Cycadales PDF

BOT1112 Gymnosperms Cycadales Lecture 04 Continued: 21.12.2023, 8-9am Order Coniferales Cycadales Ginkgoales Gnetales Order Cycadales  370 except  SPOROPHYTE Plants are low and palm-like , height 4-8 feet.  Tallest species, – up to 20 feet high...

BOT1112 Gymnosperms Cycadales Lecture 04 Continued: 21.12.2023, 8-9am Order Coniferales Cycadales Ginkgoales Gnetales Order Cycadales  370 except  SPOROPHYTE Plants are low and palm-like , height 4-8 feet.  Tallest species, – up to 20 feet high  Stem unbranched , columnar and covered with persistent leaf bases. Female reproductive structures–the megasporophylls are not aggregated in cones.  Ovules (2 or more) borne on the lower margins in ascending order  External Morphology plant shows tuberous stem when young , becoming columnar and unbranched later. Leaf– Shoot apex is protected by a rosette of brown scale leaves. Plant grows very slowly adding a new crown of leaves every 1 or 2 years , alternating with crown of scale leaves. The pinnately compound megaphyllous leaves have 80-100 pairs of leaflets arranged on the rachis Leaf base is rhomboidal in shape and attaches the leaf transversely to the stem The leaflets are thick , leathery in texture, ovate or lanceolate in shape & photosynthetic in function.  Scale leaves are very small , rough and dry , triangular in shape and brown in colour , thickly coated.  Root is of two types – normal and apogeotropic coralloid.  Normal tap-roots grow from the radicle deep inside the soil giving out lateral branches  Some of the lateral roots grow apogeotropically towards the surface of soil and branch dichotomously  The much branched mass appears like a coral on the soil surface hence called coralloid roots  Do not bear root caps  The cluster has lenticel like apertures  Become infested by N2 fixed blue-green algae (cyanobacteria); bacteria & diatoms  eg. Nostoc punctiforme, Anabaena cycadacaerum  Symbiotic relationship thus established Sexual Reproduction  Strictly dioecious plant  Male plants are rare  Male strobilus or cone borne singly at the apex of the trunk  Cone shortly stalked & large (upto 50 cm length or more)  Microsporangia confined to abaxial (lower) surface  Usually present in sori– each with 2- 6 sporangia  They contain a large number of haploid microspores (pollen grains)  Female plant do not produce cones  A whorl of spirally arranged megasporophylls arise around the short apex  Each megasporophyll resembles the foliage leaf and approximately 10-23cms.long  Lower petiolar part bears the naked ovules on the margins  Largest ovule (6 cms. X 4 cms.) seen in C. circinalis  Ovules are sessile, ovoid or spherical in shape  The thick integument is differentiated in three layers- outer and inner fleshy layers , middle stony.  Ovule is well supplied with vascular bundles. Eg. Cycas circinalis Cycas revoluta Cycas rumphii C. Pectinata C. siamensis Zamia sp. Megasporophylls of different Cycas species; the incision of the lamina varies interspecific strongly; A. Cycas revoluta; B. Cycas rumphii; C. Cycas circinalis. Cycas circinalis Cycas male cone female C. rumphii Zamia sp C.revoluta