Plant 2 Notes:
Seed Plants- Gymnosperms and Angiosperms (these are informal classification names).
Gymnosperms: Water is no longer required to reproduce since pollen grains produce a pollen tube, which allows sperm to move to the vicinity of the egg without swimming. Plants could now
venture into dryer climates. Pollen grains are airborne cells, which contain the male gametophytes. These are very effective in protecting the male gamete. They also make excellent fossils. Seeds consist of an embryo, a seed coat, and stored food.
Gymnosperm: Naked seed: four divisions: 1) Ginkgophyta: Ginkgo
2) Cycadophyta: cycads 3) Coniferophyta: Conifers
4) Gnetophyta: Gnetum, Ephedra, and Welwitschia
Only one species exists. This species is considered to be a living fossil. Male ginkgo trees produce air-born spores that germinate close to female ovules in female trees. The male gametophyte forms a pollen tube that grows toward the egg cell. Two motile multiflagellated swimming sperm cells swim down the tube to fertilize the egg. After fertilization, the female produces a seed.
Division Cycadophyta: found in tropical regions (including Florida):
The pollen is produced by cone-like strobili and carried by the wind to the female cones. There are only 100 species of cycads.
Division Coniferophyta: Cone bearers
There are nine families that contain 550 species in this division. The best-known conifers are pines, firs, spruces and hemlocks.
They produce needles or scale like leaves. The leaves usually last a year and a growing season and then are shed (not all at once). They have a well-developed vascular system (tracheids in xylem). Conifers bear separate pollen bearing cones (small male cones at the end of branches) and female cones (big cones on the older branches of the tree). Each tree is monoecious.
Formation of Seed:
Seed: This is a protective structure in which the embryonic plant can be dispersed and remain dormant until conditions are favorable.
Before we can really understand the seed, let's review a term Alternation of Generations: In some green algae, there are two generations (sporophyte and gametophyte) that are independent and usually the same size and shape. In primitive vascular plants (fern) the two generations are still separate, but the gametophyte is much smaller than the sporophyte. The gametophyte is reduced in size even further and dependent on the sporophyte in gymnosperms.
megaspores and are formed in the megasporangia.
The megasporangium contains a single megaspore mother cell. Through meiosis it becomes a megaspore that is surrounded by non-reproductive tissue (integument). The entire structure is known as an ovule.
The ovule is produced on the female cone scale, one of many in a large, scaled cone. The male cone, by comparison, is much smaller.
In the male cone, the specialized microspore mother cells in microsporangia undergo meiosis to produce haploid microspores. Each microspore matures into a pollen grain. Wind blows the pollen grains to hopefully pollinate the ovule.
Within the ovule: of the four cells produced by meiosis, three disintegrate. The remaining cell develops into a gametophyte. The gametophyte grows within the ovule and develops two or more archegonia, each containing a single egg cell. As the ovule ripens, it secretes a sticky liquid. As pollen lands on the cone, it falls between the scales and lands on the sticky liquid. This is because the shape of the cone with its scales and ovules causes air currents to be directed toward the sticky liquid. As the liquid dries, it draws the pollen grains into the ovule. A few months later, the pollen grain develops into a male gametophyte.
The male gametophyte produces two nonmotile cells (sperm) that are carried to the egg by the pollen tube, produced by a male gametophyte. The pollen tube grows through the tissues of the ovule. This takes about a year. The process takes a shorter time in white than yellow pines.
The sperm eventually meets the egg and a zygote is formed. The zygote begins to divide and forms an embryo (young sporophyte). As the ovule matures, the integuments harden into the seed coat that encloses the sporophyte and the female gametophyte tissue (which will become food for the embryo). After the cone matures, it opens and releases the seeds that are carried by the wind to a new spot.
Notice: Fertilization requires no swimming sperm.
Inside the Seed: there are three generations in one.
1) Seed Coat/Wings: integument that is hardened from the mother sporophyte. 2) Food: from the female gametophyte-- gametophyte tissue.
3) Embryo (from the next sporophyte generation) with cotyledons: seed leaves, which will appear as the first leaves.
Division Anthophyta: Angiosperms: Flowering plants.
Angiosperms: There are 250,000 species that are divided into two classes. 1) Monocotyledons
Differences between monocots and dicots:
Define: Cotyledon: (seed leaf) a leaf within which stored food is found.
