosperm, being certain to present labeled pictures of each important stage.