Seed Plants – The Gymnosperms

The seed plants evolved from fern-like non-seed ancestors. Several changes

occurred to make this novelty possible. First, two types of spores, large megaspores and

small microspores, appeared. This change is illustrated in Selaginella and some aquatic

ferns. In both, the gametophytes are reduced in size, developing within the spore walls. The

male gametophyte developing within the microspore wall became the pollen. The female

gametophyte developed within the spore wall, and the spore was retained within the

megasporangium. For fertilization to occur pollen was carried by wind to the

megasporangium, the grains germinated as a tube and the male gametes moved to the egg

cell. After fertilization, the embryo developed inside of the megasporangium, now called

the ovule. The fertilized ovule then became a seed, with an embryo inside.

This new life cycle had several advantages. First, the protected pollen grain

was blown by wind to the site of germination, reducing the requirement for water and

permitting these plants to sexually reproduce in much drier conditions. Secondly, the

development of the seed provided a means of protecting the embryo against dessication and

the storing of the embryo in dormancy until ideal conditions would trigger germination.

Finally, modifications of the seed promoted dispersal by wind or animals.

Plants with this life cycle are called gymnosperms because the ovule/seed is

produced on a leaf-like structure and is unprotected, or naked. Gymno- = naked and -sperm

= seed. We will see details of this modification and this new life cycle in two plants native

to south Florida. There are four groups of living gymnosperms: the conifers, the cycads,

Gingko, and Gnetum and its relatives. Ginkgo is represented by a single species: Gingko

biloba. It does not grow in south Florida, but it is sold in health food stores as a tonic to

improve cerebral circulation and memory in aging.

The Conifers These trees are enormously important, as the source of softwood timber used in

wood-based construction and the source of fiber in producing most of the world’s paper. They

are dominant in certain forests at temperate and higher latitudes, as in northern latitudes and the

northwest of the United States. Most of the coastal areas of south Florida, on the limestone

ridge, were covered with a pine forest, called the pine rocklands. Little of this forest remains,

having been replaced by agriculture and then commercial development. Conifers vary in their

leaves and particularly in their female cones; some being reduced to look more superficially

like a berry. They all share the same basic life cycle, that of a pine tree given as an example

below. Three conifers native to south Florida are described in detail, and several exotic species

are mentioned briefly.

Dade County Pine—

Pinus elliottii var. densa. This

extreme southern variety of the slash

pine grows in a few remaining stands

on the rock ridge of south Florida. Its

wood is very resinous, extremely

hard when dry, and very resistant of

termite attack. It was used in the

construction of homes and boats well

into the last century. Dade County

pine grows in the Ecosystem Preserve

and the parking lot just to the east. A

couple of trees also grow in the small

conifer collection NE of the north

parking lot.

Dade County Pine, continued

The fragile male cones

and small purplish female cones

develop in January-February. In a

forest the air is yellow with the wind-

carried pollen. After fertilization the

female cone scales swell and close.

Then the cones develop for the entire

year, and open to release the winged

seeds prior to the rainy season (May)

the next year. The trees always have

female cones in some stage of

development, but the male cones soon

fall off the tree after they shed their

pollen. The Dade County Pine is a

member of the pine family (Pinaceae)

along with all pines and the firs, such as

the Frazer’s fir from the Appalachian

Mountains on sale before Christmas.

Procedure – examine pine twigs and

leaves 1. Examine pine twigs having leaves

(needles) and a terminal bud. Notice

the number of needles; the length and

number of leaves distinguishes many

of the species Pinus.

Questions 1. How are the needles arranged?

2. How many leaves are in a bundle?

3. How are pine leaves different from

those of deciduous plants?

4. Why are pines called evergreens?

5. How do the structural features of pine

leaves adapt the tree for life in cold,

dry environment?

