cycas plant and plant anatomy for msc botany

msc botany

1. DISTRIBUTION It is the most widely distributed genus of order cycadales Cycas,the largest genus among old world cycads. In India,cycas grow naturally in Orissa,Assam,MeghalayTamilnadu,Karnataka,and Andaman and Nicobar island Cycas is represented by 15 species(sporne) According to willis there are 20 species. Schuster only recognised 8 species,mentioning for the rest as forms,varities,or subspecies of other species Cycas circinalis,Cycas beddomei,Cycas pectinate ,and Cycas rumphii o ccurs in wild condition in India. Cycas revolouta and Cycas siamensis are commonly cultivated in Indian gardens 1

2. VEGETATIVE STRUCTURE . Cycas is a palmlike, evergreen plant. Plant body consist of a coloumnar aerial trunk with a crown of pinnately compound leaves at its top. Roots in cycas are of two types,normal tap root and coralloid root NORMAL TAP ROOTS They are positively geotropic,grow deep into soil and possess no root hairs. Their function is to fix plants and absorb water and nutrients from soil. According to Eichler,coulter and chamberlain,Schuster and others, a tap root system persist in the adult plant According to worsdell taproots are soon replaced by adventitious roots. 2

3. ANATOMY OF VEGETATIVE PARTS NORMAL ROOT It is circular in outline. They resemble structurally with Dicotyledonae . Outermost layer is epiblema or exodermis Epiblema possess root hairs Cotex parenchymatous with intercellular spaces Cells of cortex filled with starch,some tannin filled cells mucilage cells,sometimes sphaeraphides are also present. Cotex delimited by endodermis,which is barrel shaped. Endodermis is followed by multi-layered pericycle 3

4. CORALLOID ROOT : Anatomically, the coralloid roots resemble normal roots except some undermentioned differences: 1. Secondary vascular tissue in coralloid roots is either totally absent or poorly devoleped . Presence of algal zone in middle of cortex According to Chaudhary and aktar the algal zone is not of universal occurrence in coralloid root of cycas . 4. It may be absent in such coralloid roots which go very deep into the soil 5. Algal zone consist of radially elongated ,large thin walled cells having large intercellular spaces occupied by algae life Reported algal zone of coralloid roots are anabaena cycadae,nostoc punctiforme,Oscillatoria,azotobacter,pseudomonas radicicola,and fungi. 7. Their chief function is nitrogen fixation Structure of endodermis,pericycle and vascularbundle are similar to normal roots 9. Xylem is exarch and triarch 4

5. STEM Similar to root,stem also resemble dicotyledonous stem Epidermis which is a compactly arranged thick walled cells. Presence of persistent leaf bases makes the epidermis layer discontinuous and ruptured layer Cortex is large and consist of thin walled parenchymatous cells filled with starch grains It contain numerous mucilaginous canals and girdle traces Mucilagenous cells surrounded with radially elongated epithelial cells Starch in parenchymatous cortex is the source of sago. Endodermis and pericycle not clearly demarcated Stele is ectophloic siphonostele V.B are conjoint,collateral,open and endarch Vessels absent in xylem, companion cells absent in phloem. 5

6. Between xylem and phloem lies the primary cambium which remain active for a short period of time It is replaced by another by another ring of secondary cambium somewhere in the cortex These successive cambial ring form 2-14 different vascular ringsshowing polyxylic conditions in old stem. Well devoleped medullary rays are present between vascular bundle. Pith is well devoleped & parenchymatous,it contain many mucilage canals 6

7. LEAF TRACES: It remain scattered in the cortical region of stem It constitute vascular supply to leaves from main vascular cylinder There are 4 leaf traces which forms the vascular supply to leaves Two are direct traces while two are girdle traces Direct trace originate from vascular cylinder lying in front of leaf base,while girdle trace lying opposite to that of direct traces 7

8. SECONDARY GROWTH: It is similar to that dicotyledons Interfascicular and intrafascicular cambia fuse to fom a complete cambial ring The cambiar ring cut secondary phloem outwards and secondary xylem toward inwards Well devoleped medullary rays traverse the secondary vascular tissue After a while this cambial ring stop functioning and a second cambial ring devoleps from parenchymatous cortex or pericycle Thus a polyxylic condition is devoleped . Cork cambium devoleps from outer cortex and cut cork towardouter side and secondary cortex toward inner side 8

9. T.S OF OLD STEM. ENLARGED VIEW 9

10. RACHIS: Outline of transverse section is rhombhoidal in basal region,biconvex in middle and roughly cylindrical at the apex of rachis Epidermis is the outermost layer which is heavily cuticularized Upper and lower side has irregularly distributed stomata Hypodermis is differentiated into 2-3 layers of chlorenchymatous and inner 4-6 layers of schlerenchymatous cells Schlerenchyma is poorly devolepe on lateral sides and it is also seen intermixed with chlorenchyma Ground tissue is a large region consisting of thin walled parenchymatous cells Mucilage canals and vascular bundles arranged in this region Vascular bundles are arranged in the form of inverted omega Towards tip vascular bundles are arranged in c shaped manner and number is comparitatively less 10

