Evolution of Seed Habit Part 4

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:

Fig. 9.24 comparison of dicot and monocot

Anglospermic familes

Some angoispermic families are described below:

1. Rosaceae (Rose family)
2. Solanaceae (potato family).
3. Fabaceae (pea family).
4. Caeselpiniaceae (cassia family).
5. Mimosaceae (acacia family).
6. Poaceae (grass family).

Rosaceae (rose 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.