CYCAS : Classification, Morphology, Anatomy & Reproduction in Gymnosperms | Notes

CYCAS : Classification, Morphology, Anatomy & Reproduction in Gymnosperms | Notes

The member of cycadales order are commonly known as cycads. Stick your eyes on CYCAS : Classification, Morphology, Anatomy & Reproduction in Gymnosperms |Notes. they originated from the seed ferm i.e., Cycadofilicates, toward the end of Carboniferous period and formed a dominant vegetation during Triassic period f Mesozoic era. This period is known as ‘AGE OF CYCADS’ . The order includes eleven living genera and about 100 species. 

Systamatic position

Division : Cycadophyta

  Order : Cycadales 

  Class : Cycadopsida

  Family : Cycadaceae

Genus – Cycas

Occurrence:

Sixteen species of Cycas are found in the tropical and subtropical parts of the earth. The name Cycas has been derived from a Greek word Kykas = Cocopalm. It is an evergreen plant. In India Cycas is represented by six species— C. revoluta, C. pectinata, C. siamensis, C. beddomei, C. rumphiand C. circinalis.

Cycas revoluta is the most commonly cultivated species of Indian gardens. It also occurs in China, Africa, Nepal, Sri Lanka and Japan. Its natural habitat is in open, sunny, well-drained situations.

(i) Roots

Roots are of two types, i.e., normal tap root and coralloid root. Tap root is positively geotropism, non-green, without root hair. It helps in anchorage and absorbs water and minerals. Coralloid roots are lateral branches of tap root. It grows first horizontally (diageotropic) then, repeatedly divide dichotomously, come out of the soil surface (apogeotropic) in the form of coral like mass. These are devoid of root hair and root cap.

(ii) Stem

The stem is erect, columnar, woody and un-branched. It is covered with alternate whorls of leaf bases of foliage leaves and scale leaves. The stem apex bears a crown of leaves and sex organs at maturity.

(iii) Leaves :

Leaves are dimorphic i.e. 2 types, assimilatory or foliage leaves and scaly leaves or cataphylls. Both the leaf types are born at the stem apex in alternate spirals. Foliage leaves are large (1 -3 m), green, pinnately compound. Petiole has 2 rows of spines. Rachis bears 80-100 pairs of pinnae or leaflets.

Leaflets are sessile, elongated, tough, and lanceolate in shape with a spiny apex, revolute or smooth margins. Young leaves have circinate vernation like those of ferns and always covered with brown hairs called ramenta. Scale leaves are small, dry, brown, triangular leaves covered with ramenta. They are non- photosynthetic and protect the stem apex.

Internal structure

T.S of root

Epiblema : single layer of thin walled cells. Some cells give rise to root hairs.

Cortex : multilayered zone of thin –walled parenchymatous cells. Presence of tannin cells and mucilage cells. Innermost layer is endodermis characterised by the presence of casparian bands.

Pericycle : multilayered, parenchymatous cells.

Vascular tissue : Diarch stele. Pith absent. Xylem exarch and tracheids of protoxylem have spiral thicknings and those of metaxylem have scalariform thickenings.

The mature normal root shows secondary growth which starts by the formation of cambium strips inner to the primary phloem strands, these cambium strips cut off secondary phloem towards outer side and secondary xylem  towards  the inner side.

Due to the pressure of secondary tissues, the primary phloem is crushed but the primary xylem can be seen in the center of the stele. A distinct layer of phellogen or cork cambium arises in the outer region of the cortex which give rise to cork on its outer side and phelloderm or secondary cortex on its inner side. Cork, phellogen and phelloderm are collectively known as periderm. Due to development of periderm, epiblema is ruptured.

T.S of coralloid root

Cortex differentiated into outer, inner and middle cortex. Middle cortex consists of algal zone consists of thin walled elongated cells. Blue green algae like Anabaena, Nostoc and oscillatoria sp occur which lives symbiotically.

ts of coralliod of cycas

T.S of Stem

Epidermis is the outermost layer consisting of compactly arranged thick- walled cells. Presence of several persistent leaf bases makes the epidermis a discontinuous and ruptured layer.

Cortex is large and consists of thin-walled, parenchymatous cells, filled densely with starch grains. It contains numerous mucilaginous canals and girdle traces.

Each with many radially elongated epithelial or secretory cells. Medullary rays connect the mucilage canals of the cortex with that of the pith Starch in the parenchymatous cells of the cortex is the source of ‘sago’. 