Phylum Gingophyta: The Ginkiphyta

consist of one species, Ginkgo biloba

(Maindenhari plant), a large dioecious tree that

does not bear cones. Ginkgo are hardy plants

in urban environments and tolerate insects,

fungi, and pollutants. Males are usually

planted because females produce fleshy,

smelly, and messy fruit that resembles cherries.

Ginkgo has not been found in the wild and would

probably be extinct but for its cultivation in

ancient Chinese and Japanese gardens.

Phylum Gnetophyta: This gnetophytes (71

species in 3 genera) include some of the most

distinctive (if not bizarre) of all seed plants. They

have many similarities with angiosperms, such as

flowerlike compound strobili, vessels in the

secondary xylem, loss of archegonia, and double

fertilization.

Pine life cycle In seed plants, the gametophyte gneration is greatly reduced. A germinating pollen grain is

the mature microgametophyte (male cones) of a pine. Pine microsporangia are borne in

pairs on the scales of the delicate pollen-bearing cones. Megagametophytes (female cones),

in contrast, develop within the ovule. The familiar seed-bearing cones of pines are much

heavier than the pollen-bearing cones. Tow ovules, and ultimately two seeds, are borne on

the upper surface of each scale of a cone. In the spring, when the seed-bearing cones are

small and young, their scales are slightly separated. Drops of sticky fluid, to which the

airborne pollen grains adhere, form between these scales. These pollen grains geminate,

and slender polled tubes grow towards the egg. When a pollen tube grows to the vicinity of

the megagametophyte, sperm are released, fertilizing the egg and producing a zygote there.

The development of the zygote into an embryo occurs within t the ovule, which mature into

a seed. Eventually, the seed falls from the cone and germinates, the embryo resuming

growth and becoming a new pine tree.

Pine Life Cycle diagram

Procedures and questions about

conifer reproduction

Procedure – examine pine cones 1. Examine young living or

preserved ovulate cones. These

cones will develop and enlarge

considerable before they are

mature.

2. Examine a prepared slide of a

young ovulate cone ready for

pollination. Each ovuliferous

scale of the female cone bears

two megasproangia, each of

which produces a diploid

megaspore mother cell. Each

megaspore mother cell undergoes

meiosis to produce a megaspore

that develops into a

megagametophyte. A

megagametophyte and its

surrounding tissues constitute an

ovule and contains at least one

archegonium with an egg cell.

3. Examine a prepared slide of an

ovulate cone that has been

sectioned through an ovule. An

ovule develops into a seed.

4. Examine a mature ovulate cone

and notice its spirally arranged

ovuliferous scales. These scales

are analogous to

microsporophylls of staminate

cones, but ovuliferous scales are

modified branches rather than

modified leaves. At the base of

each scale you’ll find two naked

seeds. Notice that the seeds are

exposed to the environment and

supported (but not covered) by an

ovuliferous scale.

Procedure – examine a pine seed 1. Examine a prepared slide of a

pine seed. Locate the embryo,

seed coat, and food supply.

Seeds are released when the

cone dries and the scales

separate. This usually occurs

13-15 months after pollination.

2. Examine some mature pine

seeds, noting the winglike

extensions of the seed coat.

Questions 1. On which surface of the scale

are the seeds located?

2. How large in a staminate cone

compared to a newly pollinated

ovulate cone? A mature ovulate

cone?

3. What is the make gametophyte?

4. What is the female

gametophyte?

5. What is the function of the

winglike extensions of a pine

seed?

6. How are other gymnosperms

similar to pines?

7. How are they different?

Bald Cypress—Taxodium distichum. This is a conifer in another family, the Taxodiaceae.

Its female cones are much smaller and the individual scales are rounded to produce a round

cone. It is a swamp tree, growing in stands throughout the southeast. It was once common

in a strip of swamp forest down the southeast coast of Florida, and more common along the

west coast, as in the Big Cypress National Preserve. Bald Cypress trees were planted on

pond margins at FIU soon after it opened. We now have some bald cypress “domelets”, with

cypress knees (the pneumatophores that assist in oxygen uptake to the roots) and Everglades

wading birds sitting on branches. The bald cypress is unusual among conifers in that it loses

its short needle foliage during the winter months. Few of the original cypress domes

remain; the majority of these swamp forests were logged before and during the Second

World War, partly for the construction of PT boats.

