In this article we will discuss about the asexual and sexual modes of reproduction in plants and animals.
Sexual Reproduction in Plants and Animals:
The mode of reproduction which involves the formation of male and female gametes either by the same individuals or by different individuals of opposite sex is known as sexual reproduction. These gametes fuse to form a new cell called zygote, which grows and develops into a new individual.
Sexual reproduction is exhibited by all the flowering plants and most of the animals. Before an organism starts to reproduce sexually, it has to attain a certain level of growth and maturity. This period during which an organism grows to attain the sexual maturity is called juvenile phase (in animals) or vegetative phase (in plants). This phase is followed by another phase called reproductive phase.
The end of reproductive phase is marked by the onset of another phase called senescent phase. After this phase, the organism dies. In both plants and animals, hormones regulate the transitions between these three phases. Interaction between hormones and certain environmental factors regulate the reproductive processes and the associated behavioural expressions of organisms.
Points related to the attainment of sexuality in plants are:
1. Annuals and biennial plants exhibit clear cut vegetative, reproductive and senescent phases.
2. In perennials, these phases cannot be defined clearly.
3. Some plants show unusual behaviour for flowering, e.g., Strobilanthus kunthiana (neelakurinji) plant flowers once in 12 years, whereas bamboo plants flower only once in their life time.
Sexuality in Plants:
In lower plants, the male and female reproductive structures may be present on the same plant; such plants are called homothallic or monoecious, e.g., Chara.
In some other lower plants, male and female reproductive structures may be present on different plants. These plants are known as heterothallic or dioecious, e.g., date palm.
In angiosperms, flower is the reproductive part of the plant. In some angiosperms like mustard, China rose, pea, each flower contains both stamens and pistil. Such type of flowers are called bisexual flowers.
In contrast to above, in some angiosperms such as papaya, watermelon, sunflower, rice, wheat, etc., flowers contain either stamen or pistil. Such types of flowers are called unisexual flowers. Flowers with stamen (staminate flowers) are male flowers, while flowers with pistil (pistillate flowers) are female flowers.
Sexuality in Animals:
Just like plants, animals also exhibit variation in attaining sexual maturity. Some animals remain reproductively active throughout their reproductive phase; such animals are called continuous breeders, e.g., human. On the other hand, there are some animals, which become reproductively active only during the favourable seasons in their reproductive phase; such animals are called seasonal breeders, e.g., dog, birds, frog, lizard, etc.
During the reproductive phase in mammals, the activities of ovaries, accessory ducts and hormones exhibit cyclic changes. In the non-primate animals such as dogs, tiger, cow, rats, sheep, etc., these cyclic changes are called oestrus cycle, while these changes in the primates such as monkey, apes and human etc., are called menstrual cycle. On the basis of sexuality, animals are divided into two categories, i.e., bisexual hermaphrodaac animals and unisexual animals.
(i) Bisexual:
These possess both male and female reproductive organs in the same individual, e.g., hermaphrodites (earthworms, tapeworm, leech and sponges).
(ii) Unisexual:
Both the sexes are separate with distinct male and female individuals, e.g., cockroach, dog, etc.
Asexual Reproduction in Plants and Animals:
Production of an offspring by a single parent without the formation and fusion of gametes is called asexual reproduction. It involves only mitotic cell divisions and meiosis does not occur in asexual reproduction.
Offspring produced by asexual reproduction are not only identical to parents but are also exact copies of their parent or clone of their parent. The term ‘clone’ is used to describe such morphologically and genetically similar individuals.
Asexual reproduction occurs in protozoans and some lower animals such as sponges, coelenterates, certain worms and tunicates. It is absent among the higher non-vertebrates and all vertebrates.
Asexual Reproduction in Animals:
Asexual reproduction is common among single-celled organisms and animals with relatively organisation. In protists and monerans, the organism or the parent cell divides into two, to give rise to a new individual. Thus, in these organisms cell division is itself a mode of reproduction.
It takes place in animals generally by:
(i) Fission:
It is the division of the parent body into two or more daughter individuals identical to the parent.
Fission can occur by following types:
(a) Binary Fission:
It is the division of the parent cell into two small, nearly equal sized daughter individuals. Each grows into an adult, e.g., Amoeba, Paramecium, etc.
(b) Multiple Fission:
It is the division of the parent body into many small daughter individuals simultaneously, e.g., Plasmodium (the malarial parasite), Amoeba (during unfavourable conditions).
(ii) Budding:
It is the formation of a daughter individual from a small projection, the bud, arising on the parent body.
