8. Plants Reproduction

General features of sexual systems

In most plant groups, both sexual and asexual methods of reproduction occur. Some species, however, seem secondarily to have lost the capacity for sexual reproduction. Such cases are described below

The cellular basis

Sexual reproduction at the cellular level generally involves the following phenomena: the union of sex cells and their nuclei, with concomitant association of their chromosomes, which contain the genes, and the nuclear division called meiosis. The sex cells are called gametes, and the product of their union is a zygote. All gametes are normally haploid (having a single set of chromosomes) and all zygotes, diploid (having a double set of chromosomes, one set from each parent). Gametes may be motile, by means of whiplike hairs (flagella) or of flowing cytoplasm (amoeboid motion). In their union, gametes may be morphologically indistinguishable (i.e., isogamous) or they may be distinguishable only on the criterion of size (i.e., heterogamous). The larger gamete, or egg, is nonmotile; the smaller gamete, or sperm, is motile. The last type of gametic difference, egg and sperm, is often designated as oogamy. In oogamous reproduction, the union of sperm and egg is called fertilization. Isogamy, heterogamy, and oogamy are often considered to represent an increasingly specialized evolutionary series.

In the plants included in this article—bryophytes (mosses, hornworts, and liverworts) and tracheophytes (vascular plants)—sexual reproduction is of the oogamous type, or a modification thereof, in which the sex cells, or gametes, are of two types, a larger nonmotile egg and a smaller motile sperm. These gametes are often produced in special containers called gametangia, which are multicellular. In cases in which special gametangia are lacking, every cell produces a gamete. In oogamy, the male gametangia are called antheridia and the female oogonia or archegonia. A female gametangium with a sterile cellular jacket is called an archegonium, although, like an oogonium, it produces eggs. In most of the plants dealt with in this article, the eggs are produced in archegonia and the sperms in antheridia with surface layers of sterile cells.


The plant basis

Individual plants may be either bisexual (hermaphroditic), in which male and female gametes are produced by the same organism, or unisexual, producing either male or female gametes but not both. A bisexual individual, however, is not necessarily capable of fertilizing its own eggs. In certain ferns, for example, male gametes of one individual are not compatible with the female gametes of the same individual, so cross-fertilization (with another individual of the species) is obligatory. This situation, of course, is similar in adaptive significance to cross-pollination (which leads to cross-fertilization) among seed plants.

Among the liverworts, mosses, and vascular plants, the life cycle involves two different phases, often called generations, although only one plant generation is, in fact, involved in one complete cycle. This type of life cycle is often said to illustrate the “alternation of generations,” in which a haploid individual (i.e., with one set of chromosomes), or tissue, called a gametophyte, at maturity produces gametes that unite in pairs to form diploid (i.e., containing two sets of chromosomes) zygotes. The latter develop directly into individuals, or tissues, called sporophytes, in which the nuclei of certain fertile cells, called spore mother cells, or sporocytes, give rise to haploid spores (sometimes called meiospores). These spores are lightweight and are borne by air currents; they germinate to form the haploid, sexual, gamete-producing phase, usually designated the gametophyte.

There are several variations in the above-described life cycle. The haploid gametophyte and sporophyte may be free-living, independent individuals (e.g., certain algae and yeasts), in which case the life cycle is diplobiontic, or the sporophyte may be physically attached to the gametophyte, as it is in liverworts and mosses. By contrast, the gametophytic phases develop as parasites on the sporophytes of the seed plants, as in certain algae. In further variation, the alternating phases may be similar morphologically except for the type of reproductive cells (gametes or spores) they produce (isomorphic life cycle), or they may be strikingly dissimilar, as in some mosses, ferns, and seed plants (heteromorphic life cycle). Only heteromorphic life cycles occur in liverworts, mosses, vascular plants, and certain fungi.

The differences between the gametophyte and sporophyte are often great, especially those of the diplobiontic types, so the alternates seem to be two different, unrelated individuals rather than different manifestations of the same organism.

8. Plants Reproduction

Reproduction by special asexual structures

Throughout the plant kingdom, specially differentiated or modified cells, groups of cells, or organs have, during the course of evolution, come to function as organs of asexual reproduction. These structures are asexual in that the individual reproductive agent develops into a new individual without the union of sex cells (gametes). A number of examples of special asexual agents of reproduction from several plant groups are in this section.