Phylum Cycadophyta: the cycads These gymnosperms are no longer widely distributed, only found in mostly dry tropical

regions, but they were once dominant plants. These were the primary food of the large

herbivorous dinosaurs. Most cycads are extremely tough, thorny, and often very toxic.

Fairchild Tropical Garden, and the adjacent Montgomery Botanical Center, have the largest

cycad collection in the world. Cycads have life cycles similar to the conifers, but certain

details (as the flagellate male gametes) are different. Cycad plants are female (producing

long-lived female cones) or male (producing ephemeral male cones). We illustrate the

cycad life cycle with the example of the coontie, Zamia pumila, and describe a few cycads

commonly encountered in south Florida (and on campus).

Zamia Life Cycle

Coontie—Zamia pumila. The

coontie is the only cycad native to

the United States, growing in

south Florida Pinelands. Its

rhizomes are full of starch, which

was the source of the first

manufacturing industry in south

Florida. The "trunks" were

ground up to release the starch,

the starch was then washed to

remove the toxic cycasin, and the

product dried and ground. Florida

“arrowroot” was then shipped up

the east coast for cooking and

stiffening the collars of Victorian

shirts. The coontie is a small

plant, less than half a meter high.

It grows on campus in the

Ecosystem Preserve, the Campus

Security Compound, and by the

Conservatory. Recently, the

remarkable discovery was made

that the coontie is pollinated by

beetles, that feed on both the male

and female cones.

Seed Plants – the Angiosperms – Flowering Plants

The angiosperms are seed plants, similar to gymnosperms, but with some important

evolutionary modifications. Flowers are reproductive organs derived from leaf-like

appendages. The relationship of the accessory flower organs, petals and sepals, is obvious.

The stamens and pistils can also be seen in development to originate from leaf-like

structures. In the flowering plant life cycle, the male gametophyte which develops within

the microspore wall into a pollen grain are even more reduced than in the gymnosperms.

Its movement to the ovule is often aided by appearance and scent, attracting pollinators. The

female gametophyte develops as the embryo sac, within an ovule, and within a new

structure: the ovary. In pollination the pollen grain germinates on the stigma of the pistil

and grows down the length of the style to the opening of the ovule. After fertilization, the

embryo sac and ovule develop into the seed. A second fertilization produces a nutritive

tissue, the endosperm, that surrounds the embryo. At maturity, the ovules, or seeds, are

protected within the ripened ovary wall to become a fruit. The fruit, fleshy or dry, aids in

dispersal.

Peduncle – flower stalk

Sepals – the lowermost or outermost whorls

of structures, which are usually leaflike and

protect the developing flower; the sepals

collectively constitute the calyx.

Petals – whorls of structures located inside

and usually above the sepals; the petals

collectively constitute the corolla.

Androecium – the male portion of the plant;

consists of stamens, each of which consist of

a filament atop which is located an anther;

inside the anthers are pollen grains which

produce the male gametes

Gynoecium – the females portion of the

plant; consist of one or more carpels, each

made up of an ovary, style, and stigma; the

ovary contains ovules that contain the female

gametes. The term pistil is sometimes used

to refer to an individual carpel or a group of

fused carpels.

More information about Angiosperms

Flower symmetry

The sepals and petals are usually the most

conspicuous parts of a flower, and a variety

of flower types are described by the

characteristics of the perianth (combined

calyx and corolla). In regular

(actinomorphic) flowers such as tulips, the

members of the different whorls of the flower

consist of similarly shaped parts that radiate

from the center of the flower and are

equidistant from each other. The flowers are

radially symmetrical. In other flowers such

as orchids, one or more part of at least one

whorl are different from other parts of the

same whorl. These flowers are generally

bilaterally symmetrical and are said to be

irregular (zygomorphic).

