Transcript week 5, gymnosperms, angiosperms and flowering plants

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 windcarried 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.
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.
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.
2.
3.
4.
5.
How are the needles arranged?
How many leaves are in a bundle?
How are pine leaves different from
those of deciduous plants?
Why are pines called evergreens?
How do the structural features of pine
leaves adapt the tree for life in cold,
dry environment?
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
Procedure – examine a pine seed
1.
1.
2.
3.
4.
Examine young living or
preserved ovulate cones. These
cones will develop and enlarge
considerable before they are
mature.
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.
Examine a prepared slide of an
ovulate cone that has been
sectioned through an ovule. An
ovule develops into a seed.
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.
2.
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.
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
Two classes of angiosperms
Monocots
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•
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•
•
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
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).
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.
Leaf Identification
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.
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.
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
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.
Figure 3.
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. Mothpollinated 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.
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.
Figure 5.
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 hearshaped 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 endospermabsorbing 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.
Dichotomous Key to Major Types of Fruits
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
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:
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.
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?