Evolution of Seed Habit Part 4
Double
fertilization is a special process found in angiosperms. In this two
male gametes fuses with two cells simultaneously.a male gamete (n) fuses
with egg (n) to form a diploid zygote (2n) which develops later into an
embryo and second male gamete (n) fuses with another female cell called
fusion nucleus (2n) resulting into a triploid (3n) endosperm cell,
which develops into food storing endosperm tissue. It is an important
evolutionary advancement in which food storage fertilized ovule is made
only on fertilization i.e. formation of zygote. This actually helps the
plant to economize its food resources.
Classification of angiosperms
The
class angiospermae is divided into two sub-classes, the monocotyledonae
(with one cotyledon) and the dicotyledonae (with two cotyledons),
according to the number of cotyledons in the embryo.
The
plants included in the monocotyledonae are called monocotyledonous
plants or monoocts. The plants included in the dicotyledonae are called
dicotyledonous plants or dicots. A few distinguishing characters of the
two classes are given below:
Anglospermic familes
Some angoispermic families are described below:
Rosaceae (rose family):
- Rosaceae (Rose family)
- Solanaceae (potato family).
- Fabaceae (pea family).
- Caeselpiniaceae (cassia family).
- Mimosaceae (acacia family).
- Poaceae (grass family).
A family with about 100 genera and 2000 species is distributed over most of the earth. In pakintan 29 genera and about 213 species are reported.
Familiar plants: pyrus (pear): rosa (rose); malus (apple); fragaria (strawberry) etc.
Fig. 9.25 Rosaceae:
A-twig: B-young stamen B1-enlarged open anther, showing pollen in it:
C-style hairy and stigma bilabiate: C1-enlarged bilabiate stigma.
Vegetative characters
Plants
are trees, shrubs and herbs. Stem of the shrubby plants usually have
spines. Leaves are alternate, rarely opposite, simple or compound, with
paired stipules which are sometimes adnate to the pertiole; spines may
also occur on the rachis.
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