Two classes of angiosperms

Monocots

• One cotyledon per embryo

• Flower parts in sets of three

• Parallel venation in leaves

• Multiple rings of vascular bundles in

stem

• Lack a true vascular cambium (lateral

meristem)

Dicots

• Two cotyledons per embryo

• Flower parts in sets of 4 or 5

• Reticulate (i.e., netted) venation in

leaves

• One ring or vascular bundles in stem

• Have a true vascular cambium (lateral

meristem)

A radially symmetrical flower

Photo by Gita Ramsay A bilaterally symmetrical (irregular) flower

Photo by Gita Ramsay

Angiosperm life cycle

Eggs from within the embryo sac inside the ovules, which, in turn, are enclosed

in the carpels. The pollen grains, meanwhile, form within the sporangia of the

anthers and are shed. Fertilization is a double process. A sperm and egg come

together, producing a zygote; at the same time, another sperm fuses with the

polar nuclei to produce the endosperm. The endosperm is the tissue, unique to

angiosperms, that nourishes the embryo and young plant.

Basic Leaf Information Leaves differ from stems in not having an apical meristem, so leaves are determinate (i.e.,

limited in their growth), while stems are indeterminate (theoretically capable of growing forever). In the root apical meristem, the differentiating cells produce the root cap, a structure that protects the root apical meristem as it pushes its way through the soil, and the root body, which is the part of the root that we see. Thus, the apical meristems of the root and shoot differ in their structure—the root apical meristem is internal, surrounded by cells on all sides, whereas the shoot apical meristem is external and not covered by cells. You usually need to look at sections of plants under the compound microscope to see these differences, but on some plants, such as the screw pine or Pandanus, next to the OE pond on campus, you can clearly see the root cap of the prop roots before they enter the ground. Examine plants on campus, identifying roots, stems, leaves, apical meristems and axillary buds.

Both roots and shoots can branch. The branches form more roots, if they are root branches, and more shoots, if they are shoot branches. Root branches are produced inside the root itself, breaking out through the root, while shoot branches form from axillary buds. Axillary buds are produced in the upper angle between the leaf and the stem, which is called the axil of the leaf (Figure 1).

Leaves are produced in a very organized manner at the shoot apex. This results in a predictable arrangement of the mature leaves on the stem. This arrangement is called the phyllotaxis of the leaf. Common patterns are for the plant to produce 1 leaf at a time at the apex, resulting in an alternate phyllotaxis. Sometimes twp leaves are produced at a time at the apex, with successive leaf pairs at 90o from each other. This is an opposite phyllotaxis. If more than twp leaves are produced at a time, the phyllotaxis is whorled, but this is a much more rare occurrence. See the examples in Figure 4.

One way to begin to analyze what’s what on a plant is to consider where different

parts fit into the overall ground plan of the plant. For example, a thorn that is

lateral to another structure (the stem) and has a third structure in its axil (the

axillary bud) is in the right position to be equivalent to a leaf.

Figure 4. A = palmately compound leaf, opposite leaf arrangement;

B = pinnately compound leaf, alternate leaf arrangement; C = simple, lobed, petiolate leaves, alternate leaf arrangement; D = simple leaves, opposite leaf arrangement; E = simple, lobed and toothed, petiolate leaf, opposite leaf arrangement; F = simple leaves, alternate leaf arrangement; G = simple lobed leaf, alternate leaf arrangement; H = simple linear leaf with sheathing leaf base, alternate leaf arrangement; I = simple leaves, whorled leaf arrangement; J = simple needlelike leaves, alternate leaf arrangement; K = simple bilobed leaf, alternate leaf arrangement.

Leaf Identification

Figure 1.