Endodermis and pericycle are not clearly demarcated mucilage canal is lined.

Numerous vascular bundles remain arranged in a ring. The stele is ectophloic siphonostele. Each vascular bundle is conjoint, collateral, open and endarch. The xylem consists of tracheids and xylem parenchyma.

Protoxylem contains tracheids with spiral thickenings while the metaxylem has scalariform thickenings. Vessels are absent. The phloem is located outside the xylem and consists of sieve tubes and phloem parenchyma. Companion cells are absent.

Between the xylem and phloem lies the primary cambium, which remains active only for a short period. It is soon replaced by another ring of secondary cambium somewhere in the cortex.

ts of stem of cycas

Leaf Traces

The leaf traces remain scattered in the cortical region of the stem and constitute the vascular supply to the leaves from the main vascular cylinder.

Normally, there are four leaf traces which form the vascular supply to the leaf. Two of these are direct traces, while the remaining two axe girdle traces.

The direct traces originate from the vascular cylinder lying in front of the leaf base while the girdle traces develop from the vascular cylinder lying opposite to that of direct traces. They proceed together and curve soon in opposite directions, and by girdling round the vascular cylinder they enter in the leaf base.

In the cortical region the girdle traces also remain connected with other leaf traces. At the time of their entrance in the petiole, the leaf trace bundles subdivide and form many petiole bundles. Such type of unique girdle traces of Cycas, which also occur in Magnoliaceae show a close relationship of Cycadales of Gymnosperms & Magnoliaceae of dicotyledons.

Secondary Growth

It is similar to that of dicotyledons. In the beginning, Cycas is monoxylic, i.e. contains a single ring of vascular bundles. But one or more concentric rings of vascular bundles appear outside the primary ring of bundles in the older stems showing polyxylic condition.

By the activity of inter-fascicular and intra-fascicular cambia, which unite to form a cambium ring, the secondary growth is initiated. This cambium ring cuts secondary phloem towards outer side and secondary xylem towards inner side.

After a short while this cambium ring stops functioning and a second cambium ring develops either in the parenchymatous cortex or in the region of pericycle This cambium ring also behaves in the similar fashion.

Cambial rings towards the periphery of the stem form lesser number of vascular bundles. The cork cambium develops on the outer region of the cortex and cuts cork towards outer side and secondary cortex towards inner side.

Rachis

The outline of transverse section is rhomboidal in the basal region of the rachis, biconvex in the middle cambium and roughly cylindrical at the tip region or at the apex of the rachis. Two arms of the bases of leaflets are present on the rachis, one on each side.

Epidermis is the outermost layer of the rachis consisting of thick-walled cells. It is heavily circularized. On its upper as well as lower sides are present irregularly distributed sunken stomata. Hypodermis is present below the epidermis.

It is differentiated into outer 2-3 layers of chlorophyll-containing thin-walled cells of chlorenchyma and inner 4-6 layers of thick- walled lignified cells of sclerenchyma. Sclerenchyma is poorly-developed on the lateral sides. It is also seen intermixed with chlorenchyma.

Ground tissue is a large region consisting of thin- walled, parenchymatous cells. Many mucilaginous canals and vascular bundles are present in this region.

Vascular bundles are arranged in the shape of an inverted Greek letter Omega (Ω). Towards the tip of the rachis the bundles are arranged in C-shaped manner and their number is comparatively less. Each vascular bundle remains surrounded by a bundle sheath. It is conjoint, collateral and open.

The xylem in each vascular bundle is present towards inner side. It consists of tracheids and xylem parenchyma. Cambium separates the xylem from the phloem. Vessels are absent.

The vascular bundles are diploxylic, i.e. consists of two types of xylem viz. centripetal xylem and centrifugal xylem. Phloem, present towards the outer side of the vascular bundle, consists of sieve tubes and phloem parenchyma. Companion cells are absent.

The vascular bundles show different structure at different levels of rachis starting from the base up to the apex, especially with regard to their diploxylic nature.

Leaflet

Cycas leaflets are large, tough, thick and leathery. In a vertical section the leaflet is differentiated into a swollen midrib portion and two lateral wings. In C. revoluta and C. beddomei the wings are curved downward or revoluted at the margins but in C. circinalis, C. rumphii, C. pectinata and C. siamensis the margins are flat.

Epidermis

It is the outermost layer consisting of thick-walled cells. It is surrounded by a thick layer of cuticle. Upper epidermis is a continuous layer while the continuity of the lower epidermis is broken by many sunken stomata.

