Doc: Cycas - Class 11 PDF Download

• Kingdom           →         Plantae
• Sub kingdom    →         Embryophyta
• Division            →        Tracheophyta
• Class                →       Gymnospermae
• Subclass           →       Cycadophyta
• Order               →         Cycadales
• Family             →          Cycadaceae
• Genus             →         Cycas

Common name : Sago palm or palm fern.

1. Cycas is the most widely distributed genus of the order Cycadales. There are about 20 species which occur in wild state in China, Japan, Australia, Africa, Nepal, Bangladesh, Burma and India. Four species of Cycas – C. circinalis, C. pectinata, C. rumphii and C. beddomei – occur in natural state in India, chiefly in Assam, Orissa, Meghalaya, Andaman and Nicobar Islands, Karnataka and Tamil Nadu. C. revoluta and C. siamensis are widely grown in gardens.
2. C. rumphii – This species occurs in Andaman and Nicobar Islands.

Economic Importance

1. A tincture prepared from the seeds of C. revoluta is used to relieve headache, giddiness and sore throat.
2. Leaves of C. revoluta are rich in nitrogen and they are used as green manure for rice, sweet potato and sugarcane.
3. The starch of stem cortex & pith (usually) of C. revoluta is utilized for the production of sago

[I]  External morphology :

Cycas is an evergreen slow-growing palm-like small tree with an average height of 1.5-3m. It is commonly found in xerophytic habitats. It also grows well under cultivation in gardens. The sporophytic plant body is differentiated into roots, stem and leaves.

1.   Root : There are two types of roots in Cycas :

(i) Normal tap roots       
(ii) Coralloid roots.

(a)  Normal tap roots : These roots are positively geotropic and their main function is anchorage and absorption of water and mineral nutrients.

(b)  Coralloid roots : These are specialized apogeotropic roots which grow on the surface of the soil. They are repeatedly dichotomously branched and appear as coralline masses. They are more frequent on young plants. The coralloid roots possess lenticels, which help in respiration.

2.   Stem : In mature plants, the stem becomes thick, columnar and woody. It is covered with persistent and woody leaf bases.

1. The stem is usually unbranched.

3.   Leaves : Cycas has dimorphic leaves : (i) Foliage or Assimilatory leaves and 

(ii) Scale leaves.

(i)   Foliage or Assimilatory leaves : Large, pinnately compound foliage leaves form a crown at the top of stem. The leaflet has a single mid vein, lateral veins are absent. Young leaves covered with ramenta.

• Circinate vernation is found in young leaf let.

(ii)  Scale leaves : The scale are small, rough, dry and triangular. They are thickly covered with ramenta. They are incapable of carbon assimilation and their main function is to protect apical meristem and other aerial parts.

• The foliage and scale are arranged in close alternate whorls at the stem. Usually a single crown of leaves is formed in a year.

[II]  Internal Anatomy :

(a)  Internal Structure of Normal Tap Root : 

1. Vascular tissue forms a central diarch stele. The xylem is exarch and tracheids of the protoxylem have spiral thickenings and metaxylem tracheids have sclariform thickenings.
2. The mature normal root shows secondary growth.

(b)  Internal structure of Coralloid root : Coralloid root's cortex is differentiated in to three distinct regions.

Middle cortex → It has algal zone. The algal zone consists of a single layer of loosely connected thin walled and radially elongated cells. In the algal zone blue-green algae, such as Anabaena cycadae, Nostoc punctiforme and Oscillatoria spp. occur which live symbiotically. These help in nitrogen fixation. Pericycle is present inside inner cortex.

1. Vascular Bundle : coralloid roots are triarch and exarch. Root hair and root cap absent in coralloid roots

(c)  Internal structure of Stem : Its internal structure is similar to that of a dicotyledonous stem.