11. Each vascular bundle surrounded by bundle sheath It is conjoint collateral and open Vascular bundles are diploxylic It consist of centripetal and centrifugal xylem Cambium separate xylem and phloem 11

12. VASCULAR BUNDLES AT THE BASE OF RACHIS: Only centrifugal xylem is well devoleped Its protoxylem toward centre showing endarch condition centripetal xylem not devoleped VASCULAR BUNDLES IN THE MIDDLE REGION: Both centripetal and centrifugal xylem present showing diploxylic nature Centripetal xylem present just opposite to protoxylem of centrifugal xylem VASCULAR BUNDLE AT THE APEX OF RACHIS Centripetal xylem is well devoleped,triangular and exarch Centrifugal xylem much reduced and present as 2 patches on each side of protoxylem elements of centripetal xylem Centrifugal xylem is totally absent at the extreme tips of rachis 12

13. LEAFLET They are large,tough,thick and leathery In a vertical section leaflet is differentiated into swollen midrib portion and two lateral wings In c.revolouta and c.beddomei the wings are curved downwards In c,circinalis,c.rumphii,c.pectinate,and c.siamensis the margins are flat Outerepidermis surrounded by thick layer of cuticle Upper epidermis is a continuous layer Lower epidermis is discontinuous due to presence of stomata Stomata is haplocheilic Sclerenchymatous hypodermis present below epidermis It is absent below lower epidermis,but in midrib it is several cell thick 13

14. Continous layer of palisade present below hypodermis Mesophyll well differentiated into palisade and spongy parenchyma Spongy parenchyma present only on wings directly above lower epidermis Transfusion tissue consist of two small groups of short and widetracheids like cells with reticulate thickening or bordered pits on their walls. Few layers of transversely elongated cells are present in both wings between palisade and spongy parenchyma that represent accessory transfusion tissue Vascular bundle is one present in midrib region It is conjoint collateral open and diploxylic Triangular centrifugal xylem is well devoleped with endarch protoxylem Phloem is arc shaped and remain separated by cambium 14

15. MALE REPRODUCTIVE STRUCTURES MALE CONE It is large,conical or ovoid,compact solitary and shortly stalked structures which is generally terminal in position Several perpendicularly attached microsporophylls are arranged around the cone axis in closely set spirals All microsporophylls in male cones are fertile except those in base and apex 2.MICROSPOROPHYLLS They are flat,leaf like ,woody and brown coloured structures with narrow base and expanded upper portion called apophysis at the apex on the abxial surface present thousands of microsporangia in 3-5 groups Each such group is called sorus In between these structures are one or two celled hair like structures Wall layers of sporangia include outer thick epidermis,middle thin walled cells and innermost layer tapetum 15

16. Many pollengrains are present in each sporangium. Each sporangium provided with radial line of dehiscence help in dispersal of spores Each pollengrains is rounded,unicellular structure surrounded by outer thick exine and inner thin intine Cytoplasm soround the centrally located nucleus A large vacuole is also present 16

17. 3. DEVOLEPMENT OF MICROSPORANGIUM It is eusporangiate type Few hypodermal sporangial initial divide periclinally to form outer primary wall cells and inner primary sporogenous cells Primary wall cells divide periclinally and anticlinally to form 5-7 cells thick wall of sporangium Primary sporogenous cells divide to form microspore mother cells MMC divide meiotically to form haploid microspores arranged tetrahedrally Tapetum is utilized for spore formation Haploid chromosome number is 11 in cycas ,in cycas revolouta it is sometimes 12 17

18. 2.STRUCTURE OF OVULE: Cycas ovules are orthotropus,unitegmic and short stalked The single integument cover the ovule except at a single mouth like opening called micropyle The integument consist of three layers Outer fleshy sarco testa,middle stony layer called sclerotesta and inner fleshy layer which remain in close association with nucellus Nucellus grow out into a beak like portion called nucellar beak This protrude in micropylar canal Certain cells at the top of nucellus dissolve and form a cavity like structure called pollen chamber Pollen grains are received at pollen chamber At maturity papery nucellus encloses female gametophyte 3-6 archegonia are present in female gametophyte 18