The stomata are of haplocheilic type (perigenous) in Cycas circinalis, C. revoluta and C rumphii.

Hypodermis

Hypodermis is sclerenchymatous and present below the epidermis. It is absent below the lower epidermis but in the midrib region it is several-celled thick.

Mesophyll tissue

Mesophyll is well-developed and remains differentiated into palisade and spongy parenchyma. A continuous layer of palisade is present below the sclerenchymatous hypodermis. Its cells are radially elongated and filled with chloroplasts. The palisade may be a continuous layer over the midrib as in Cycas beddomei, C. media, C. pectinata and C. revoluta, or it may be a discontinuous layer as in C. circinalis and C. rumphii.

Spongy parenchyma is present only in the wings, directly above the lower epidermis. Its cells are oval, filled with chloroplasts, and loosely arranged having many air-filled intercellular spaces.

Transfusion tissue consists of two small groups of short and wide tracheid-like cells with reticulate thickenings or bordered pits on their walls.

Few layers of transversely elongated cells are present in both the wings just in between the palisade and spongy parenchyma. This represents the accessory transfusion tissue or secondary transfusion tissue.

Vascular bunbles

Vascular bundle is one, and present in the midrib region of the leaflet. It is conjoint, collateral, open and diploxylic. The triangular centrifugal xylem is well-developed with endarch protoxylem. It is represented by two or sometimes more small groups on either side of the protoxylem.

Phloem is arc-shaped and remains separated by cambium. Phloem consists of sieve tubes and phloem parenchyma. Companion cells are absent. The portion of the midrib in between the palisade layer and lower hypodermal region is filled with parenchymatous cells. Some of these cells contain calcium oxalate crystals.

Reproduction in Cycas

(i) Vegetative Reproduction

The most common method of vegetative propagation in Cycas is by bulbils. The bulbils develop from the axil of the scaly leaves. They are more or less oval structures with a broad base narrowing towards the apex. Several scaly leaves are arranged spirally and compactly over a dormant stem in a bulbil.

On detachment from the stem, a bulbil starts germination by producing many roots towards the lower side and a leaf towards the upper side. A bulbil from male plant will develop only into the male plant, while from the female plant will form only the female plant because Cycas is a strictly dioecious plant.

Male Cone

The male cone or male strobilus is a large, conical or ovoid, compact, solitary and shortly-stalked structure, which is generally terminal in position. It sometimes attains a length of as much as 1.5 metre. In the centre of the cone is present a cone axis.

Several perpendicularly attached microsporophylls are arranged around the cone axis in closely set spirals. At the base of male cone are present many young leaves. All the microsporophylls in a male cone are fertile except a few at its basal and apical parts.

The terminal growth of the stem is checked for sometimes when a male cone appears at its apex. It is because of the fact that the apical meristem is used up during the development of the male cone. Cones of some species of Cycas are amongst the largest cones in the plant kingdom.

male of cycas

Microsporophylls, Microsporangia & Microspores

Microsporophylls are flat, leaf-like, woody and brown-coloured structures with narrow base and expanded upper portion. The upper expanded portion becomes pointed and is called apophysis. Narrow base is attached to the cone axis with a short stalk.

Each microsporophyll contains two surfaces, i.e. an adaxial or upper surface and an abaxial or lower surface. On the adaxial surface is present a ridge-like projection in the middle and an apophysis at the apex.

On the abaxial surface are present thousands of microsporangia in the middle region in the groups of 3-5. Each such group is called a sorus. In between these groups are present many hair-like structures, which are very soft and one or two- celled structures.

Microsporangia

In T.S. of a microsporophyll, there are present many microsporangia on the abaxial side. Each shortly-stalked, oval or sac-like microsporangium is surrounded by 5-6 layers. The wall layers of each sporangium include an outer thick epidermis or exothecium, middle zone of thin-walled cells and an innermost layer of tapetum.

Microspore

Many pollen grains or microspores are present in each sporangium. In the expanded region of microsporophyll are present many mucilaginous canals and vascular bundles. Each sporangium is provided with a radial line of dehiscence, which helps in the dispersal of spores.

Each microspore or pollen grain is a rounded, unicellular and uninucleate structure surrounded by an outer thick exine and inner thin intine. Cytoplasm surrounds the centrally located nucleus. A large vacuole is also present

Female Reproductive Organs

True female cone or strobilus is absent in Cycas. Female reproductive organs are present in the form of megasporophylls. Many megasporophylls are present around the apex of the monopodial trunk of the female plant above each crown of foliage and scaly leaves.

