Plant Growth and Reproduction
Patterns of Growth
This chapter explores growth patterns of annuals, biennials, and perennials, and the role of hormones in plant growth and development.
24-1 Patterns of Growth
Plants are classified into three main groups: annuals, biennials, and perennials depending on how long it takes them to produce flowers and how long they live.
Annuals, Biennials, and Perennials
Annuals: Plants that grow from seed to maturity, flower, produce seeds, and die within one growing season.
Examples: marigolds, corn, and peas.
The term "annuals" comes from the Latin word "annus," meaning year.
Biennials: Plants that typically live for two years.
First year: grow roots, stems, and leaves.
Leaves and stems die back in winter, but roots remain alive.
Second year: new leaves and stems grow, and the plant produces flowers. After flowering and seed production, the plant dies.
Examples: foxglove, sugar beets, carrots, and turnips.
The word "biennials" comes from the Latin word "bi" meaning two.
Perennials: Plants that live for more than two years.
Examples: peonies, dawn redwoods, trees, and shrubs.
Peonies can outlive the person who planted them.
Dawn redwoods can live for hundreds or thousands of years.
Woody plants: Trees and shrubs whose stems contain woody tissue.
Herbaceous plants: Plants whose stems have little or no woody tissue.
In cold climates, herbaceous plants die back to their roots every year.
Examples: chrysanthemums and tulips.
In tropical climates, above-ground parts can grow throughout the year.
Growth of Stems
Stems grow longer at the tip as cells in the apical meristem divide.
Newly formed cells grow larger in the zone of elongation (behind the meristem).
Farther back is the zone of maturation, where cells differentiate into various cells of the stem.
In herbaceous plants, almost the entire stem is composed of primary tissue produced by the apical meristem.
In perennial plant stems, new tissue is added by vascular cambium and cork cambium to increase thickness.
The addition of new tissue in cambium layers increases the thickness of the stem that is needed to support a larger plant.
Growth in Dicot Stems
In woody dicot stems, the vascular cambium between the xylem and phloem tissues remains alive through the plant's first winter.
In the following spring, the vascular cambium becomes active and its cells begin to divide.
Secondary xylem cells are formed on the surface of the vascular cambium that faces the center of the stem.
Secondary phloem cells are formed on the surface that faces the outside of the stem.
Secondary layers spread sideways from each vascular bundle, uniting the circle of vascular bundles into a solid ring.
Xylem tissue is on the inside of the ring, and phloem tissue is on the outside, with the vascular cambium in-between.
The new ring of xylem tissue formed during every growing season plus the older xylem become the wood of trees.
Sapwood: Xylem that still conducts water.
Heartwood: Oldest xylem cells that are clogged with tars and resins, no longer conduct water, but give strength and support to the tree.
Tree rings (annual rings) are formed because the vascular cambium makes larger xylem cells during the spring and smaller xylem cells during late summer and fall. The large cells of spring wood look lighter.
Secondary phloem cells are added by the vascular cambium to the inner surface of the previous year's phloem. The phloem layer doesn't become as thick as the xylem layer because the cambium makes only one new phloem cell for every six or eight xylem cells.
Phloem tissue forms the inner half of tree bark carrying sugars and other products of photosynthesis from leaves to other plant parts.
A tree will die if the phloem in tree bark is damaged or removed in a ring around the trunk.
Growth in Monocot Stems
In monocots (e.g., corn and lilies), xylem and phloem tissues are arranged in vascular bundles scattered throughout the stem.
Most monocots lack a vascular cambium, so no new xylem and phloem cells can be produced.
Once the apical meristem of a monocot produces a stem, that stem cannot grow thicker.
Few monocots grow more than several meters tall.
Palms are a few tall monocots, and they remain short for several years while producing leaf after leaf.
The apical meristem and the stem below it grow wider meanwhile. After a stem that is strong enough to serve as a trunk is produced the monocot tree begins to grows taller.
The stem of a palm does not increase greatly in width once the tree begins to grow tall.
Growth of Roots
Roots, like stems, grow in length as their apical meristem produces new cells near the root tip.
The fragile new cells are covered by a tough root cap that protects the root as it forces its way through the soil.
As the root grows, the root cap secretes a slippery substance that lubricates the progress of the root through the soil.
New root cap cells are continually added by the meristem, which is constantly being scraped away.
Most of the actual increase in root length occurs in the zone of elongation immediately behind the meristem.