1. Epidermis is usually discontinous due to the presence of persistent leaf bases.
2. Cortex composed of parenchymatous cells, rich in starch grains. The cortex is also traversed by several mucilagenous canals and many leaf traces.
3. Endodermis and pericycle both are indistinct.
4. There are several vascular bundles arranged in a ring forming an ectophloic siphonostele.
5. The vascular bundles are conjoint, collateral, endarch and open.
6. In a transverse section of stem, numerous leaf traces can be seen in the cortical region. They are vascular strands which supply the leaves.
7. Each leaf receives four leaf traces. (Two direct & Two girdle traces)
8. There is a parenchymatous pith in the centre of stem. The pith cells are rich in starch and several pith cells also contain tannin and mucilagenous substances.
9. The stem of Cycas shows normal secondary growth in early stages, similar to that of dicotyledonous stem.
10. Note : Young stem of Cycas which is monoxylic earlier becomes polyxylic later on.

(d) Internal structure of leaf : -

(i)   Rachis : - In a transverse section rachis is circular in outline. It has two rows of leaflets inserted on the adaxial side.

• Ground tissue containing mucus canals present below hypodermis.
• There are several vascular bundles in the ground tissue and they are arranged in a typical inverted omega-like arc.
• The vascular bundles are conjoint, collateral and open, and each is surrounded by a single layered bundle sheath.
• Xylem is diploxylic in V.B. i.e. centrifugal & centripetal xylem present. Centrifugal  xylem is more at basal part of rachis while centripetal xlyem is more towards tip.

(ii)  Leaflet :.

• Upper and lower epidermis is present. stomata are present only at lower epidermis .
• Leaf mesophyll is modified in pallisade and spongy tissue.
• There are groups of tracheid-like cells on the lateral sides of the vascular xylem in the mid-rib region. These cells have reticulate thickenings and bordered pits on their walls. They represent transfusion tissue.
• In the wing region long colourless and transversely elongated parenchyma cells are present in between palisade and spongy parenchyma. They represent secondary or accessory transfusion tissue.
• Accessory transfusion tissue helps in lateral conduction of water and nutrients because lateral veins are lacking in cycas.
• A single vascular bundle is present in the midrib region. The vascular bundle is conjoint collateral, open and xlyem is diploxylic.
• It has a large and triangular centripetal xylem (Adaxial in position) and two small groups of centrifugal xylem (Abaxial in position).
• A thin layer of cambium is present between the xylem and phloem.

REPRODUCTION

Cycas reproduces by vegetative and sexual means.

 I.  Vegetative reproduction :

• Vegetative propagation takes place by resting adventitious buds or bulbils. They develop in the basal part of the stem from parenchymatous cells of the cortex, in the crevices between persistent leaf bases.
• It is a common method of propagation in C.revoluta.. Bulbils formed on male plants give rise to male plant and those developed on female plants form female plants.
• In C.circinalis, vegetative propagation takes place by suckers which develop from the roots. They grow horizontally at the ground for some distance and then form new plants.

II.  Sexual reproduction : -

• Species of Cycas are strictly dioecious.

1. Male strobilus : The strobilus (cone) develops at the apex of the stem in between the crown of foliage leaves. In its development, apical meristem of the stem is utilised and as a result, the future stem becomes sympodial.

•  The male cone is shortly stalked, compact, oval or conical woody structure.It is 40-80 cm in length, the largest among the plant kingdom.
• The microsporophyll is a woody, more or less horizontally flattened and nearly triangular structure. It is differentiated into a proximal wedge-shaped fertile part which expands distally from a narrow point of attachment and a distal sterile part, tapering into an up curved apophysis. The upper (adaxial) surface of the fertile part of the microsporophyll is sterile, while on the lower (abaxial) surface there are 700-1000 microsporangia, arranged in definite groups, known as sori. (Singular - Sorus). Each microsporangium is globular and sac-like structure. The wall of microsporangium is trilayered.                      
• Tapetum is innermost and it covered sporogenous tissue
• Types of development of microsporangium → Eusporangiate type.

