Formation of a seed: The ovule develops into a seed after fertilization. The integument forms a seed coat. The zygote develops into a dicot embryo differentiated into radicle, hypocotyle, stem tip and two cotyledons (Fig. 15.14B). The endosperm is already present in the ovule. It provides nourishment to the developing embryo.

(A) L.S. of an ovule showing       (B) Structure of a seed
female gametophyte and
germinating pollen
(diagrammatic)

Figure 15.14

In Cycas, 2-5 zygotes are formed in a single ovule. Therefore 2-5 embryos start developing simultaneously. This condition is called polyembryony. However, in a mature seed only one embryo is present. Others degenerate during seed formation. The Cycas the seed is naked, endospermic and dicotyledonous. The seed germinates and produces a new Cycas plant. It may be a male or a female.

Cycas shows a heteromorphic alternation of generations. The dominant generation is diploid, represented by the plant. It ends with meiosis during spore formation. The haploid generation has male and female gametophytes. This ends with fertilization and again a new diploid generation begins (Fig. 15.15).

Table of Contents

15.0 - Introduction
15.1 Pteridophyta : General Account
15.2 Gymnosperms
15.3 Angiosperms : Dicotyledons
15.4 Angiosperms : Monocotyledons
15.5 Vascularization
15.6 Development of seed habit
15.7 Development of Flower and Fruit

Chapter 16