Trait Monocot Dicot
Leaf Veins Parallel Branching or net like Cotyledans (first leaf) One Two
Floral parts Multiples of 3 Multiples of 4 or 5 Vascular System Unorganized Organized
Roots Net-like or fibrous Tap root Secondary growth Usually absent present
There are exceptions to each of these characteristics. Any plant with two or more characteristics of the same list is almost certainly a member of the class exhibiting the characteristics in question. If a plant is a monocot for any one characteristic, then the plant is a monocot for the other
characteristics. A plant cannot be both a monocot and a dicot.
Stamens: the male part of the flower, lie inside the corolla. Each slender stalk is called a filament, and on top of the filament is the anther. The anther is the spore (pollen) producing structure. A group of stamens is called the androecium.
The carpels, which are the center of the flower, are called the gynoecium. Each individual structure containing one or more carpels is called the pistil. Each pistil has three parts:
1) Stigma: the top, which is sticky and collects pollen. 2) Style: stalk that supports the stigma
3) Ovulary: the base, which contains the ovules.
A typical flower contains sepals (bud covering which protects the flower), petals (may attract insects), stamens, and carpels. This is a complete and perfect flower. An incomplete flower is lacking petals. An imperfect flower is lacking either the androecium or the gynoecium.
There are two types of reproduction in plants: asexual and sexual.
Asexual: most asexual reproduction takes place in the form of vegetative propagation. Plants form new plants from portions of their own roots, stems, or leaves. We do this artificially by taking cuttings.
New plant growth can also emerge from stolons (runners), which have hidden buds along horizontal stems that sprout, sending young stems upwards.
Sexual reproduction in flowering plants. How does a flower know when to flower?
The critical factor is the length of night. The sensitivity to light and darkness cycle is called
photoperiodicity. The most effective light for the established photoperiod is red or orange-red. The actual signal that induces the flower to develop is delivered from the leaves to the other parts of the plant and is hormonal in nature. We have not found a inducing hormone, but a flower-repressing hormone has been found.
In the ovulary megasporogenesis occurs:
In the ovulary, ovules develop. These consist of a stalk and surrounding tissues called the nucellus and one or two integuments (skin like protective coverings) with a small opening called the
In each ovule, a large cell called a megaspore mother cell will undergo meiosis and produce only one haploid cell that survives. This haploid cell will undergo three mitotic divisions to give eight haploid nuclei that will be incorporated into seven cells and become the female gametophyte or megagametophyte. One cell has two nuclei and is the endosperm mother cell. The two bottom cells near the egg are synergids, they attract and guide the pollen tube. The three top cells are antipodal cells. Their function at the present is unknown.
In the anthers microsporogenesis occurs:
The anthers contain four chambers called pollen sacs. Within these pollen sacs are numerous microspore mother cells, which will undergo meiosis.
Each of four haploid nuclei produced undergoes mitosis, giving two nuclei: the generative nucleus and the tube nucleus. When the mitotic event is finished, a tough coating is formed, and a pollen grain is the finished product.
Pollination and fertilization.
Pollination occurs when the pollen is carried to the stigma by wind insects, animals, rain… When the pollen lands on the stigma, it germinates. When the pollen germinates, it produces a pollen tube (from the tube nucleus), which grows down through the stigma and into the long style (enzymes digests the tissue ahead). As the tube grows, the generative nucleus stays close to the tip of the tube and divides producing two sperm nuclei.
Double Fertilization: The tube penetrates the ovule at the micropyle. The sperm nuclei enter the embryo sac. One nucleus fertilizes the egg cell, forming a diploid zygote, and this will develop into the plant embryo. As fertilization occurs, there is an increase in calcium in the egg. This helps prevent other sperm nuclei from entering, which is polyspermy. This type of thing also happens in animal eggs. The second sperm nucleus penetrates the endosperm mother cell and fuses with the two nuclei present. This produces a triploid nucleus and forms the starch and food regions of the seed in seventy percent of angiosperms. The rest have a different pattern of megagametophytes. This is double fertilization—the egg cell and the endosperm mother cell must both be fertilized.
swells, and seeds and fruit develop.
The triploid cell divides by mitosis, no cytokinesis, to produce a ‘super cell’ with a liquidy
consistency. Finally cytokinesis occurs and the cells produce cellulose cell walls. This ‘firms up’ the endosperm. The endosperm acts as food for the embryo.
