Seed Plants – The Gymnosperms

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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 worlds 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.
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Dade County PinePinus 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 Frazers fir from the Appalachian
Mountains on sale before Christmas.
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• Procedure examine pine twigs and leaves
• 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
• 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?

• 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.

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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
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Procedures and questions about conifer
reproduction

• Procedure examine pine cones
• 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 youll 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
• 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
• On which surface of the scale are the seeds
  located?
• How large in a staminate cone compared to a newly
  pollinated ovulate cone? A mature ovulate cone?
• What is the make gametophyte?
• What is the female gametophyte?
• What is the function of the winglike extensions
  of a pine seed?
• How are other gymnosperms similar to pines?
• How are they different?

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Bald CypressTaxodium 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.
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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
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CoontieZamia 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.
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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

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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
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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.

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• 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 structurethe 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.
•  

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One way to begin to analyze whats 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. 
Leaf Identification

• 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.
•  

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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
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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.
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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 cant 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.  
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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.
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• 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.
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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 ovarys 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 stonya 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

• 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

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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.

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.
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.
21
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
• What is the function of the stomata?
• Do epidermal cells of leaves have a cuticle? Why
  is this important?
• What is the significance of chloroplasts being
  concentrated near the upper surface of the leaf?
• 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.