Airborne spores characterize most nonflowering land plants, such as mosses, liverworts, and ferns. Although the spores arise as products of meiosis, a cellular event in which the number of chromosomes in the nucleus is halved, such spores are asexual in the sense that they may grow directly into new individuals, without prior sexual union.

Among liverworts, mosses, lycopods, ferns, and seed plants, few-to many-celled specially organized buds, or gemmae, also serve as agents of asexual reproduction.

The vegetative, or somatic, organs of plants may, in their entirety, be modified to serve as organs of reproduction. In this category belong such flowering-plant structures as stolons, rhizomes, tubers, corms, and bulbs, as well as the tubers of liverworts, ferns, and horsetails, the dormant buds of certain moss stages, and the leaves of many succulents. Stolons are elongated runners, or horizontal stems, such as those of the strawberry, which root and form new plantlets when they make proper contact with a moist soil surface. Rhizomes, as seen in iris, are fleshy, elongated, horizontal stems that grow within or upon the soil. The branching of rhizomes results in multiplication of the plant. The enlarged fleshy tips of subterranean rhizomes or stolons are known as tubers, examples of which are potatoes. Tubers are fleshy storage stems, the buds (“eyes”) of which, under proper conditions, can develop into new individuals. Erect, vertical, fleshy, subterranean stems, which are known as corms, are exemplified by crocuses and gladioli. These organs tide the plants over periods of dormancy and may develop secondary cormlets, which give rise to new plantlets. Unlike the corm, only a small portion of the bulb, as in lilies and the onion, represents stem tissue. The latter is surrounded by the fleshy food-storage bases of earlier-formed leaves. After a period of dormancy, bulbs develop into new individuals. Large bulbs produce secondary bulbs through development of buds, resulting in an increase in number of individuals.

sprouted potato tuber
8. Plants Reproduction

General features of asexual systems

Asexual reproduction involves no union of cells or nuclei of cells and, therefore, no mingling of genetic traits, since the nucleus contains the genetic material (chromosomes) of the cell. Only those systems of asexual reproduction that are not really modifications of sexual reproduction are considered below. They fall into two basic types: systems that utilize almost any fragment or part of a plant body and systems that depend upon specialized structures that have evolved as reproductive agents.

Reproduction by fragments

In many plant groups, fragmentation of the plant body, followed by regeneration and development of the fragments into whole new organisms, serves as a reproductive system. It is common horticultural practice to propagate desirable varieties of garden plants by means of plant fragments, or cuttings. These may be severed leaves or portions of roots or stems, which are stimulated to develop roots and produce leafy shoots. Naturally fallen branches of willows (Salix) and poplars (Populus) root under suitable conditions in nature and eventually develop into trees. Other horticultural practices that exemplify asexual reproduction include budding (the removal of buds of one plant and their implantation on another) and grafting (the implantation of small branches of one individual on another).

asexual reproduction
asexual reproduction Production of new individuals along a leaf margin of Kalanchoe pinnata.Eric Guinther

Fragments of the plant bodies of liverworts and mosses regenerate to form new plants. In nature and in laboratory and greenhouse cultures, liverworts fragment as a result of growth; the growing fragments separate by decay at the region of attachment to the parent. During prolonged drought, the mature portions of liverworts often die, but their tips resume growth and produce a series of new plants from the original parent plant.

In mosses, small fragments of the stems and leaves (even single cells of the latter) can, with sufficient moisture and under proper conditions, regenerate and ultimately develop into new plants.

8. Plants Reproduction

plant reproductive system

plant reproductive system, any of the systems, sexual or asexual, by which plants reproduce. In plants, as in animals, the end result of reproduction is the continuation of a given species, and the ability to reproduce is, therefore, rather conservative, or given to only moderate change, during evolution. Changes have occurred, however, and the pattern is demonstrable through a survey of plant groups.