Depending on the place where the bud arises, budding can be of two types:
(a) External budding (Hydra)
(b) Internal budding
(iii) Spore Formation:
These are mitospores or asexually produced spores, formed to overcome unfavourable conditions. Spore formation is mainly found in lower fungi.
Following types of spores are found in various organisms as:
(a) Zoospores:
They are spores with motile flagella that occur in, e.g., Chlamydomonas, Albugo, Achyla. These spores bear 1-2 flagella born anterior or posterior. These flagella help the zoospores to swim in aquatic habitat for proper dispersal.
(b) Conidia:
They are non-motile exogenous spores, which develop through constrictions on the tips of the hyphae, e.g., Penicillium, Aspergillus.
(c) Gemmules:
A mass of cells enclosed within a common opaque envelope, which is released by sponges is called gemmule. In all freshwater and a few marine sponges, gemmules or internal buds are formed. Each gemmule has a mass of archeocytes surrounded by the protective covering to tide over seasonal drought or adverse environmental conditions. A mass of archeocytes come out via micropyle or a minute pore and grows into a sponge.
Asexual Reproduction in Plants:
It is commonly known as vegetative propagation. It is the formation of new plants from the vegetative parts like roots, stems, leaves, etc. The structural unit that is employed in place of seed is called a propagule.
Vegetative propagation in plants is of two types:
(i) Natural Vegetative Propagation:
This can take place by the following methods
(a) Root Tubers:
The buds present on the roots grow into leafy shoots called slips above the ground and adventitious roots at their bases. Each slip gives rise to a new plant, e.g., sweet potato, dahlia, guava, yam, Tinospora.
(b) Underground Stems:
Vegetative propagation carried out through underground stem.
It can be further categorized into following types:
i. Suckers:
They arise from the base of the erect shoot, grow horizontally in the soil and then come out to form new aerial shoots. These shoots become independent when suckers break away from the parent plant, e.g., mint. Chrysanthemum.
ii. Rhizomes:
These serve as a means of vegetative reproduction by perennating under unfavourable conditions and producing new aerial shoots during favourable conditions, e.g., ginger, turmeric, banana, lotus, water hyacinth.
iii. Bulbs:
They are highly reduced underground stems, e.g., onion, lilies, garlic.
iv. Tubers:
They are the modified underground stem branches having several buds, e.g., each eye of the potato is a bud, which grows into a new potato. The potato crop is raised by the tubers not by seeds.
(c) Creeping Stem:
Vegetative propagation by the means of creeping stem, can be categorized under:
i. Runners:
They are modified stems, which produce adventitious roots at nodes. Each node gives rise to an aerial shoot which becomes a new plant, e.g., Oxalis, Centella.
ii. Offsets:
They are one internode long runners, which develop tuft of leaves at the apex, e.g., Pistia, water lettuce.
(d) Aerial Steam:
Aerial modified stems of cacti develop new plant when the stem segments fall on the ground and act as the means of vegetative propagation, e.g., Opuntia.
(e) Leaves:
Some plants develop adventitious buds on their leaves, which get detached and develop new plants, e.g., Bryophyllum, Kalanchoe, Begonia.
(f) Bulbils:
These are fleshy buds produced in the axil of foliage leaves in place of axillary buds. They grow to form new plants when shed/fall on the ground, e.g., lily, Oxalis, Agave, Allium sativum. The site of origin of the new plantlets in the plant given above arise invariably from the nodes present in the modified stems. When these nodes come in contact with damp soil or water, they produce roots and new plant.
(ii) Artificial Vegetative Propagation:
The man-made methods are called artificial methods of vegetative propagation.
These include:
(a) Cuttings:
When a small piece of any plant organ stem/root or leaf is used for propagation, e.g., leaves (Bryophyllum), roots (tamarind), stems (sugar cane, grapes, rose and Bougainvillia).
(b) Layering:
In this method, roots are artificially induced on the stem branches before they are detached from the parent plant, e.g., Jasmine, grape, litchi, orange, etc.
(c) Grafting:
In this method, parts of the two plants are joined in such a way that they grow as one plant. The rooted supporting portion on one plant called stock is joined with the twig of another plant called scion, e.g., rose, apple, plum, peach, mango.
(d) Micro-Propagation:
This method includes propagation of plants by culturing the cells, tissues and organs called as tissue culture. Initially, this forms an undifferentiated mass of cells called callus. Later on, this callus differentiates to produce a large number of plantlets.