evolution of gymnosperms

2. By Rose Mary Martin

3. 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.

4.  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 .

5.  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 .

6. 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 .

7.  2. Gymnosperm eustele
has no leaf gaps and has
evolved through
longitudnal dissection of
th eprotostele .

8. 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 .

9.  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

10. 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.

11.  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.

12. 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

13.  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

14.  3. Trigonocarpales
 Single integument is free
from nucellus .
 Cupule have not far been
discovered .
 Nucellar apex is complex .
 Has distinct pollen
chamber .
Genoseprma kidstonii

15. 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 .

16. Various stages in telome concept

17. 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