Reproduction in Cycas

description of different types of reproductive organs, developmental stages and process of reproduction in Cycas. Various internet sources have been used.

1. Reproduction in Cycas By- Dr. Debashree Kakati Asst. Prof. Sipajhar College Darrang , Assam

2. Points to be discussed- Types of reproduction Bulbil Structure of male cone Development of microspoangium Development of megaspoangium Fertilization Embryogeny Seed and seed germination

3. Vegetative reproduction: Adventitious buds or bulbils - In Cycas reproduction takes place mostly by vegetative means. The bulbils is a adventitious bud develop from the basal part of stem at the axile of leaf especially from cortical cells. They are more or less oval shaped. Several scale leaves are arranged spirally and compactly over a dormant stem in a bulbil . Upon detachment from the stem, a bulbil germinates to produce a new plant. A bulbil from male plant produces a new male plant while a bulbil from female plant produces a new female plant . Fig: Single bulbil

4. Sexual Reproduction: Male Strobillus /Male cone: The male plant of Cycas produces male strobilus (cone) at the apex of the stem in between the crown of foliage leaves . Each male cone is a shortly stalked compact, oval or conical woody structure, 40-80 cm in length. On the axis of the cone are many perpendicular and spirally arranged microsporophylls . Each microsporophyll is a woody, brown coloured , more or less horizontally flattened structure with a narrow base, an expanded upper portion and sterile apophysis at the apex. Each microsporophyll has adaxial (upper) and abaxial ( lower, basal) surfaces . Numerous microsporangia in groups of 2, 4 or 5 occur on its basal surface called sori . A microsporangium has 5-6 wall layers which surround the sporogenous cells containing a large prominent nucleus and dense cytoplasm. Pollen grains or microspores are produced at the end of meiotic division of microspore mother cells found in the microsporangium.

5. Microsporangium

6. Development of microsporangia ( Microsporogenesis ): Development of microsporangia is eusporangiate i.e .development starts with many initial cells). Cells of hypodermis of microsporophyll divides by transverse or periclinical divisions to produce sporangial initial cells . S porangial initial cell divide tansversly to produce- Outer primary wall cell and Inner primary sporogenous cell Primary wall cells divide to produce 4-8 layered outer wall . Primary sporogenous cells divide to produce mass of sporogenous tissue or spore mother cells or archesporium . Outermost layer of the sporogenous tissue forms tapetum – the nourishing layer. As development proeeds the sporogenous cell round off and becomes isolated as spore mother cells which begin to float in a nutritive medium formed by disorganisaition of tapetum . Spore mother cells divide by reduction division (Meiosis) followed by mitosis division so as to form four microspore s or pollen grains or spore tetrads , . Thus microspores get developed which are haploid (n) and regarded as first cell of male gametophyte.

7. Development of Male gametophyte (Before pollination): Microspore or pollen grain is the first cell of the gametophyte. It has an outer exine and inner intine wall. The microspore germinates in situ i.e . while within the microsporangium. Each microspore divides asymmetrically into a 2-cells: a smaller prothallial cell and a larger antheridial cell . The prothallial cell does not divide further while the antheridial cell divides into a smaller generative cell near the prothallial cell and a larger tube cell . P ollination takes place at 3-celled stage (a prothallial cell, a generative cell and a tube nucleus) ā Fig A- Two celled pollen Fig B- 3 celled pollen exine Intine Development of male gametophyte of Cycas : Each microspore on pollen develops into male gametophyte partly even before the release of pollens from microsporangium.

8. Pollination: In Cycas pollination is anemophilous (by wind). The 3-celled microspores liberate from mega-sporangia are blown away by wind. Finally microspores reach on ovules and get enlarged in the pollination drop (ooze) of micropyle of ovule. As the ooze dries up, the microspores are drawn into the pollen chamber. Development male Gametophyte (After pollination ): After pollination development of male gametophyte occurs in the micropyle . The exine ruptures and the intine grows out in form of a pollen tube. The pollen tube acts as a haustorium , i.e. absorb food while penetrating through the nucellus and hang in the archegonial chamber. In the pollen tube, generative cell divides into a stalk cell and a body cell ( spermatogenous cell ). Finally , the body cell divides into two male gametes or antherozoids or sperms . Thus , a fully developed male gametophyte consists of a disorganized prothallial cell, stalk cell, tube nucleus and 2 male gametes. Each male gamete appears top-shaped with characteristics 5-6 spiral bands of cilia. The size of male gamete in Cycas varies from 180-210µm (largest, 400«m reported from Chigua , a cycad ) can be seen with naked eye.