In the zone of maturation, these cells take on the structures and functions of mature root cells.
25-1 Cones and Flowers as Reproductive Organs
The cones of gymnosperms and the flowers of angiosperms are plant structures that are specialized for the purpose of sexual reproduction.
It is within cones and flowers that the vital process that ensures the continuation of the species takes place.
Cones and flowers are as important to the survival of plant species as roots, stems, and leaves are to the survival of individual plants.
All plants have life cycles in which a diploid sporophyte generation alternates with a haploid gametophyte generation.
Gametophyte plants produce male and female gametes; when gametes join, they form a zygote which develops into the next sporophyte generation.
Life Cycle of Gymnosperms
Familiar gymnosperms such as pine trees are diploid sporophytes, each of which has grown from a zygote contained within a seed.
Years after the seed germinates, a pine tree matures and produces male and female cones.
Male Cones: Carry structures called microsporangia that produce male gametophytes called pollen grains.
Female Cones: Carry structures called megasporangia, which produce female gametophytes. These in turn, produce ovules (the structures in which egg cells form).
Pollen grains released from male cones are carried by the wind to female cones, where they may be caught by a sticky secretion.
If a pollen grain lands near an ovule, the grain splits open and begins to grow a pollen tube.
Two haploid sperm are located within the pollen tube.
Inside an ovule is an unfertilized egg. The pollen tube grows into the ovule, and eventually the two sperm break out of the tube.
One sperm fertilizes the egg; the other sperm disintegrates.
The zygote that is formed grows into an embryo encased within what later develops into a seed.
The seed contains an embryo plant as well as a supply of food for the embryo when it begins to grow.
Life Cycle of Angiosperms
Angiosperms (flowering plants) are the dominant form of plant life on Earth today.
They have evolved a life cycle that liberated the reproductive stages of these plants from standing water, allowing them to spread over most of the earth.
Each flower represents proof of a plant's survival and offers assurance that a plant species will produce more of its own kind.
Structure of a Flower
Flowers are actually miniature stems that produce four kinds of specialized leaves: sepals, petals, stamens, and carpels.
These specialized leaves are arranged in circles and have been modified during the course of evolution to serve different purposes related to reproduction.
The outermost circle of flower parts consists of several sepals that are green, resemble leaves, enclose the flower bud before it opens and protect the flower while it is developing.
All the sepals in a flower together form the calyx.
Petals: The second circle of flower parts that are found just inside the sepals, they are often brightly colored. All of the petals in a flower form the corolla.
Brightly colored flower parts act as a kind of "flower advertisement," attracting insects and other pollinators to the flower.
Because they produce no gametophytes, the sepals and petals of a flower are called sterile leaves.
Fertile leaves are located inside the petals and contain the structures that produce male and female gametophytes.
Male Gametophytes (Stamens)
The first circle of fertile leaves consists of stamens.
Each stamen has a long, thin filament that supports an anther.
Inside the anther are microsporangia in which the male gametophytes (microspores) are produced.
In most species of angiosperm, each flower has several stamens.
Female Gametophytes (Carpels)
The centermost circle of flower parts consists of carpels.
Carpels are produced from fertile leaves that have rolled up into fertile leaves, rather than on the outer surface of cones, as they are in gymnosperms.
A single flower may contain one or more carpels; when multiple carpels are present in a flower, they may be either separate or fused.
One or more carpels form the pistil.
The pistil consists of a base called the ovary, a stalk called the style, and the stigma (located at the top of the style).
The stigma is the surface upon which pollen is deposited by wind or animal pollinators; it is sticky or contains many small projections that help catch pollen.
Female Gametophyte Development
Located inside each ovary is one or more megasporangia called ovules.
A single diploid () cell called the megaspore mother cell grows inside each ovule.
Each megaspore mother cell produces a female gametophyte in a series of steps.
First, the megaspore mother cell undergoes meiosis, producing four haploid () cells, three of which die.
Next, the remaining haploid cell divides mitotically to produce eight nuclei. These eight nuclei and the membrane that surrounds them are called the embryo sac, and it is the entire female gametophyte.
Inside the embryo sac, there are eight nuclei that move around:.
Two nuclei locate themselves in the center of the sac (polar nuclei).
Three nuclei clump together at each end.
One of the three nuclei in the group closest to the opening in the ovule enlarges to become the egg nucleus.
The two other nuclei flank the egg nucleus.