Reproduction in plants is either asexual or sexual. Asexual reproduction in plants involves a variety of widely disparate methods for producing new plants identical in every respect to the parent. Sexual reproduction, on the other hand, depends on a complex series of basic cellular events, involving chromosomes and their genes, that take place within an elaborate sexual apparatus evolved precisely for the development of new plants in some respects different from the two parents that played a role in their production.

In order to describe the modification of reproductive systems, plant groups must be identified. One convenient classification of organisms sets plants apart from other forms such as bacteria, algae, fungi, and protozoans. Under such an arrangement, the plants, as separated, comprise two major groups—the nonvascular bryophytes (mosses, hornworts, and liverworts) and the vascular tracheophytes. The vascular plants include the seedless lycophytes and ferns (both groups are considered lower vascular plants) and the two groups of seed plants, the gymnosperms and angiosperms.

A comparative treatment of the two patterns of reproductive systems will introduce the terms required for an understanding of the survey of those systems as they appear in selected plant groups.

8. Plants Reproduction

Asexual Reproduction

Asexual reproduction only requires DNA from one parent. It creates offspring that are genetically identical to the parent. Genetically identical offspring are called clones. Clones lack genetic diversity. This makes them more susceptible to disease. It also makes them less adaptable to changes in the environment. 

There are different methods of asexual reproduction. They include vegetative propagation and fragmentation.

Vegetative propagation does not require seeds or spores. Instead, offspring grow from a part of the parent plant. In different plants, vegetative propagation happens in different ways. Here are a few examples.

  • Garlic, onions and tulip plants all reproduce using true bulbs. These short underground stems are also called scaly bulbs. They have a basal plate that is usually surrounded by modified leaves. These leaves form a papery covering called a tunic. New bulbs grow off of the parent bulb’s basal plate.
  • Crocuses reproduce using corms, which are similar to true bulbs. However, a corm doesn’t have as many layers. Corms are used up during the growing season and get replaced by one or more new corms. 
  • Potato plants reproduce using tubers. These underground growths produce new plants from stems or growing points called eyes. 
  • Ginger plants reproduce using rhizomes. These stems that grow sideways along the soil or just below the surface. They branch apart to produce new points of growth. 
  • Strawberry plants reproduce using stolons. They look like branches growing along the ground. Stolons anchor themselves to the ground and develop roots. And these roots grow into new plants.
8. Plants Reproduction

Plant Reproduction

Plants are living organisms. That means they need to reproduce in order to pass on their genes to future generations. Plants can create offspring through either sexual or asexual reproduction.

Sexual Reproduction

Sexual reproduction requires genetic material (DNA) from two parents. The parent plants have male and female sex cells, called gametes. The genetic material from the male and female gametes combines to produce offspring. This process is called fertilization.

Seeds produced through fertilization contain genetic material from both parents. As a result, the offspring are not genetically identical to either of the parent plants. This genetic diversity can help them survive if the environment changes. 

Flowering plants reproduce sexually through a process called pollination. The flowers contain male sex organs called stamens and female sex organs called pistils. The anther is the part of the stamen that contains pollen. This pollen needs to be moved to a part of the pistil called the stigma

Parts of a flower
Parts of a flower

Plants can either self-pollinate or cross-pollinate. Self-pollination happens when a plant’s own pollen fertilizes its own ovules. Cross-pollination happens when the wind or animals move pollen from one plant to fertilize the ovules on a different plant. The advantage of cross-pollination is that it promotes genetic diversity. Some plants have features that prevent self-pollination, such as pollen and ovules that develop at different times. 

Pollinators are animals that carry pollen between plants. Many pollinators are insects, like bees, butterflies, moths and beetles. Some birds, including hummingbirds, also play a part. Likewise, certain mammals, like bats and rodents, move pollen between plants. The colours and smells of flowers often attract pollinators. Pollen will stick to a pollinator’s body as it feeds on the flower’s nectar

Fertilization is the next step after pollination. Once it reaches the pistil, the pollen needs to fertilize an egg inside the stigma. This egg is called an ovule. 

Fertilization creates fruits that contain seeds. Some fruits are fleshy, like oranges and watermelons. Others are dry, like acorns or walnuts. These fruits are an attractive food for various animals. After digesting fruit, animals expel waste that contains seeds. This way, seeds can take root and grow in places far from the plants that produced them!