Asexual means without sex and this method does not involve the fusion of gametes in most cases. There are many different forms of asexual reproduction employed by different kinds of organisms including plants. Some of these methods are:
(i) Binary fission in bacteria
It is a type of asexual reproduction in which one unicellular organism divides into two by simple division (fig 16.1). Bacteria belong to kingdom monera and exhibit binary fission. Under favorable conditions one bacterium would divide into two bacteria after about every 20 minutes. They gave a single circular chromosome made of DNA. Steps involved in binary fission are:
(1) when the DNA molecule replicates, it results in the formation of two chromosomes.
(2) These two chromosomes move towards opposite sides.
(3) The middle portion of the cell membrane invigilates inwards from the two sides and when meet in the centre it separates the two halves of the bacterial cell.
(4) New cell wall is deposited between two cross cell membranes.
(5) The daughter bacteria grow to their normal size, and then separate from one another.
(ii) Budding in yeasts
Yeast, a fungus, consists of a single microscopic spherical cell. The cells of yeast reproduce by budding. It is a special type of asexual reproduction, in which a small bud or out-growth is formed on one side of cell. The nucleus divides by mitosis into two nuclei, and one of the two nuclei is passed on to the bud. Bud enlarges and is finally cut off as an independent cell. When the process of budding is rapid, the individual cells so formed do not separate at once and as a result, small groups of attached cells may some times be seen.
“Practical work: observation of budding in yeast using prepared slides”
1. How many buds are present on a single yeast cell?
2. Can you find nucleus in any bud?
(iii) Spore formation in Rhizopus
You have already studied the structure of Rhizopus. Asexual reproduction in Rhizopus is by the formation of spores in sporangia which are borne on the tips of sporangiophores. Spores are single celled, thick walled, and are produced in sporangium without sexual process. These spores are released from the sporangium and are carried by wind. Each spore when exposed to suitable conditions forms new Rhizopus mycelium.
(Fig.16.3 Sporangium and spore formation in Rhizopus)
“Practical work examination of sporangia and spores of Rhizopus, from prepared slides and fresh material”
Study the prepared slides of Rhizopus showing sporangia and spores under the microscope.
In chapter 5, you have already learned to prepare slide of Rhizopus from fresh material. Make a slide and study it under microscope.
Draw labeled diagrams of sporangia and spores.
1. What is the shape of sporangium?
2. What is the shape of spores?
3. How many spores are present within a sporangium?