About
the Author
Sample Entry from
The
Plant Kingdom
A
Handbook
(of
Introductory Botany)
(c)
1994 D.D.
As Also Published in
The Tree of Life
A
Computer Database
(of
Introductory Botany & Zoology)
(c)
1992 D.D.
Division
Anthophyta [Angiosperms]
ORGANISMS
Angiosperms
are the most diverse group of all living plants (in terms of the number of
different species).
Dicots:
Magnolia; ranunculus; cabbage; cacti; cotton; tomato; willow;
mints; rose; apple; melons; legumes such as beans and peas; wisteria;
carrot; sunflower; and other typical flowering, fruiting plants, including
"hardwood" trees and shrubs.
Monocots: Aloes, asparagus, garlic, hyacinths, Joshua tree, lilies,
onions, and tulips; palms; pineapple; crocus, gladiolus, and iris;
orchids; grasses, including bamboo, sugar cane, and grains (such as rice,
wheat, corn, oats, barley, and rye); etc.
Although their amino acids must be supplemented in our diets with
the amino acids from legumes (in order for our bodies to metabolically
synthesize proteins), grains have always been "the staff of
life" for human civilizations.
ENVIRONMENT(S)
Typically on
land, sometimes on or in water and occasionally on other plants.
In particular, grasses are dominant in more terrestrial habitats
than are dominated by any other group of plants in the
"Cenozoic" (the modern geologic era).
GROSS
STRUCTURE
Cellulosic
walls give shape to individual cells.
Forming
either a fibrous or taproot system, roots anchor the plant to the soil,
absorb water (with solutes) from the soil, and conduct water (with
solutes) up to shoots and photosynthesized foods down from shoots.
Although they do not have normal buds like shoots, some roots do
form shoots and additional roots, from "adventitious" buds.
New growth
arises on shoots from buds, formed at the shoot tips and in the axils
(where the leaves or side-branches meet the stem).
A sequence of short days followed by low temperatures may induce
the scale-protected buds of woody plants to go and stay dormant, as in
fall and over winter.
Young,
"primary" growth -- cell-division in the "apical meristems",
of root- and shoot-tips, coupled with cell-elongation -- produces growth
in length; whereas "secondary" growth -- arising from
cell-divisions in the "vascular cambium", between the wood and
the bark of roots and stems --
produces growth in girth. In
such monocot trees as palms, primary growth continues at the top of the
shoot -- there is no true secondary growth.
Produced from
shoot tips in definite, species-specific patterns, angiosperm leaves are
typically flat and thin, with "petioles" (leafstalks) and
"veins" (which strengthen the blades and, being composed of
vascular tissue, conduct water with solutes, from the roots, as well as
foods, photosynthesized in the leaves).
Dicots are
very small to very large plants with fleshy to woody roots, fleshy to
woody stems (one trunk in trees, more in shrubs), typically broad
evergreen or deciduous leaves with net-patterned veins, and flowers whose
parts are typically arranged in groups of 4 or 5.
Monocots are
small to large plants with typically fleshy roots, typically fleshy stems,
typically strap-like leaves with parallel-patterned veins, and flowers
(beardlike in grasses) whose parts are typically arranged in groups of 3.
MATERIAL
& ENERGY INTAKE/OUTPUT
Although a
few angiosperms are parasitic on other plants (such as mistletoe on
various trees) and others (such as the Venus flytrap) are
"saprophytic" (living on decaying organic matter), in most
angiosperms light-energy is photosynthetically captured, by chloroplasts
(especially in the "parenchyma" cells between the upper and
lower "epidermis" of leaves).
Some grasses perform an extraordinarily efficient type of
photosynthesis.
Water vapor
and gases flow especially through "stomata" pores (each
regulated by a pair of "guard cells") in leaves --
carbon dioxide flows in and oxygen flows out for photosynthesis, during
the day; and oxygen flows in and carbon dioxide flows out for respiration,
both day and night. "Transpiration"
is the evaporation of water from plant tissues, especially leaves. A waxy "cuticle" covering over the epidermal cells
helps prevent water loss from shoots, bark helps prevent water loss from
any woody stems of dicots, and hardened outer tissues help prevent water
loss from the shoots of monocots. In
addition, these protective outer structures can act as barriers to
pathogens, parasites, and predators (which can also be repelled by such
external growths as spines or thorns).