Following the double fertilization (the formation of the diploid embryo and the triploid endosperm), the 3n cell divides mitotically to produce endosperm. The zygote divides through mitosis to form the embryo; the cells begin to differentiate into three tissues: protoderm, procambium, and ground meristem.
As the differentiation continues, the result is the formation of three distinct embryonic tissues. This gradual change is called morphogenesis.
In the early stages of embryonic growth, cell division takes place throughout the body of the growing plant. As the plant grows and tissues differentiate, the cell division is restricted to certain parts of the plant body, especially to the apical meristems. The apical meristem is located at the tips of the roots and shoots.
The endosperm is the source of food for the monocot, and it is usually starchy. The radicle is the embryonic root.
The hypocotyl is the shoot below the cotyledon. The epicotyl is the shoot above the cotyledon. The silk scar is where the style was attached. The plumule is the shoot with miniature leaves.
The micropyle is where the pollen tube enters. The hilum is where the seed attaches to the ovulary.
The cotyledons absorb food from the endosperm initial growth.
The seed coat is the tough outer covering that prevents water loss and protects against mechanical damage.
The plumule is the shoot with miniature leaves
The seed and fruit:
Seed: consists of the embryo, a food supply, and a protective covering. The stored food consists of or is derived from the endosperm.
The seed coat is outer layer or layers (integuments) of the ovule. This will protect the ovule. The pericarp is the wall of the fruit. This is the thickened outer wall of the ovary/ovulary. The fruit develops from the wall of the ovulary and other tissues.
Types of Fruit:
Fruits come in many different forms and this affects how the seeds are dispersed. Fruits are usually classified as simple, aggregate or multiple. This classification depends on the arrangement of the carpels.
Simple fruits develop from one carpel or the fused carpel of a simple pistil.
Aggregate fruits have several separate carpels of a single flower. e.g. raspberry, strawberry. Multiple fruits consist of the carpels of more than one flower. e.g. pineapple: the ovularies of the individual flowers fuse as they mature.
Seed Dispersal: The seeds need to be dispersed away from the mother plant to reduce competition for resources. In this competition, the child will always lose. Seeds are dispersed in a number of ways (each plant has it’s own adaptation for dispersing seeds). Seeds can be dispersed by floating in the water, floating in the air (gliding), the wind can disperse the seed, animals can eat and excrete the seed, the seed can attach to the animals fur or body, or the seed can be flung by the parent. There are a number of examples of these types of dispersal strategies.
Adaptations to Seasonal Changes: Dormancy and the life cycle:
Annuals: the entire life cycle of the plant takes place within a single growing season. Only the seeds are left after the one growing season. Seeds are resistant to cold, desiccation and other environmental factors. The seeds bridge the gap between generations. Annuals are usually soft stemmed. Herbaceous: soft stemmed (non-woody).
Biennial: the period from seed germination to seed formation spans two growing seasons. The first growing season results in a short stem and rosette of leaves near the soil and a root. The root is modified for food storage (beets and carrots). In the second growing season the stored food reserves in the root are mobilized for flowering, fruiting, and seed formation, after which the plant dies.
Perennials: vegetative structures persist year after year. Herbaceous perennials remain dormant as modified underground structures during unfavorable seasons. Woody perennials survive above ground and only flower when they become adult plants. These plants have an advantage in that height can be added during each growing season. The leaves of these plants can overtop their neighbors. Deciduous plants drop their leaves annually.
Some seeds remain dormant for long periods of time before germination. The seed coat plays a major role in dormancy. Sometimes the seed coat acts as a mechanical barrier by preventing the entry of water and gases. Growth can be initiated when the coat is worn away, burned by fire, abraded by sand or soil, partially digested.
Germination cannot begin until the seed has imbibed the water required for metabolic activities. Hydration causes the seed to expand and the seed coat ruptures. Metabolic changes in the embryo cause the embryo to grow. The embryo produces the enzyme alpha amylase. This enzyme breaks down the starch that is stored in the endosperm or cotyledon. The nutrients move to the growing areas of the embryo.
In dicots, the first organ to emerge is the radicle. The hypocotyl grows upward, and once stimulated by the light, straightens, thus raising the cotyledons and the epicotyl. The epicotyl now spreads its first leaves.