Sexual Reproduction in Flowering Plants 5 Flashcards
development of fruit
As ovules mature into seeds, the ovary develops into a fruit, i.e., the
transformation of ovules into seeds and ovary into fruit proceeds
simultaneously. The wall of the ovary develops into the wall of fruit called
pericarp. The fruits may be fleshy as in guava, orange, mango, etc., or
may be dry, as in groundnut, and mustard, etc. Many fruits have evolved
mechanisms for dispersal of seeds.
what are false fruits
In most plants, by the time the fruit develops from the ovary, other
floral parts degenerate and fall off. However, in a few species such as apple,
strawberry, cashew, etc., the thalamus also contributes to fruit formation.
Such fruits are called false fruits (Figure 1.15b). Most fruits however
develop only from the ovary and are called true fruits
what is parthenocarpy
Although in most
of the species, fruits are the results of fertilisation, there are a few species in which fruits develop without fertilisation. Such fruits are called
parthenocarpic fruits. Banana is one such example. Parthenocarpy can
be induced through the application of growth hormones and such fruits
are seedless.
Seeds offer several advantages to angiosperms
Seeds offer several advantages to angiosperms. i) Firstly, since
reproductive processes such as pollination and fertilisation are
independent of water, seed formation is more dependable.
ii) Also seeds have
better adaptive strategies for dispersal to new habitats and help the species to colonise in other areas.
iii)As they have sufficient food reserves, young
seedlings are nourished until they are capable of photosynthesis on their
own.
iv)The hard seed coat provides protection to the young embryo.
v)Being
products of sexual reproduction, they generate new genetic combinations
leading to variations.
vi)Seed is the basis of our agriculture. Dehydration and dormancy of
mature seeds are crucial for storage of seeds which can be used as food
throughout the year and also to raise crop in the next season
How long do the seeds remain alive after they are dispersed?
This
period again varies greatly. In a few species the seeds lose viability within
a few months. Seeds of a large number of species live for several years.
Some seeds can remain alive for hundreds of years.
describe some of the oldest seeds
The oldest is that of a lupine, Lupinus
arcticus excavated from Arctic Tundra. The seed germinated and flowered
after an estimated record of 10,000 years of dormancy. A recent record of
2000 years old viable seed is of the date palm, Phoenix dactylifera
discovered during the archeological excavation at King Herod’s palace
near the Dead Sea.
what is apomixis
Although seeds, in general are the products of fertilisation, a few flowering
plants such as some species of Asteraceae and grasses, have evolved a
special mechanism, to produce seeds without fertilisation, called apomixis.
apomixis is a
form of asexual reproduction that mimics sexual reproduction.
what are the 2 mechanisms of apomixis
There are
several ways of development of apomictic seeds. I,i) n some species, the
diploid egg cell is formed without reduction division and develops into
the embryo without fertilisation.
ii) More often, as in many Citrus and Mango varieties some of the nucellar cells surrounding the embryo sac start
dividing, protrude into the embryo sac and develop into the embryos. In
such species each ovule contains many embryos.
what is polyembryony
Occurrence of more
than one embryo in a seed is referred to as polyembryony.
what are hybrid plants and why are they desirable? what is the major draw back to it
Hybrid varieties of several of our food and vegetable crops are being
extensively cultivated. Cultivation of hybrids has tremendously increased
productivity. One of the problems of hybrids is that hybrid seeds have
to be produced every year. If the seeds collected from hybrids are sown,
the plants in the progeny will segregate and do not maintain hybrid
characters. Production of hybrid seeds is costly and hence the cost of
hybrid seeds become too expensive for the farmers.
If these hybrids are
made into apomicts, there is no segregation of characters in the hybrid
progeny. Then the farmers can keep on using the hybrid seeds to raise
new crop year after year and he does not have to buy hybrid seeds every
year. Because of the importance of apomixis in hybrid seed industry,
active research is going on in many laboratories around the world to
understand the genetics of apomixis and to transfer apomictic genes
into hybrid varieties.
