Cycas

Cycas is a gymnosperm. It is the only living genus identified in the family Cycadaceae. It contains more than 100 species. It is mainly distributed in the eastern and southeastern asian regions. Many Cycas species are native to China, Australia and India.

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Cycas are perennial evergreen trees. They appear like a palm tree. They are characterised by the presence of naked seeds, i.e. ovules are not enclosed within the ovary. The main body part is the diploid sporophyte. Cycas are dioecious, i.e. the male and female plants are separate.

Table of Content

Classification
Morphology of Cycas
Reproduction and Life Cycle
Economic Importance of Cycas
Frequently Asked Questions

Classification

Kingdom	Plantae
Subkingdom	Tracheobionta
Superdivision	Spermatophyta
Division	Cycadophyta
Class	Cycadopsida
Order	Cycadales
Family	Cycadaceae
Genus	Cycas

They belong to the subkingdom Tracheobionta, which comprises all vascular plants. The superdivision spermatophyta includes all seed plants.

Some of the common species of Cycas are:

C. revoluta (sago palm)
C. circinalis (fern palm) – endemic to India
C. media (Australian nut palm)
C. rumphii

Structure

The main plant body of Cycas is a diploid sporophyte. It is a vascular plant and is differentiated into roots, stems and leaves. Some of the important features of Cycas are:

Morphology of Cycas

It is an evergreen and perennial plant. Some species grow as tall as 40 feet in height.
It is a dioecious plant, i.e. male and female reproductive parts are borne on different trees.
It possesses two kinds of roots, tap root and coralloid root.
The primary roots are tap roots. The main functions of these roots are anchorage and absorption of water and minerals.
The coralloid roots develop from the normal roots. The coralloid roots are associated with cyanobacteria that perform biological nitrogen fixation.
The stem or caudex is mostly unbranched, thick and woody. The surface of the stem is rough due to persistent leaf bases.
Leaves form a crown at the top of the stem. There are two types of leaves found in Cycas, foliage leaves and scaly leaves.
The scaly leaves are small and brown in colour. They have a protective function. They protect reproductive structures and the shoot apex. The scaly leaves alternate with the foliage leaves.
The green foliage leaves are pinnately compound. Each leaf has multiple leaflets, which are arranged on a long rachis.
The leaves have circinate vernation and have a midrib without lateral veins.
The foliage leaves are not permanent and fall off, leaving leaf bases.
Xylem contains tracheids and xylem parenchyma but lacks vessels.
Phloem consists of sieve tubes and phloem parenchyma. Companion cells are absent.
They show secondary growth.
They are heterosporous. The megaspores and microspores are produced in the megasporangia and microsporangia, respectively. The sporangia are borne on spirally arranged megasporophylls and microsporophylls.
In Cycas, the megasporophylls and microsporophylls are present on different trees, hence they are dioecious.
The megasporophyll does not form a cone or a strobili.

Reproduction and Life Cycle

Cycas shows vegetative as well as sexual reproduction. Vegetative reproduction occurs through bulbils or adventitious buds. Sexual reproduction occurs through the formation of seeds.

Vegetative Reproduction

The adventitious buds or bulbils develop in the axils of scaly leaves. They detach from the stem and develop into new plants. The bud from the male plant develops into a male plant and the bud from the female plant develops into a female plant.

Sexual Reproduction

Cycas is heterosporous. It produces two kinds of spores. It is dioecious, i.e. male and female reproductive parts are borne on different plants. The sexual reproduction is oogamous, i.e. the egg is quite large in size and non-motile compared to male gametes.

Development of Male Gametophyte

The microspores are produced in the microsporangia, which are borne on microsporophylls. Microsporophylls are arranged spirally in the acropetal succession on the axis to produce a cone or compact strobili. The male cone of Cycas is the largest in the plant kingdom.

The microspores are haploid and produced by meiosis in microspore mother cells. Microspores develop into male gametophytes, which are reduced and are called pollen grains. The development of pollen grains is initiated within the sporangia. The microsporangia dehisce at the 3-celled state of pollen grains that consist of a prothallial cell, a generative cell and a tube cell. Further development of the male gametophyte occurs after pollination. Pollen grains are wind pollinated.

Development of Female Gametophyte

The megaspores are produced in the megasporangia, which are borne on megasporophylls. Megasporophylls are arranged spirally in the acropetal succession on the crown of leaves but do not form a cone or strobili. The ovules or megasporangia of Cycas are the largest in the plant kingdom.

Megaspores are haploid and are produced after meiosis in the megaspore mother cell. One of the megaspores develops into a multicellular female gametophyte within the megasporangium. The female gametophyte bears two or more archegonia that are female sex organs. The female gametophyte is retained within the megasporangium.

Fertilisation

Pollen grains are carried towards the ovule by air. The pollen tube grows towards the archegonia in the ovules and discharges its contents , i.e. male gametes, near the mouth of the archegonia. The male gamete fuses with the egg nucleus to form a diploid zygote. The zygote gives rise to the embryo and the ovule develops into a seed. The seeds are naked (not covered), which is the characteristic of gymnosperms.

Economic Importance of Cycas

Cycas are used for decorative purposes, horticulture and in ceremonies. They are also used as food and medicine.

Many species of Cycas are used as ornamental plants.
Starch is obtained from Cycas revoluta (sago palm) and some other species.
Some parts of the Cycas species plants are also used to prepare alcoholic drinks.
Some of the Cycas species produce edible seeds.
Leaves are used for decorative purposes and to prepare baskets, hats, etc.
Seeds and raisins obtained from some species are used to treat ulcers, sores and swellings.
Juice of young leaves is used to treat vomiting and flatulence.
Young succulent leaves of various species are often cooked as vegetables.
Cycas seeds are also used to treat hypertension and musculoskeletal disorders.

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Frequently Asked Questions

What is a distinguishing feature between Cycas and the extinct Cycads?

The female cone in a Cycas species is megasporophyll that is held in a lax rosette manner in comparison to Cycads that have a tightly held female cone.

Is Cycas edible?

The Cycas seeds contain cycasin toxin and should not be consumed.

Is Cycas a flowering plant?

Cycas is a gymnosperm that produces naked seeds, but does not produce flowers and fruits.