Countless
"root hairs", growing on the tips of roots just behind the
cell-elongation zone, absorb most of the water and dissolved minerals for
vascular plants. The soil --
consisting of both inorganic and organic matter, both living and
non-living -- provides roots with support, air, water, and minerals.
The chemical elements required for plant life are easily remembered
by reciting this little saying: "C.
Hopkin's Cafe, managed by my cousins Mo and Clyde" -- C [carbon], H
[hydrogen], and O [oxygen] are provided by carbon dioxide and water; and
provided by soil minerals are P [phosphorus], K [potassium], N [nitrogen],
S [sulfur], Ca [calcium], Fe [iron], Mg [magnesium], B [boron], Mn
[manganese], Cu [copper], Zn [zinc], Mo [molybdenum], and Cl [chlorine]
(Additional elements are sometimes required, by certain plants in certain
soils.).
INTERNAL
TRANSPORT
Typically,
sugars and other food molecules, photosynthesized in leaves, are "translocated"
through phloem tissue, down to roots and out to other food-consuming
"sinks": Theoretically,
food molecules are actively transported into and out of the living phloem
cells; and water osmotically follows, creating positive water pressure
within the phloem and within growing, food- consuming cells.
In contrast,
water and dissolved solutes, absorbed in roots, move through non-living
xylem tissue, up to leaves: Theoretically, water molecules are
"transpired" (evaporated by solar energy) through the stomata;
and a column of water (held together by its hydrogen bonds) cohesively
follows, under negative pressure (tension) within the xylem.
The phloem of
angiosperms typically includes sieve-tube members (which are connected
end-to-end via sieve plates into sieve tubes and are typically regulated --
and probably powered -- by "companion cells"); and the
xylem of angiosperms typically includes not only non-living tracheids
(communicating via pit pairs in their side walls) but also non-living
vessel elements (connected via perforations in their end walls into
vessels).
Within the
young, "primary" stem are the following tissues (from the
outside in): The "epidermis", composed of water-conserving
epidermal cells, guard cells, and/or epidermal hairs; the
"cortex", composed of supportive "collenchyma" cells,
hardened "sclerenchyma" fibers and "sclerids", and
food- and water-storing "parenchyma" cells; the vascular tissue,
composed of food-conducting phloem (consisting of supportive fibers, the
conductive sieve-tube members with companion cells, and food-storing
parenchyma cells) and water- (and solute-) conducting xylem (consisting of
supportive fibers, the supportive and conductive tracheids, the conductive
vessel elements, and food-storing parenchyma); the "pith",
composed mostly of food- and water-storing parenchyma with supportive
sclerids; and "pith rays", extending from the pith to between
the vascular bundles and composed of food- and water-storing and
-conducting parenchyma cells.
Typically,
the vascular tissues in primary stems are grouped together in bundles
(typically with the phloem to the outside and the xylem to the inside) -- forming strands running the length of the stem and arranged in a circle in
the cross-section of the stem. In
monocots, the vascular bundles are distributed throughout the
cross-section of the stem, not just in a circle.
When present,
secondary growth in the stems of dicots is laid down by a cylinder of
vascular cambium formed from the meristematic tissue remaining between the
primary phloem and xylem (within each vascular bundle) plus some of the
parenchyma cells in the pith rays (between the vascular bundles):
Laid-down to the outside is "secondary phloem", which
(with corky tissues produced from "cork cambiums" arising to the
exterior) form the bark; and laid-down to the inside (in annual rings, in
Temperate climates) is "secondary xylem", consisting of young,
active "sapwood" (which eventually matures into inactive
"heartwood"). Secondary tissues are typically not produced in
monocots, although there is a fibrous -- not grained -- "wood"
in such monocots as palm trees.
The young,
primary tissues in the root are (from the outside in) the protective
epidermis, the food-storing cortex (with the filtering
"endodermis" as its innermost layer of cells), the "pericycle"
(which may eventually produce side roots and secondary tissues), and the
vascular tissues -- the water- (and solute-) conducting primary xylem
(star-shaped in cross-section) and the food-conducting phloem (lying
between the arms of the xylem).