Flowers, thus, have a number of functions. They provide plants with the opportunity to spread

genes, since both the pollen and seeds can leave the parent plant. Because they enable the plant

to reproduce sexually, flowers mix male and female genes and contribute to genetic diversity.

Through the production of fruits they help to disperse the next generation, and through

provisioning of the seeds, they help that generation to begin to grow.

There is enormous variation in flower structure among species. They can lack sepals

and/or petals, or these whorls can resemble each other, as in many monocots, such as lilies.

The parts of a whorl can fuse to each other, as in the tubular corollas of sunflowers, or to

adjacent whorls, as when stamens are attached to the corolla. A fundamental difference is in

the position of the carpels in relation to other parts of the flower. If the sepals, petals, and

Plants: Reproduction

Flowers and Inflorescences

Flowers are short shoots (rosettes) specialized for sexual reproduction. The stem is called the

receptacle and bears leaf homologues. Although the number of parts can vary, flowers can have

up to 4 whorls of “leaves”. The first 2 whorls, the sepals and petals, are sterile and are often

modified for protection of the developing flower and/or for attraction of pollinators (Figure 1).

The term for all of the sepals is calyx, while the term for all of the petals is corolla. The last two

whorls, the stamens and carpels, are the fertile parts. The stamens are usually differentiated into

the filament and anther (Figure 1). The anthers are the site of meiosis and produce the pollen

or male gametophyte. The carpels are usually differentiated into the stigma, which receives the

pollen, the style that supports the stigma, and the ovary (Figure 1). The ovules are inside the

ovary. Meiosis also occurs in the ovules, producing the female gametophyte, which, after double

fertilization, makes the embryo and endosperm. The ovules mature into the seeds, while the

ovary, sometimes with additional parts, matures into the fruit.

stamens are inserted on the top of the ovary, the ovary is said to be inferior and the flower is

epigynous (Figure 2). The individual flowers of the sunflower provide an example. If the

sepals, petals, and stamens are inserted below the ovary, the ovary is superior and the flower is

hypogynous (Figure 2). Bean flowers are hypogenous, as are those of Brassica. Sometimes the

other floral parts are fused halfway to the ovary, or fuse to themselves, forming a cup that comes

up partway around the ovary. These flowers are perigynous.

The number of parts per whorl also varies. In general, monocots have parts in 3s or multiples of

3, while dicots have parts in 4s or 5s or multiples of these numbers. The overall symmetry of a

flower can be radial (actinomorphic), with the whorls distributed evenly around the receptacle,

as in strawberry flowers or the flowers of Brassica (Figure 3). Alternatively, the flower can have

bilateral symmetry (be zygomorphic), in which case it has a distinct top and bottom, as in orchid

flowers or bean flowers (Figure 3).

Figure 2.

Because one of the functions of flowers is to enhance pollination (the transfer of pollen from

the anthers to a stigma), the structure of flowers varies with the type of pollinator. Wind

pollinated flowers are generally not colorful (the wind can’t see), very small, have no or

reduced sepals and petals, and may separate the anthers and stigmas into different flowers.

They also produce huge amounts of pollen. Animal-pollinated flowers are often more colorful,

have sepals and petals, and vary in size, color, and symmetry depending on the type of

pollinator. Because hummingbirds see red, hummingbird-pollinated flowers are often red,

whereas bee-pollinated flowers tend to be yellow or blue, because bees see these colors. Moth-

pollinated flowers are often white, but have strong scents that are emitted at night, as moths are

sensitive to odor and are active at night.

Flowers have to both attract pollinators and provide them with a reward, so that they will visit

other flowers of the same species. Common rewards are pollen itself, which is often rich in

proteins and lipids, and nectar, which may be secreted by glands in the flower.

Figure 3.

Figure 5.