Microsporogenesis : Microsporogenus cell  microsporemother cell  microspore 

1. The megasporophylls of Cycas are not organised into cones and instead they occur in close spirals in acropetal succession around the stem apex of the female plant. New megasporophylls are produced every year like the foliage leaves. The growth of the apical meristem of the female plants is monopodial, the axis continues to grow as it produced foliage leaves and megasporophylls.
2. The megasporophylls are considered to be modified leaves. They are flat and dorsiventral structures, measuring 15-30 cm in length. A megasporophyll is differentiated into a basal stalk and an upper pinnate lamina. Ovules are formed on the lateral sides of the stalk.             
3. Largest ovule in plant kingdom is present in Cycas thaursai.

Structure Of Ovule : 

1. The ovule of Cycas is orthotropous and unitegmic. It is sessile or shortly, stalked and largest in the plant kingdom, about 6 cm in length and 4 cm in diameter. The ovule consists of a large nucellus surrounded by a single integument. The integument remains fused with the body of the ovule except at the apex of the nucellus, where it forms a nucellar beak and micropyle.
2. The integument is very thick and is differentiated into three distinct layers – the outer and inner layers are fleshy, whereas the middle layer is hard and stony. The inner fleshy layer remains fused with the nucellus and it is short-lived. Mucilage canals and tannin cells are frequently present in the integument. Some cells of the nucellar beak dissolve to form a pollen chamber that lies in the central region of the beak. The young ovule is green and is covered by minute multicellular hairs. At maturity it becomes red or orange in colour and hairs disappear.
3. The ovule is supplied by three vascular strands from megasporophyll.

Gametophyte

1. Cycas is heterosporous, it produces two types of spores – microspores and megaspores. The microspore develops into male gemetophyte and the megaspore in female gametophyte.

A.  Male gametophyte :

The male gametophyte develops partially inside the micro sporangium before pollination and partially within the pollen chamber of the ovule after pollination.

1. Development of male gametophyte before polination : - The development of the male gametophyte before pollination takes place inside the microsporangium. The microspore divides by a transverse wall in to two unequal cells, a small prothallial cell and a large antherdial cell. The prothallial cell does not divide further. The antheridial cell divides to form a small generative cell (adjacent to the prothallial cell) and a large tube cell. Thus the microspore becomes 3-celled. The dispersal of microspores takes place at the 3-celled stage. Further development occurs in the pollen chamber of the ovule after pollination.

2.   Pollination : The 3-celled microspores are shed in the air after the dehiscence of the sporangium. They are very light in weight and are carried by air currents.  

• In synchronization with the time of dispersal of the microspores, some cells of the nucellar beak disorganise to form a viscous fluid. This fluid oozes out from the micropyle in the form of a pollen drop. Some of the microspores, carried by air current, are entangled in the pollen drop. As the pollen drop dries up, the microspores are sucked into the pollen chamber through the micropylar canal. As the result of drying of the viscous fluid, the micropylar canal of the pollinated ovules is plugged.

3.  Development of male gametophyte after pollination : Further development of the 3-celled male gametophyte takes place within a week of pollination in the pollen chamber of the ovule. The generative cell divides to form a stalk cell and a body cell, about the same time, the exine ruptures and the intine protrudes out in the form of a pollen tube. The pollen tube penetrates the nucellar tissue and grows towards the female gametophyte. In Cycas, the pollen tube is haustorial in nature.  

• The stalk cell does not divide, whereas the body cell divides to form two male gametes (antherozoids) just before fertilization. (Mature male gametophyte of Cycas is 5-celled) The antherozoids swim freely in the cytoplasm of the pollen tube. There is an interval of about four months between the pollination and fertilization.
• The male gametes are naked, top-shaped structures, measuring 180-210 µm and multi ciliated/ multiflagellated. 

