7. The reproductive organs are usually grouped into compact cones or strobili (exception-female reproductive organs in Cycas).

8. Pollination is mostly anemophilous (by wind).

9. The endosperm (representing the female gametophyte) is a pre-fertilization tissue and is haploid.

10. Archegonia are highly reduced and simple.

11. Vessels are absent from xylem.

(b) Cycas is among the simplest of living gymnosperms. There are about twenty species of Cycas  in the world. Cycas revoluta and Cycas circinalis are two of them.

Cycas is a slow growing small tree, about 3-6 meters in height. It grows under xerophytic conditions. The plant body is a diploid sporophyte and resembles a small palm tree. It has a tap root system, columnar stem and a dense crown of green leaves (Fig. 15.8A).

The stem is thick, cylindrical and column-like. It is usually unbranched and has a growing apex (apical bud). The stem surface is covered with a thick, protective armor of the bases of fallen leaves.

The tap root (primary root) is very thick and bears secondary roots. Some of the secondary roots are negatively geotropic. These grow upwards to the soil surface and form a special type of root called coralloid roots (Fig. 15.8). These roots form clusters due to repeated irregular branching. Coralloid roots (also called root tubercles) are enlarged, roughly cylindrical and without root-hairs. They have an algal zone in the region of the middle cortex. Nitrogen-fixing blue-green alga (Anabaena) is present in the intercellular spaces of the algal zone. The alga provides nitrogen compounds to Cycas and in return, it gets space, shelter, water and nutrients from Cycas. Thus, the association is symbiotic.

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