Meiosis in Anthers Stamens produce the male gametophytes of flowering plants. This is an important

stage in the life cycle because pollen often leaves the parent plant, providing one of the few

times plants can move genes around. The stamens are subdivided into the filaments and

anthers. The anthers bear 4 microsporangia internally. The microsporangia produce

microspore mother cells that undergo meiosis, producing 4 pollen grains per microsporocyte.

These microspores are initally held together in groups of 4 by the original mother cell wall.

This wall enentually breaks down, however, and the microspores are released. Each

microspore will divide once to make the pollen vegetative cell and generative cell. The

generative cell will divide to produce the two sperm that fertilize the egg cell and polar nuclei

in double fertilization. This second division happens late in the life of a pollen grain, often

occurring after pollination!

Because these different parts of the life of a pollen grain look different, you can assess the

developmental stage of the pollen by squashing the anthers and seeing whether the pollen is in

groups of 4 (tetrads, which occur immediately post-meiosis), or is single with a heavy wall,

which is older pollen that will soon be dispersed (Figure 5).

Remember the difference between pollination and fertilization. In pollination pollen

is transferred from anthers to the stigma. The pollen germinates on the stigma, grows down

the style, and passes into the micropyle of the ovule. It grows through the nucellus, releasing

two sperm into the embryo sac. Fertilization comes at this point: one sperm fertilizes the egg

and thus forms the first cell of the daughter embryo; the other sperm fuses with the polar

nuclei, producing the triploid endosperm.

A seed is a mature ovule that includes a seed coat, a food supply, and an embryo. The stages of embryo development in the seed of Capsella (a dicot) is show to the right/blow. The developing embryo grows, absorbs the endosperm, and stores those nutrients in “seed leaves” called cotyledons. Development includes the following stages:

• Proembryo stage –. Initially the embryo consists of a basal cell, suspensor, and a two celled proembryo. The suspensor is the column of cells that pushes the embryo into the endosperm. Note that the endosperm is extensive but is being digested.

• Globular stage – A stage that is radially symmetrical and has little internal cellular organization.

• Heart-shaped stage – Differential division produces bilateral symmetry and two ctyledons forming the hear-shaped embryo. The enlarging cotyledons store digested food from the endosperm. Tissue differentiation begins, and root and shoot meristems soon appear.

• Torpedo stage – the cotyledons and root axis soon elongate to produce an elongated torpedo-stage embryo. Procambial tissue appears and will later develop into vascular tissue.

• Mature embryo – has large, bent cotyledons on either side of the stem apical meristem. The radicle, later to form the root, is differentiated toward the suspensor. The radicle has a root apical meristem and root cap. The hypocotyl is the region between the apical meristem and the radicle. The endosperm is depleted and food is stored in the cotyledons. The epicotyl is the region between the attachment of cotyledons and stem apical meristem; it has not elongated in the mature embryo.

(a) A garden bean (dicot seed); will absorb the

endosperm before germination; (b) a corn seed

(monocot); the single cotyledon is an endosperm-

absorbing structure called a scutellum.

Fruits

Simply stated, fruits are ripened ovaries. Once fertilization occurs the ovules develop into

seeds, and the ovary wall develops into the fruit wall. The wall develops from leaf-like

structures, called carpels. A fruit may develop from a single, or many, carpels. How the

carpels fuse together determines the numbers of chambers in the fruit, from one to many, and

each of these may contain one to many seeds. Under exceptional circumstances the fruit may

develop in the absence of seeds (as a seedless grape or naval orange), a process called

parthenocarpy. It is possible to examine a fruit to determine the ovary’s position in the

flower. If scars or parts of old petal and sepals are at the tip of the fruit, the flower was inferior

(as an apple). If at the base then superior (as an orange). If the ovary wall is fleshy, the fruit is

a berry, if dry at maturity and breaks open, the fruit is a capsule. Sometimes the ovary wall

develops into a fruit of different layers, including an inner one that is stony—a drupe (like a

peach). Sometimes accessory parts form part of the flesh of the fruit, an accessory fruit or

pome (like an apple). Sometimes the flower forms multiple pistils, and the ovaries fuse

together to form an aggregate fruit (like a raspberry). Sometimes the ovaries of separate

flowers fuse together to form a compound or multiple fruit, such as a pineapple. You can

quickly find a great diversity of types of fruits by examining the produce in a supermarket,

looking at the fresh fruits and nuts.

