Figure 15.11                            Figure 15.12
A megasporophyll           Transverse section of                                     a mature ovule of Cycas                                                              

The megasporophylls (Figure 15.11): These develop in loose spirals around the apex of the stem in female Cycas. Hence, growth of the stem in female plant is monopodial. There is no compact female cone in Cycas. A megasporophyll may be 10-12 inches long. It is narrow in the lower fertile region and is broad and expanded in the upper sterile region called the apophysis. The margin of the apophysis may be serrate or may have long finger -shaped lobes. The fertile region bears 2-10 ovules in two lateral rows. The ovules are sessile, naked and large. The surface of the megasporophyll is covered with soft velvety hair.

Structure of an ovule (megasporangium) (Figure 15.12): The ovule is egg-shaped and consists of a nucellus and a single integument. The nucellus forms the main body of the ovule. It has thin walled colorless cells. The integument is thick and differentiated into three layers: the outer and inner layers are fleshy and the middle layer is hard and stony. Micropyle is present in the integument at the apex of the ovule. The nucellus forms a nucellar beak which projects into the micropyle. A cavity develops in the nucellar beak, called a pollen chamber. Pollen grains are deposited here during pollination.

A diploid megaspore mother cell is differentiated in the tissue of a nucellus. It divides by meiosis and forms a linear tetrad of four haploid megaspores. Of these, the upper three megaspores degenerate. The basal (chalazal) megaspore is functional and forms the female gametophyte.

The two types of spores produce two types of gametophytes.

Microspore and male gametopohyte (Fig. 15.13): The microspore produces the male gametophyte. At the time of liberation from the male cone, each microspore contains a 3-celled male gametopohyte: a prothallial cell, a generative cell and a tube cell. Cycas is wind pollinated. The 3-celled pollen grains (microspores) are carried along by wind and are deposited in the pollen chamber of the ovule. The pollen grains germinate here. Each pollen forms a pollen tube which penetrates the nucellus tissue and grows towards the female gametophyte. The pollen tube contains two male gametes (sperms or spermatozoids). These are large, top-shaped and multiflagellate.

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