Unlike in the stems, there is no pith in the center of the roots of
dicots and many monocots.
When present,
secondary growth in roots is laid-down by a vascular cambium formed from
"procambium" cells (meristematic cells remaining between the
primary xylem and phloem) plus cells of the pericycle (around the tips of
xylem arms): Like in the
stem, to the outside is laid-down secondary phloem, which (with tissues
from cork cambiums, arising to the exterior) forms the bark; and to the
inside is laid-down secondary xylem (wood).
In healthy
plants, a copious production of rubbery "latex" or other sap can
kill invading pests.
INTERNAL
CONTROL
Genetic, as
typically determining leaf shape and flower structure -- both features
are typically useful in plant identification (although leaf shape
sometimes varies with the age of the plant or with environmental
conditions).
Hormonal: In angiosperms (and many other plants) the effects of such
environmental stimuli as light, temperature, and even touch are typically
transmitted by hormones, which --
in various concentrations and combinations, in various tissues, at
different times -- also regulate and coordinate virtually all aspects of
plant growth and development. There are 5 basic types of hormones.
"Auxins"
typically promote organ formation, tissue organization, cell division,
cell elongation, metabolism, translocation, growth movements, and
"apical dominance" (of topshoots over sideshoots) and typically
inhibit leaf abscission.
"Gibberellins"
typically promote cell elongation, cell division, metabolism, and
flowering and typically inhibit dormancy and organ formation.
"Cytokinins"
typically promote cell division, cell enlargement, organ formation, and
nutrient movement and typically inhibit dormancy, apical dominance, and
death of tissues.
"Abscisic
acid" typically promotes dormancy, flowering, abscission, and closure
of the stomata and typically inhibits the action of gibberellins.
"Ethylene"
gas typically promotes growth movements, abscission, death of tissues, and
ripening of fruits (Ethylene is the gas used commercially to ripen
bananas, etc.).
REPRODUCTION
Asexual, via
vegetative body parts.
Sexual:
Typically "monoecious" but often "dioecious"
(with separate sexes -- that is, with male and female plants).
Like other higher plants, the diploid sporophyte is dominant to the
dependent haploid gametophytes.
Flowers -- keys to angiosperm success
-- arise on shoots from
floral buds (evolved from leaf buds), which form at the shoot tips or in
the axils (where the leaves or side- branches meet the stem).
Flowers may be produced either alone or together, in well-defined
"inflorescences". Many
plants -- the so-called "long day" and "short day"
plants -- produce flowers only in response to appropriate night- length.
Mounted on
the typically swollen "receptacle" of a "pedicel"
(flower stalk), a flower is typically composed -- from outside to inside --
of a "perianth", typically several "stamens", and
one to many "pistils" ("Complete" flowers have all
parts, "perfect" flowers have the parts of both sexes.).
The perianth is composed of the "calyx" (the assemblage
of "sepals", floral bracts) and, within this, the
"corolla" (the assemblage of "petals", which are also
modified leaves). A stamen is
composed of the typically bulbous "anther" (bearing pollen),
atop the typically long "filament".
A pistil (consisting of one or more fused "carpels",
modified leaves) is composed of the typically sticky "stigma"
(which receives the pollen), at the end of the typically long
"style" (through which the pollen tube will grow), at whose base
is the typically swollen "ovary" (which eventually contains the
seeds and ripens into the fruit).
The stamen is
a microsporophyll (modified leaf). Within the pollen sacs (microsporangia)
in the anthers, many microspore mother cells ("pollen mother
cells") each produce --
via meiosis -- four haploid microspores, each of which forms -- via
mitosis -- 2 cells or nuclei within:
A "generative" cell/nucleus and a "tube"
cell/nucleus. The outer wall
of the microspore hardens (and sometimes becomes sculptured), thus forming
a grain of pollen (the young male gametophyte), which is released and
disseminated. In typical
dicots and some monocots, showy, perfumed flowers attract insects, birds,
bats, and other small animals (with whom they have often co-evolved -- see honey bees, in "Arthropods"), which drink from the floral
nectaries and, as they visit other flowers, coincidentally spread the
pollen. In some dicots and such monocots as grass plants, the flowers are
not showy or perfumed; and the pollen is typically spread by the wind.