19. MALE GAMETOPHYTE DEVOLEPMEN T Microspore is the first cell of male gametophyte It divide in to two and forms smaller prothallial cells and larger antheridial cells Antheridial cell divide to form generative cell and tube cell Shedding of microspores takes place at this three celled stage POLLINATION Microsporangia dehisce by longitudinal slit Microspores are blown by wind for pollination Nucellar beaks of ovule dissolve to form a drop of mucilage This mucilaginous drop that oozes out from micropyle to represent pollination drop Microspores are entangled in this pollination drop and which is withdrawn in micropylar chamber through micropylar canal Mycropylar chamber dry at the top of nucellar beak get sealed up due to drying of mucilage,which completes the process of fertilization 19

20. MALE GAMETOPHYTE DEVOLEPMENT[after pollination] At nucellar tissue of ovule ,exine break and intine come out as pollination tube Generative cell divide to form a stalk cell and a body cell stalk cell enlage in size by the accumulation of starch grains Body cell become large sized,prothallial cell push into stalk cell and presses it against the cell Further devolepment takes place after a gap of 4 months During this period pollen tube penetrate through nucellus and hangs down in the archegonial chamber Two blephaeroplast apper one at each pole of the nucleus of the body cells in a transverse position,they give rise to cilia Body cell divide to form sperm mother cells Each have a single nucleus,small amount of cytoplasm and blepharoplast Sperms are liberated into pollentube by breaking of sperm mother cells 20

21. FEMALE GAMETOPHYTE DEVOLEPMENT: functional gametophyte is the first cell of female gametophyte It is haploid in nature By absorbing surrounding cells of nucellus it enlarges in size Itdivide mititically to form several free nuclei A vacuole formed in the centre pushes the free nuclei and cytoplasm towards periphery The wall formation starts from periphery to centre and soon a cellular tissue the female gametophyte or endosperm devolep A archegonium devoleps from a single superficial initial near the micropylar end of endosperm It divide periclinally to form outer neck cell and inner central cell Primary neck cells divides to form the neck of archegonium Nucleus of inner central cell divides to form ventral canal nucleus and egg nucleus A mature archegonium consist of two neck cells and egg Archegonial neck open into archegonial chamber formed by depressions endosperm 21

22. FERTILIZATION Pollen tube containing sperms and tube nucleus grow downward penetrating the tissue of pollen chamber and enter into a cavity that is partly formed by pollen chamber and archegonial chamber In the end pollen tube burst releases the sperms into the archegonial chamber One of the sperm enter into archegonium through neck and reaches upto egg and fuse to form zygote EMBRYOGENY Diploid zygote is the first cell of sporophyte It contain dense cytoplasm and large nucleus Nucleus undergoes repeated free nuclear divisons to form hundreds of nuclei Wall formation starts from base and progress toward suppern side proembryo It is meristematic in nature Three regions differentiate in this proembryo 22

23. HAUSTORIAL REGION: it is the upper part of proembryo it absorbs food materials for the devoleping embryo cells of this region posses feebly devoleped walls which soon disappears SUSPENSOR ZONE It is the middle zone Its cells elongate enormously and form long and coiled suspensor It pushes the embryo into food containing cells of endosperm EMBRYONAL ZONE Cells of this zones are present at the tip of suspensor It is the basal portion of the embryo Its cells divide to form proper embryo By the cessation of growth in the centre and its continuation in the margins two cotyledons are differentiated Plmule differentiate in the central depressed zone Radicle differentiate below hypocotyl A pad of tissue devoleps at radicle forming coleorhiza which protect radicle 23

24. STRUCTURE OF SEED Mature fleshy cycas seeds consists of orange red coloured outer thick testa or seed coat formed of 3 layered integument of ovule Nucellus cells are used up during embryo devolepment therefore they are represented by a thin layer Embryo remains embedded in the well devoleped endosperm It consist of plumule radicle and two large cotyledons. In mature cycas seed parent sporophytic generation is represented by integument gametophytic generation is represented by endosperm and new sporophytic generation is represented by sporophyte. 24

25. GERMINATION OF SEED There is no resting period for cycas Germination in cycas is epigeal Seed coat breaks open and radicle comes out through micropyle to form primary roots Root take downward curve and grow rapidly into soil Cotyledon absorb nutrients from endosperm for terminating seedlings When endosperm exhaust cotyledons dry up ,plumule come out and starts forming scaly leaves and foliage leaves Crown of leaves formed after several years 25

26. ECONOMIC IMPORTANCE OF CYCAS Cycas is used a source of food,it is used in the preparation of starch and alcoholic drink Starch from stem is used to make sago In Japan seeds and stem of cycas revolouta is used for making wine Juice obtained from young leaves of cycas circinalis is used for skin diseae,vomiting of blood,and stomach disorder Cycas revolouta and cycas circinalis are grown as ornamentals in the garden cycas leaves being very large is usd for making baskets, mats etc Wood of cycas revolouta is used for making small boxes and dishes Cycas circinalis seeds are used in democratic Kampuchea as a fish poison. 26