1. Fleshy fruits

A. Simple fruits (i.e., from a single ovary)

1. Flesh mostly of ovary tissue

a) endocarp hard and stony;

ovary superior and single-seeded

(cherry, olive, coconut): drupe

b) endocarp fleshy or slimy; ovary

usually many seeded (tomato, grape,

green pepper): berry

2. Flesh mostly of receptacle tissue

(apple, pear, quince): pome

B. Complex fruits (from more that 1 ovary)

1. Fruit from many carpel son a singlr

flower (strawberry, raspberry);:

aggregate fruit

2. Fruit from carpels of many flowers

fused together (pineapple): multiple

fruit

II Dry fruits

A. Fruits that split open at maturity (usually

more than one seed)

1. Split occurs along two seems in the

ovary. Seeds borne on one of the halves

of the split ovary (pea and bean pods,

peanuts): legume

2. Seeds released through pores or

multiple seams (poppies, irises, lilies):

capsule

B. Fruits that do not split open at maturity

(usually one seed)

1. Pericarps hard and thick, with a cup at

its base (acorn, chestnut): nut

2. Pericarp thin and winged (maple, ash,

elm): samara

3. Pericarp this and not winged

(sunflower, buttercup): achene

(cereal grains): caryopsis

Dichotomous Key to Major Types of Fruits

Features of Mature Woody Stems

Examine the features of a dormant twig. A terminal

bud containing the apical meristem is at the stem tip

surrounded by bud scales. Leaf scars from shed

leaves occur at regularly spaced nodes along the

length of the stem. The portion between the stem and

nodes are called internodes. Vascular bundles scars

may be visible within the leaf scars. Axillary buds

protrude from the stem just distal to each leaf scar.

Search for clusters of bus scale scars. The distance

between clusters or from a cluster to the terminal bud

indicates the length of yearly growth.

This is a cross-section of a sunflower

stem. An epidermis covers the stem.

The epidermis is coated with a waxy,

waterproof coating called the cutin.

Below the epidermis is the cortex, which

stores food. The pith in the center of the

stem also stores food. Also note the

vascular bundle composed of phloem

and xylem. Xylem transports water and

minerals; phloem transports most organic

compounds in the plants.

The shoot apex – Examine a living coleus

plant and not the arrangement of leaves on

the stem. Examine a prepared slide of a

longitudinal section of the root tip of Coleus

(above). Note that the dome-shaped shoot

apical meristem is not covered by a cap as

the room apical meristem would be. The

shoot apical meristem produced young

leaves (leaf primordia) that attach to the

node. An auxiliary bud between the young

leaf and the stem for a branch or flower.

Internal Anatomy of Leaves

Examine the diagram above of the internal anatomy of a leaf. Note that the leaf is only10-15 cells

thick – pretty thin for a solar collector! The epidermis contains pores called stomata, each

surrounded by two guard cells. Just below the upper epidermis are closely packed cells called

palisade mesophyll cells; these cells contain about 50 chloroplasts per cell. Below the palisade

layers are spongy mesophyll cells with numerous intercellular spaces.

Questions:

1. What is the function of the stomata?

2. Do epidermal cells of leaves have a

cuticle? Why is this important?

3. What is the significance of

chloroplasts being concentrated near

the upper surface of the leaf?

4. Based on the arrangement of vascular

tissues, how could you distinguish the

upper versus lower surfaces of a leaf?

A stoma. Unlike the other epidermal cells,

the guard cells flanking this stoma contain

chloroplasts. Water passes out through the

stomata, and carbon dioxide enters by the

same portals.