Typically
growing on a stalk within a carpel (a megasporophyll modified leaf) of the
ovary is the "ovule", consisting of a pair of
"integuments" (with a "micropyle" opening at their
lower end) surrounding "nucellus" tissue (the megasporangium),
embedded within which are megaspore mother cells, each of which produce --
via meiosis -- four haploid megaspores, three of which typically die
and one of which develops into a typically round "embryo sac"
(the female gametophyte), typically consisting of an egg cell (near the
micropyle end of the embryo sac), a pair of accompanying "synergid"
cells (the evolutionary remnants of an archegonium), a central
"endosperm mother cell" (containing two haploid
"polar" nuclei), and three "antipodal cells" (opposite
the micropyle-end of the embryo sac).
Viable seeds
(that is, containing embryos) are occasionally formed "parthenogenetically"
(that is, without sexual fertilization), in some species.
Typically,
after a pollen grain reaches the sticky stigma, it germinates into a
pollen tube and grows through the style to the ovary.
Typically, the generative cell, inside the grain of pollen, divides
into two sperm nuclei, which (with the tube nucleus) travel within the
growing pollen tube (the mature male gametophyte, without an antheridium)
through the micropyle, into the ovule, and through the nucellus tissue:
Because the sperm nuclei are delivered directly to the embryo sac,
the sperms do not need to be -- and, in fact, are not -- flagellated
(Unlike the sperm of most lower plants, they do not have to swim through
environmental water.). Unlike
all other plants, there is a "double fertilization" in
angiosperms: One sperm
nucleus fertilizes the egg -- thus forming a diploid zygote -- and the
other sperm nucleus fertilizes the two haploid nuclei in the endosperm
mother cell -- thus forming a triploid "primary endosperm
cell". The zygote
develops into the embryo sporophyte.
The primary endosperm cell develops into the "endosperm",
which (as the rest of the female gametophyte decomposes) nourishes the
developing embryo (pushed by an attached "suspensor" to well
within the endosperm). Especially in grains, the endosperm may persist to
eventually nourish the seedling (which will germinate after a period of
dormancy); although in most dicots, the endosperm is entirely consumed by
the developing embryo: Food is eventually stored in the
"cotyledons" (seedling leaves) -- two in dicots, one in
monocots. The nucellus is
usually consumed by the developing embryo but sometimes persists as a
food-storing "perisperm"; and the integuments typically form a
hard coat around the seed, borne within the ovary, which ripens as the
fruit (typically developing only if the flower has been pollinated and the
egg, fertilized).
A
"simple fruit" (maturing from a single ovary, which may however
be composed of several carpels, such as the segments of an orange) may be
fleshy (such as an apple) or dry; and a dry fruit may be either
"dehiscent" (such as a bean pod), opening when ripe, or
"indehiscent" (such as a walnut), not opening when the seeds are
mature. An "aggregate
fruit" (such as a raspberry) is composed of several ovaries growing
separately within a single flower, borne on a single receptacle (which is
the edible part of a strawberry -- its little hard "seeds" are
the actual fruits); and a "multiple fruit" (such as a pineapple)
is composed of several ovaries from several flowers growing together as
one.
Typically,
fleshy fruits attract hungry animals, who carry the seeds in their gut to
new locations; and hard fruits (such as the grain of a grass plant or the
winged pod of a maple tree) help spread seeds on the fur or feathers of
animals or on the currents of wind or water.
Typically,
the dormancy of a seed is broken (that is, its germination is triggered)
by an environmental condition --
or a sequence of environmental conditions -- that signals a favorable
place and/or time: Factors
involved include light, temperature, moisture, and/or abrasion (which
naturally occurs after the seed has been washed downstream from the parent
plant and which allows chemical inhibitors, such as the traces of cyanide
found in almond seeds, to be leached-out of the seedcoat by rainwater).
All in all,
angiosperms have become the dominant terrestrial flora on Earth because of
their adaptations for reproductive success:
Flowers -- enhancing cross-pollination and, thus, genetic
diversity -- and fruits -- enhancing seed dispersal.
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