9. Fig: Successive stages of development of male gametophytein Cycas sperm Male gametophyte

10. Structure of Megasporophyll : The female plant produces megasporophylls that are not organized into cones and instead they occur in close spirals in acropetal succession around the stem apex. The megasporophylls of a year occupy the region between the successive whorls of vegetative leaves . Each megasporophyll is a modified foliage leaf ranging from 15-30 cm. in length. It has a proximal petiole, middle ovule bearing part and upper pinnately dissected sterile region. The middle fertile part bears 2-12 sessile rounded ovules in two rows. Megasporangium or Ovule: The ovules are orthotropous , unitegmic and sessile or shortly stalked. The Cycas ovule is largest in plant kingdom with 6-7 cm in diameter. In young stage ovules are green covered with brown hairs but after fertilization hairs are lost and appear orange to red in colour . Each ovule consists of a large nucellus surrounded by a single integument. The integument consists of three distinct layers: outer and inner fleshy layers and middle stony layer. The integument remains fused with the body of the ovule except at the apex of the nucellus where it forms a nucellar beak and an opening called micropyle . Some cells in the nucellar beak dissolve to form a pollen chamber. The opposite end of the microphyle is called chalaza .

11. Fig: Different types of megasporophylls found in different species of Cycas

12. Fig- LS of ovule of Cycas

13. Development of megasporangium ( Megasporogenesis ): The Cycadean ovule is composed of nucellus , which is the female gametophyte (FG), surrounded by an envelope, the integument. The ovule arises from the hypodermal cell of the megasprophyll edges. Cells divide rapidly and form the nucellus . These cells grows rapidly from the base of the nucellus and surrounds it all around except at the top forming micropyle . Now deep within the nucellar tissue a cell enlarges in size with clear nucleus and dense cytoplasm known as megaspore mother cell . It divides twice, the first division is reduction division and second is mitosis division to form a linear tetrad of megaspores. Thus four megaspores so formed are haploid in nature and are first cells of female gametophyte ( embryosac , endosperms and archegonia ). The upper 3 megaspores towards micropyle degenerate while the lower most acts as functional megaspore (embryo sac cell).

14. Development of Female gametophyte Formation of female gametophyte is monosporic , i.e develops from a single megaspore. Functional megaspore undergoes free nuclear division followed by wall formation to form a cellular female gametophyte or endosperm . Some superficial cells of the female gametophyte at the micropylar end enlarge and develop into 2-8 archegonial initials . The nucleus of an archegonial initial divides forming a primary neck cell and inner large central cell. The former divides to form two neck cells . The central cell divides to form an ephemeral venter canal nucleus and a large egg cell . Soon the venter canal nucleus disorganizes. Thus, a mature archegonium has two neck cells and an egg. The egg cell in Cycas is largest in the plant kingdom It should be noted that in gymnosperms during formation of endosperm nucellus is utilized and the endosperm develop before fertilization and is haploid (n) while in angiosperms it is triploid (3n) and formed after fertilization.

15. Fertilization: In the archegonial chamber, the tip of pollen tube burst to discharge its contents. One of the sperms enters the archegonium . When moving toward s egg, the sperm lost cilia and cytoplasmic membrane. So the fusion of a male nucleus and egg nucleus occurs to form a zygote (2n). It is noted that in Cycas fertilization exhibits both siphonogamy ( i . e. formation of pollen tube) and zoidogamy ( i.e. participation of ciliated male gametes). Fig: pollen tube Fig: closed micropyle after pollination showing pollen tubes and archegonia for fertilization

16. Embryogeny : The zygote or oospore is the first cell of sporophyte generation . The oospore undergoes free nuclear division followed by wall formation to form a small cellular mass called pro-embryo . The pro-embryo differentiated into a basal embryonal zone , middle suspensor and upper haustorium . The haustorial region remains in contact with the free-nuclear region and soon disappear. The cells of embryonal zone divide and re-divide to form embryo proper which is differentiated into two cotyledons, plumule and radicle . The suspensor becomes enlarged and coiled to push the embryo into the nutritive endosperm. The endosperm forms a pad like tissue called coleorhiza which protects the tip of radicle . Since , a single Cycas ovule contains 2-8 archegonia; the same number of embryo develops. All degenerate except one embryo that reaches maturity . A simple polyembryogeny is possible when eggs in more archegonia get fertilized.

17. Seed formation: As a result of post-fertilization changes the entire ovule becomes a seed. The embryo grows slowly, takes about a year, to mature. On maturity of the seed, the integument is an outer fleshy layer brightly coloured , followed by a stony layer and thin inner fleshy layer. The nucellus is absorbed by the developing gametophyte and become thin and papery present in the form of a cap at the micropyler end. At this stage the female gametophyte occupies a large volume serving as food for the developing embryo. Zygote develops into the embryo with radicle , plumule and two cotyledons. Fig: Seed of Cycas

18. Seed Germination: The testa of Cycas seed emits pleasant odour and sweet in taste. This causes its dispersal by birds. The seed remain variable for a few months. Under favourable condition, the seed germinates into a sporophyte . The seed germination in Cycas is hypogeal i.e. the cotyledon remains underground enclosed in the endosperm and absorb food for the growing embryo. The plumule form leafy shoot and radicle elongates into a tap root .