Plant Reproduction
What Are the Methods of Asexual and Sexual Reproduction in Plants?
Asexual Plant Reproduction
Teacher Note: Practices
In this concept, students will construct an explanation of the various ways that plants reproduce, based on valid and reliable evidence obtained from multiple and independent student-generated sources, including their own investigations, models, simulations, text, and media. To begin, have them write a summary paragraph about how plants reproduce. Have them return to and revise their summary at the end of each class session. To help them connect to the topic, have students catalog common vegetables, fruits, and other plant-based products into categories: those that reproduce asexually and those that reproduce sexually. Then, discuss why each method of reproduction lends itself to commercial production. Further study can include how agriculturists use methods such as cross pollination or fragmentation to further the commercial viability of these products.
As students read and comprehend complex texts, view the videos, and complete the interactives, labs, and other Hands-On Activities, have them summarize and obtain scientific and technical information. Students will use this evidence to support their initial ideas on how to answer the Explain question or their own question they generated during Engage. Have students record their evidence using My Notebook.
Plants can reproduce either asexually or sexually. Asexual reproduction results in offspring that are genetically identical to the parent. Asexual reproduction is based on mitosis. Plants use a wide variety of methods to reproduce asexually. There are two different types of asexual plant reproduction: vegetative propagationglossary term (opens in a new window) and apomixisglossary term (opens in a new window).
Vegetative propagation, or vegetative reproduction, takes place without any seeds or spores. In this form of reproduction, a part of the plant is removed and simply grows into an entirely new individual. Plants can reproduce this way because they have tissues that continue to grow and differentiate throughout their lifetimes. Plant vegetative propagation can involve the roots, shoots, or leaves.
Taproots are used by some plants as a structure for vegetative propagation. For example, radishes and carrots produce side roots that may break off and grow as separate plants. Other plants, such as asparagus, use adventitious roots to spread. Some plants develop root tubers. These are lateral roots that become swollen as food storage organs. Plants that produce root tubers include sweet potatoes and dahlias. Commonly, the lateral roots that connect them to the parent plant break. Each root tuber becomes a separate plant.
Some stems grow horizontally, below or across the surface of the soil. Horizontal stems take two forms. Short, swollen stems called rhizomes are typical of plants such as ginger, iris, and many species of orchid. Runners, or stolons, are thin horizontal stems that grow rapidly. Strawberries and many grasses produce stolons. Both types of horizontal stem produce stems and adventitious roots at nodes along their length. In time, these separate from the parent plant, becoming independent individuals. Stem tubers, such as potatoes, are a type of thickened stolons. At the end of the growing season the parent plant dies, leaving its stem tubers to develop into new plants the following year.
Plants such as gladioli or crocuses have short, round, underground stems called corms. Corms develop daughter corms around their base that separate when the parent corm disintegrates. Bulbs reproduce in a similar manner. This is best seen in garlic. If you pull apart a garlic bulb, you will often find newly developing daughter bulbs inside. On a larger scale, when some trees fall or make other contact with the ground, they may grow new vertical shoots that generate new individuals.
Plants that use their leaves to reproduce use two main strategies. The simplest occurs when a leaf falls or is blown some distance. In a few species, if conditions are right, the leaf may produce roots and a shoot. Other plants, such as the succulent genus Bryophyllum, grow small plantlets around the edges of their leaves. When these mature, they drop off and can grow into new plants.
Vegetative propagation is widely used in agriculture. Farmers divide and plant rhizomes, tubers, bulbs, and other organs of plant asexual reproduction. Planting fragments, planting cuttings, grafting, and layering are artificial methods of vegetative reproduction. Some plant cultivars that are seedless can only be reproduced through these artificial methods. Plant tissue culture is a procedure in which parts of plants, sometimes a few cells, are grown in nutrient medium in a lab. These cells develop into individual plants.
Vegetative propagation provides plants with a number of advantages. Plants can rapidly occupy vacant habitat using horizontal stems. Underground structures are protected from surface conditions such as grazing, cropping, or fire that may wipe out the above ground plant structures. Parent plants can also provide nutrients to their offspring, allowing them to get established. The downside of vegetative propagation is that it results in offspring that are genetically identical. Disease agents that infect a parent plant are more likely to infect its clone.
The second type of asexual reproduction used by plants is called apomixis. Apomixis is a reproductive process in which a plant’s ovule gives rise to its own embryo. The ovule is the structure inside a seedglossary term (opens in a new window) plant that contains the female gametophyteglossary term (opens in a new window). The ovule matures into a seed on its own, and there is no direct mixing of sperm and egg. Dandelions are examples of plants that undergo apomixis. Apomixis should not be confused with self-pollination.
Sexual Plant Reproduction
In animals, sexual reproduction involves the production of female and male gametes during meiosis. Each gamete is haploid (1n). It has half the number of chromosomes of its diploid (2n) parent’s cells. These gametes join during fertilization to produce a zygote, the first cell of a new diploid individual. The genome of this offspring consists of a mixture of genes from both gametes.
In plants, the process of sexual reproduction is different. In plants, meiosis does not immediately lead to gamete formation. Plants have two separate stages in their life cycles. They have a diploid stage called a sporophyteglossary term (opens in a new window) (2n) and a haploid phase, a gametophyte (1n). This type of life cycle is said to have two alternating generations.
Trees, herbs, and ferns are the types of plants we are most familiar with. These are diploid stages of a plant’s life cycle. They are all sporophytes. At some stage in their life cycle, their reproductive structures produce spores by meiosis. Because they are produced by meiosis, spores are haploid (1n). Spores can be male or female.
These spores grow by mitosis into a haploid organism called a gametophyte. Depending on the sporeglossary term (opens in a new window), some gametophytes are female and some are male. At a certain point in the life cycle, the male gametophytes produces sperm (1n) and the female gametophytes produce eggs (1n). A sperm will fertilize an egg, producing a zygote (2n). This zygote is the first cell of the diploid sporophyteglossary term (opens in a new window). Remember, sporophytes produce spores (2n). Gametophytes produce gametes (1n)—egg and sperm.
Conifers also alternate their generations. Conifer trees are the sporophyte generation, but what does the gametophyte look like?
Conifer gametophytes are tiny and can only exist connected to the parent plant. The cones are the reproductive structures of conifers. They produce the male and female spores. These spores give rise to male and female gametophytes. These produce male and female gametes, which when fertilized produce a zygote that develops into an embryo. The embryo becomes part of a seed. This all happens inside the cone. The seed is released from the cone and may grow where it lands.
A similar process occurs inside the flowers of flowering plants.
Teacher Note: Concepts
This item requires students to suggest the cause-and-effect relationships of vegetative propagation in a complex, historical, human-designed agricultural system. The task is to understand the conditions described, identify the appropriate advantages and disadvantages, and explain their answer. After students complete this item individually, have them discuss their responses in small groups and form a consensus on any variances in their responses. Debrief with a short discussion, and consider having interested students conduct additional research on the famine.
