Page 125 - NCERT Science Class 10 English Medium

reproduction incorporates such a process of combining DNA from two
different individuals during reproduction.
But this creates a major difficulty. If each new generation is to be the
combination of the DNA copies from two pre-existing individuals, then
each new generation will end up having twice the amount of DNA that
the previous generation had. This is likely to mess up the control of the
cellular apparatus by the DNA. How many ways can we think of for
solving this difficulty?
We have seen earlier that as organisms become more complex, the
specialisation of tissue increases. One solution that many multi-cellular
organisms have found for the problem mentioned above is to have special
lineages of cells in specialised organs in which only half the number of
chromosomes and half the amount of DNA as compared to the non-
reproductive body cells. This is achieved by a process of cell division
called meiosis. Thus, when these germ-cells from two individuals combine
during sexual reproduction to form a new individual, it results in re-
establishment of the number of chromosomes and the DNA content in
the new generation.
If the zygote is to grow and develop into an organism which has
highly specialised tissues and organs, then it has to have sufficient stores
of energy for doing this. In very simple organisms, it is seen that the two
germ-cells are not very different from one another, or may even be similar.
But as the body designs become more complex, the germ-cells also
specialise. One germ-cell is large and contains the food-stores while the
other is smaller and likely to be motile. Conventionally, the motile germ-
cell is called the male gamete and the germ-cell containing the stored
food is called the female gamete. We shall see in the next few sections
how the need to create these two different types of gametes give rise to
differences in the male and female reproductive organs and, in some
cases, differences in the bodies of the male and female organisms.

7.3.2 Sexual Reproduction in Flowering Plants
The reproductive parts of angiosperms are located in the flower. You
have already studied the different parts of a flower – sepals, petals,
stamens and pistil. Stamens and pistil are
the reproductive parts of a flower which
contain the germ-cells. What possible
functions could the petals and sepals serve?
The flower may be unisexual (papaya,
watermelon) when it contains either stamens
or pistil or bisexual (Hibiscus, mustard)
when it contains both stamens and pistil.
Stamen is the male reproductive part and it
produces pollen grains that are yellowish
in colour. You must have seen this yellowish
powder that often sticks to our hands if we
Figure
Figure
7.7
Figure 7.77.7 touch the stamen of a flower. Pistil is present
Figure 7.7
Figure 7.7
Longitudinal section of in the centre of a flower and is the female
flower reproductive part. It is made of three parts.
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