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CYCAS 1.pdf

This document describes the Cycas plant. It belongs to the division Cycadophyta and order Cycadales. There are 15-20 Cycas species found in tropical and subtropical regions, including several common Indian species. Cycas plants are low and palm-like with a columnar trunk bearing pinnately compound leaves. They reproduce both vegetatively through bulbils and sexually as dioecious plants with male and female cones developing on separate plants. Fertilization involves motile sperm fusing with eggs inside the ovules to form seeds containing embryo sporophytes.

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CYCAS Systematic Position Division: CYCADOPHYTA Class: CYCADOPSIDA Order: CYCADALES Family: CYCADACEAE Genus: CYCAS (Greek word Kycas= Cocopalm)
Distribution & Occurrence  Includes 15 -20 Species  Occurs wild or cultivated in tropical and sub-tropical regions  Common Indian Species:  Cycas circinalis  C. rumphii  C.beddomei  C.revoluta  C.siamensis  C.pectinata
GENERAL MORPHOLOGY  Plants are low and palm-like, height 4-8 feet  Plant body consists of a columnar aeriel trunk with a crown of pinnately compound leaves at its top.
Roots – 2 types  Normal tap roots  Positively geotropic  Grow deep into soil  Possess no root hairs  Function: fixation and absorption ◼ Coralloid roots ◼ Arise from normal roots near the ground surface ◼ Infected with bacteria ,fungi and algae ◼ Grow horizontally in the soil become swollen at tips ◼ Divide repeatedly to form bunches- greenish or brownish ◼ Coral like appearance
Stem  Thick, woody and unbranched  Tuberous when young but columnar erect and stout at maturity  Aerial part remains covered by thick large rhomboidal leaf bases  Larger leaf bases – foliage leaves  Smaller ones – scale leaves
Leaves – 2 types  Foliage leaves / assimilatory fronds  Green , large, pinnately compound leaves with spiny petiole and large strong rachis  Rachis bears many leaflets  Scaly leaves / Cataphylls  Dry , brown coloured ,somewhat triangular leaves  Present at the apex of the stem
Anatomy – Coralloid root  Anatomically Coralloid roots resemble normal roots except some differences:  Secondary vascular tissue is totally absent  Cortex is wider  Presence of a greenish algal zone in the middle of the cortex
Anatomy leaflet  Leaflet is thickly cutinized and leathery  Possesses all xerophytic characters  Sunken stomata and thickened hypodermis present  Well developed palisade layer in mesophyll  Between the palisade and lower mesophyll layers, there are transversely running long colourless cells in 3-4 layers extending from mid-rib to near leaf margin  These constitute the transfusion tissue  Mid rib bundle consists of a broad triangular centripetal xylem and two small patches of centrifugal xylem  Phloem abaxially placed
Reproduction –Vegetative  Vegetative reproduction is by means of bulbils  Develop in crevices of scale leaves and leaf bases at the basal part of an old stem  Produces new plant on detachment
Reproduction –Sexual  Strictly dioecious plant  Male strobilus or cone borne singly at the apex of the trunk  Apical shoot apex utilized in the development of male cone, hence branching sympodial  Cone shortly stalked & large(upto 50cm length or more)  Cones or strobili are found at the apex of the trunk
 Numerous micro-sporophylls spirally arranged around the central axis  Each micro-sporophyll is narrow below and broad above terminating into projection–the apophysis  Microsporangia confined to abaxial(lower) surface  Usually present in sori–each with 2-6 sporangia  They contain a large number of haploid microspores (pollengrains)
Pollination -Development of male gametophyte after pollination  The pollen grains are carried by wind(Anemophily) and caught by pollination drop secreted by ovule.  Pollination is direct.  The pollination drop is dehydrated and the pollen grains are sucked in to the pollenchamber.  Pollen grains take rest for some time in the pollen chamber
 During the germination of pollen grain the exine is ruptured and the inner intine comes out in the form of a tube like structure known as pollentube.  At this time the generative cell divides and forms a larger,upper body cell and smaller,lower stalk cell.  The pollen tube acts as haustorium to absorb food materials from the nucellus besides as sperm carrier.  During the germination of pollen grain the exine is ruptured and the inner intine comes out in the form of a tube like structure known as pollentube.  At this time the generative cell divides and forms a larger,upper body cell and smaller,lower stalk cell.  The pollen tube acts as haustorium to absorb food materials from the nucellus besides as sperm carrier.  The body cell divides and forms two naked,top shaped,motile,multiciliated antherozoids.The cilia are in 4–5spirals.  The male gametes of Cycas are 180–210μ in size and largest in the plant kingdom.
Female Reproductive Structures  Female plant do not produce definite cones  A whorl of spirally arranged megasporophylls arise around the short apex  Each megasporophyll resembles the foliage leaf and approximately 10-23 cms. long  Lower petiolar part bears the naked ovules on the margins
Ovule Structure  Ovules are orthotropous, sessile, ovoid or spherical in shape and unitegmic.  The thick integument is differentiated in threelayers- outer and inner fleshy layers, middle stony  The integument remains fused inside with nucellar tissue except at the position where it forms the micropylar opening.  Ovule is well supplied with vascular bundles.
Megasporangium  The megaspore develops in the nucellus by meiotic division and goes on to form female gametophyte tissue.  2-3 archegonia are formed in this haploid tissue which is food laden.  Egg cell in the venter of archegonia,undergoes fertilization by the motile spermatozoid forming diploid zygote
Young Sporophyte–Embryo  Embryo development is meroblastic.  Pro embryo shows upper haustorial part, middle elongating suspensors and the basal meristematic embryonal region.
Seed  A mature embryo is straight and has a short hypocotyl.  Embryonal axis has plumule at one end and radicle at the other.  Radicle is covered by coleorhiza.  Number of cotyledons may be 2-3..  Nucellus is completely absorbed in the seed.  Mature seed is large 2.5–5cm wide and usually orange or red in colour.  Germination is epigeal type
Germination of Seed (Seedling):  The seeds have no resting period and germinate immediately if placed on moist soil.  The seed is large and longer than broad.  It absorbs water from the moist soil and the coleorhiza comes first and the tip of the primary root makes its way through the coleorhiza.  The major part of the cotyledons comes out of the seed  The leaves begin to develop singly on the new plants  It has been observed that for a number of years only one leaf is developed at a time.
CYCAS 1.pdf