B.   Development of Female Gametophyte :

1. Four megaspore are formed by meiotic division in megaspore mother cell. Megaspore mother cell is originated from nucellus. Out of these four megaspore, three megaspore towards micropyle end degenerate and only one remain functional. Free nuclear division occur in functional megaspore. Cell wall formation occur and multicellular female gametophyte or endosperm is formed.
2. Archegonia are formed from archegonial initial cell which is present toward micropyle.
3. The mature archegonia consists of 2(generally)-4 neck cells and an egg. The venter is surrounded by a nutritive jacket of cells formed by the gametophyte cells. This jacket is called archegonial jacket. Neck canal cells are absent.

Note : - The number of archegonia in an ovule varies in different species of cycas, for example, 3-6 in

1. C. rumphii, 3-8in C.circinalis and  2-8 in C.revoluta.

 Different stages of development of female gametophyte

[III] Fertilization – Zoodiosiphogamous type

Embryo development : Meroblastic and endoscopic

1. The Cycas proembryo differentiated into three regions : (i) the uppermost haustorial region at the micropylar end, (ii) the middle suspensor region, and (ii) the basal embryonal region.
2. The cells of the haustorial region absorb nutrients from the micropylar end and provide them to the developing embryo.
3. The cells of the suspensor region are elongated and much coiled. They push the embryo deep into the female gametophyte (endosperm).
4. The embryonal region forms the main part of the embryo. The embryo takes about one year for its complete development.
5. Two cotyledon are present in the embryo of cycas.

A-G Cycas : Stages in the development of embryo

Seed :

1. The ovule is transformed into seed after fertilization. The inner fleshy layer of the ovule wall and the major part of the nucellus are consumed in providing nourishment to the developing embryo. Thus both these parts persist in the seed in the form of a thin layer.
2. The mature seed is red, orange or dark brown in colour. It is surrounded by a fleshy seed coat formed by the three layers of the integument of the ovule. The outer fleshy layer of the integument forms sacrotesta and the middle stony layer sclerotesta, while the inner layer remains thin and papery. The seed coat has a sweet taste and pleasant odour. These provide attraction to birds and animals which help in dispersal of seeds. There is a straight embryo with two unequal cotyledons inside the seed coat.
3. The seed of Cycas represents three generation. The seed coat, formed by the integument of the ovule, represents the first sporophytic stage, the endosperm is the gametophyte stage and the embryo is the next (new) sporophytic stage.  

Germination of seed :

The seed germinates without undergoing any rest period. It remains viable only for few months. The germination is hypogeal.  

Alternation of generation :

The Cycas proembryo differentiated into three regions: 

(i) the uppermost haustorial region at the micropylar end,
(ii) the middle suspensor region, and
(iii) the basal embryonal region.

• The cells of the haustorial region absorb nutrients from the micropylar end and provide them to the developing embryo.
• The cells of the suspensor region are elongated and much coiled. They push the embryo deep into the female gametophyte (endosperm).
• The embryonal region forms the main part of the embryo. The embryo takes about one year for its complete development.

[II] Seed : The ovule is transformed into seed after fertilization. The inner fleshy layer of the ovule wall and the major part of the nucellus are consumed in providing nourishment to the developing embryo. Thus both these parts persist in the seed in the form of a thin layer.

• The mature seed is red, orange or dark brown in colour. It is surrounded by a fleshly seed coat formed by the three layers of the integument of the ovule. The outer fleshy layer of the integument forms sacrotesta and the middle stony layers clerotesta, while the inner layer remains thin and papery. The seed coat has a sweet taste and pleasant odour. These provide attraction to birds and animals which help in dispersal of seeds. There is a straight embryo with two unequal cotyledons inside the seed coat.
• The seed of Cycas represents three generation. The seed coat, formed by the integument of the ovule, represents the first sporophytic stage, the endosperm the gametophyte stage and the embryo the next (new) sporophytic stage.

[III] Germination of seed : The seed germinates without undergoing any rest period. It remains viable only for few months. The germination is hypogeal.

• Height of Cycas media → 20 meters
• In Cycas, the radicle is enclosed in a pad-like hard covering known as Coleorrhiza.  
• Multiseriate bordered pits are present in Cycas tracheids.