The three nuclei (cells) at the opposite end of the embryo sac die.
The female gametophyte now contains a female gamete (egg nucleus) ready to be fertilized.
Male Gametophyte Development
The male gametophyte is even smaller than the female gametophyte.
Inside the anthers, microsporangia called pollen chambers produce many diploid () microspore mother cells.
Each microspore mother cell divides by meiosis to produce four haploid () microspores.
Each microspore ultimately becomes a single pollen grain.
The wall of each pollen grain thickens to protect the pollen grain's contents from dryness and physical damage when it is released from the anther.
The nucleus of the pollen grain undergoes one mitotic division, producing two haploid nuclei: the tube nucleus, and the generative nucleus, the tube nucleus disintegrates, and the generative nucleus divides to form two sperm cells.
The pollen grain (the entire male gametophyte) usually stops growing until it is deposited on a stigma.
Eventually the anther dries out, its pollen chambers split open, and mature pollen grains are released.
Pollination
At this point in the reproduction process, pollen must be transferred from an anther to a stigma.
The transfer of pollen from anther to stigma is called pollination.
Self-pollination: Pollen falls from the anther to the stigma of the same flower; some plants, such as the peas that Mendel worked with, allow this process to occur.
Cross-pollination: Pollen from one flower is transferred to the stigma of a flower on another plant. The majority of flowering plants have evolved complicated methods of reproduction that ensure that seeds will form only when cross-pollination occurs. Because sexual reproduction allows the exchange of genetic material between individuals, it increases variation in offspring, which makes it more likely that at least some individuals will survive to reproduce.
Fertilization
Once a pollen grain has landed on the stigma of an appropriate flower, it begins to grow a pollen tube.
The generative nucleus within the pollen grain divides and forms two sperm nuclei.
The pollen tube now contains a tube nucleus and two sperm nuclei.
Following a chemical trail, the pollen tube grows down the style and reaches the ovary and enters the ovule through a small hole.
When the pollen tube reaches the female gametophyte (embryo sac), the sperm nuclei enter, and both participate in a process called double fertilization (occurs only in angiosperms).
During this process, one sperm nucleus fuses with the egg nucleus to form the zygote.
The other sperm nucleus fuses with the two polar nuclei, forming the triploid () endosperm.
It is the endosperm that provides food for the embryo, which is produced when the zygote begins to grow.
Three important examples of endosperms that humans eat are corn, wheat, and rice.
Fertilization causes rapid changes to occur in the ovule, ovary, and other structures of the flower.
Parts of the ovule toughen to form a seed coat that protects the delicate embryo and its tiny food supply.
The ovary wall thickens and joins with other parts of the flower stem to become the fruit that holds the seeds.
A fertilized flower produces hormones that induce the plant to pour energy into the developing fruits and seeds. If a flower is not fertilized, these hormones are not produced, so the flower withers and falls away.
Formation of Seeds
The development of seeds was a major factor in the success of angiosperms on land.
Seeds provide nourishment and protection for delicate embryos.
Angiosperm seeds have either one or two seed leaves called cotyledons (contain stored food that is used when a seed germinates, or begins to grow).
Monocots, such as corn, have one cotyledon.
Dicots, such as beans, have two.
The length of the stem above the cotyledon(s) is called the epicotyl (develops into the plant's stem).
At the tip of the epicotyl is the tissue that will become the apical meristem.
The length of the stem below the cotyledon(s) is called the hypocotyl.
At the very base of the hypocotyl is a region called the radicle, which contains the apical meristem of the root and will become the primary root of the plant.
In many plants the food stored in the endosperm is almost completely used up by the time the seed is mature.
In these seeds the food used by the embryo during germination is stored in large cotyledons; the two halves of a bean, for example, are actually two cotyledons.
In other plants, such as corn and coconuts, much endosperm remains in the mature seed.
In a coconut, the "milk" is liquid endosperm and the "meat" is solid endosperm.
In seeds that retain a great deal of endosperm, the cotyledons look more like typical leaves produced by the plant.
Thick seed coats protect seeds from dryness, salt water, and other adverse environmental conditions.
Tough seed coats protect seeds from the animal's teeth as well as from the strong chemicals present in its digestive system.
Passing through an animal's digestive system provides an additional benefit to the seeds by lessening competition for available food and water between the adult plant and its seeds.
The animal distributes seeds in other areas that may provide a suitable environment for the seeds' survival.