Similar to foliage leaves, megasporophylls also remain spirally arranged at the apex of the stem but their number is very large and thus they appear like a rosette. Vegetative leaves and fertile megasporophylls are produced in an alternate succession without showing any effect on apical meristem.

Usually the megasporophylls in Cycas are produced only once in a year. From the apex of the main stem the megasporophylls arise in an acropetal succession.

female of cycas

Megasporophyll

Each megasporophyll is considered a modification of foliage leaf. It reaches up to 30 cm or more in length in different species. It is a flat body consisting of an upper dissected or pinnate leafy portion, middle ovule-bearing portion and proximal petiole. Petiole varies in length in different species.

The middle part is comparatively wider than petiole and bears ovules arranged in two pinnate rows. The number of ovules varies between 2-12 in different species. The ovules are green when young but at maturity they are fleshy and bright orange or red-coloured structures.

The upper, conical sterile part of the megasporophyll is pinnately divided in Cycas revoluta, C. pectinata and C. siamensis.

Cycas thouarsi contains the largest ovule amongst the living gymnosperms measuring about 70 cm.

Structure of Ovule

Cycas ovules are orthotropous, unitegmic and shortly-stalked. Generally, one or sometimes a few more ovules develop fully on a megasporophyll. Many un-pollinated ones remain small and ultimately abort.

Outer surface of the ovule may be smooth as in C. circinalis or covered with orange-yellow hairs as in C. revoluta. After fertilization these hairs are lost, the ovule changes into seed and its colour changes from orange-yellow to bright red.

The single integument is very thick and covers the ovule from all sides except a mouth-like opening called micropyle. The integument consists of three layers:

(i) Outer, green or orange, fleshy layer called sarcotesta,

(ii) Middle, yellow, stony layer called sclerotesta, and

(iii) inner fleshy layer.

Several tannin cells and mucilage canals are present in the parenchymatous region of sarcotesta. Some pigments are also present in sarcotesta and epidermis. The sclerotesta consists of lignified thick-walled cells. The inner fleshy layer consists of parenchymatous cells, and it remains in close association with the nucellus.

Nucellus

The nucellus grows out into a beak-like portion called nucellar beak. The latter protrudes into the micropylar canal. Certain cells at the top of the nucellus dissolve and form a cavity like structure called pollen chamber (Fig. 8.43). Pollen grains are received in the pollen chamber after pollination.

The nucellus gets reduced in the form of a thin papery layer in mature seeds and encloses the massive female gametophyte (endosperm). An enlarged megaspore or the embryo-sac is present within the nucellus. The endosperm is formed by the repeated divisions of the megaspore nucleus followed by free cell formation.

Just below the pollen chamber is present an archegonial chamber. 3-6 archegonia are present in the female gametophyte near the archegonial chamber. The latter remains filled with a fluid.

Oospore is the first cell of the sporophytic generation.

Proembryo

Oospore undergoes free nuclear divisions followed by wall formation to form a small cellular mass called pro-embryo.

Proembryo differentiated into- basal embryonal zone, middle suspensor, and upper haustorial region.

Haustorial region remains in contact with the free- nuclear divisions and soons disappear.

Cells of embryonal zone divide to form embryo proper which is differentiated into two cotyledons, plumule and radicle.

Suspensor becomes enlarged and coiled to push the embryo into the nutritive endosperm.

Economic Importance of Cycas

1.   Cycas plants are highly valued for their ornamental looks, and hence grown in homes and garden as an ornamental plant.

2.   A starch called ‘sago’ is obtained from the stem pith of Cycas revoluta. That is why this species of Cycas is also known as “Sago Palm”. Sago starch is used in the preparation of “Sabudana”.

3.   In Sri Lanka, the starch obtained from the seeds is used in cakes.

4.   Soft young leaves of Cycas are used as vegetable also in some parts of the world.

5.   Seed of some species of Cycas are used as fodder for animals.

6.   Boiled seeds of Cycas rumphi are eaten by inhabitants of Andaman.

7.   Its leaves are used for making mats, hats and baskets.

8.   Extract of young Cycas leaves is used in the treatment of many skin diseases, blood vomiting and stomach disorders.

9.   The decoction of seeds is used as purgative.

10.   Tincture prepared from its seeds is used in some areas in headache, nausea, bad throat etc.

11.   The terminal buds of Cycas circinalis are used to treat ulcerated wounds and swollen glands.

Life cycle of cycas

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