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phylogenetic evolution of gymnosperms

This document discusses the evolution and development of gymnosperms from earlier plant groups like pteridophytes. Some key points include: - Gymnosperms are thought to have evolved from pteridophytes, with coditales stock coming directly from pteridophytes. Coditales then gave rise to coniferales and ginkogales. - Male and female gametophytes of gymnosperms differ from homosporous pteridophytes in being endosporous and reduced. The male gametophyte develops from pollen grains. - There are different theories on the origin of ovules, including the axial, sui-geneges, and

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By Rose Mary Martin
Introduction • Heredity and environment determine the form ,structure and life-history of a plant . • Heterospory has arisen independently in various groups of plants . • At earlier time the cycadofilicales were mistaken for ferns until their seeds were discovered . • There ar elines if evolution in the gymnosperms which does not mean that each family is derived from the one before it.
 A free-nuclear stage in the embryo of gymnosperms ,immediately following fertilization , occurs in all gymnosperms expect Sequoia and perhaps Gnetum.  The free- nuclear periods in the young gametophyte and embryo do not indicate relationships.  We have assumed that gymnosperms have come from the pteridophytes .  WETTSTEIN,S view is that both cycadophytes and coniferophytes show an unmistakable pteridophyte ancestry .
 In some way coditales , the lowest of the coniferophytes stock , came from the pteridophytes .  Then the cordaitalse gave rise to the coniferales and probably also to the ginkogales .  Bennettitales and Cycadales arose ,independently from the Cycadofilicales  There is no doubt that cycadales arose from Filicales .  Swimming sperms is a universal pteridophyte character ,which Ginkgo has inherited directly or indirectly from pteridophytes .
ORIGIN OF EUSTELE  Living gymnosperms have a eustele consisting of discrete strands .  Two views are prevalent regarding the origin of the eustele  1. the appearance of pith at the center in the basic protostele condition is the first step in the stelar evolution .  this led to siphonostelic condition .  pith appeared by enclosure of cortical tissue within the stele with the correlated appearance of leaf gaps.  So it is extrastelar in origin .
 2. Gymnosperm eustele has no leaf gaps and has evolved through longitudnal dissection of th eprotostele .
ORIGIN OF MALE GAMETOPHYTE  The male gametophytes of modern gymnosperms differ from the homosporous pteridophytes in being endosporous ,much reduced and possessing a siphonogamous outgrowth called pollen tube.  The male gametophyte is the outcome of germination microspores or pollen grains.  Germination is partially precocious and partially it takes place on the nucellus of the ovule .  It is extremely reduced structure compared to massive structures of bryophytes and pteridophytes .  It is devoid of orderly arrangement of cells .
 Some sort of comparison can be made with the male gametophyte of heterosporous pteridophytes like selaginella and isoetes Germinated pollen grains of some gymnosperms A. Male gamatophyte of Cordainthus B. Male gametophyte of Physostoma elegans C. Stephanosperma akenoides D. Cy-cadeoidea dacotensis
ORIGIN AND EVOLUTION OF OVULE  The nucellus of an ovule contains a megaspore that develops into the female gametophyte  It is surrounded by one or more integuments  Regarding the morphological nature ,three main views has been put forward :-  A. the axial theory ,which held that nucellus is of the nature of a bud bearing integuments that are regarded as lateral foliar appendages.  B. the sui-geneges theory that regarded the ovule to be an independent structure borne on either foliar or axial organs.
 C.the foliar theory regarded the ovule to be homologous to ,with three lobed leaflet of the female sporophyll,the two lateral lobes of the leaflet are fused to form the outer integument ,the nucellus is an emergence borne on the cup shaped terminal lobe.
STRCTURE OF PALAEOZOIC OVULE  1. Cardicarpales  the integument is free from nucellus and consist of two or three layers  a.)outer fleshy- sacrotest  b.)middle stony –sclerotesta  c.)inner fleshy –endotesta  Nucellus appeared as a thin band of tissues surrounding the female gametophyte .  Nucellus is free from the integument at the base . Lyrasperma scotica
 2.Lagenostomaleans  The nucellus and the integuments are united and are radially symmetrical .  A protective structure called cupule is present .  Cupule is found external to integument . Lagenostoma lomaxi
 3. Trigonocarpales  Single integument is free from nucellus .  Cupule have not far been discovered .  Nucellar apex is complex .  Has distinct pollen chamber . Genoseprma kidstonii
TELOME CONCEPT  Proposed by Smith ,Long and Andrews  Reduction in number of megaspore into one .  Elaboration of megasporangial apex to form pollen catching device.  Surrounding sterile telomes (units of integument )underwent modification to form integument.  Partial fusion of the encircling telomes to form a cupule – like strcture .  One or more terminal ovules become enclosed by further sterile telomes to form second integument or cupule .  Integument becomes fused with nucellus .
Various stages in telome concept
ORGIN OF SECONDARY WOOD  The secondary wood of progymnosperm Teraxylopteris shows gymnospermous characters, like presence of trachieds with bordered pits aligned in radial rows , the pits are crowded and multiseriate  Trachieds appear rectangular in cross section .  This clearly reveal that the secondary xylem of the gymnosperms had evolved in the progymnosperms of the upper devonian
phylogenetic evolution of gymnosperms