The document discusses progymnosperms, an extinct group of woody, spore-bearing plants that are believed to have evolved from trimerophytes, leading to the emergence of gymnosperms around 350 million years ago. It covers their characteristics, including the development of vascular cambium and the origins of gymnosperm stele, as well as the evolution of homospory and heterospory. Key examples such as archaeopteris are highlighted, along with the evolution of integument in pteridosperm ovules.

1. UNIVERSITY OF
LUCKNOW
Faculty of Science
Department of Botany
Topic- Progymnosperm
Present by-
Madhesh kumar rai (011)
(M.Sc, 2nd sem.)
Plant science

2. Contents of the Topic
 Introduction of the Progymnosperms
 Characteristics of the Progymnosperms
 Origin of gymnosperm stele
 Homospory
 Anisospory
 Incipient Heterospory
 Heterospory
 Evolution of integument

3. Introduction
 The stratigraphically oldest known examples belong to
the Middle Devonian order the Aneurophytales, with
forms such as Protopteridium, in which the vegetative
organs consisted of relatively loose clusters of axes.
 Tetraxylopteris is another example of a genus lacking
leaves.
 In more advanced aneurophytaleans such as
Aneurophyton these vegetative organs started to look
rather more like fronds, and eventually during Late
Devonian times the anuerophytaleans are presumed to
have given rise to the pteridosperm order, the
Lyginopteridales.

4. INTRODUCTION
 The gymnosperms are believed to have appeared
during Upper Devonian and Lower
Carboniferous in Palaeozoic era approximately
350 million year ago.
 The progymnosperms are an extinct group of
woody, spore-bearing plants that is presumed to
have evolved from the trimerophytes, and
eventually gave rise to the gymnosperms.
 They have been treated formally at the rank of
division Progymnospermophyta or class
Progymnospermopsida (as opposite).

5. Characteristics
 In Late Devonian times, another group of
progymnosperms gave rise to the first really large
trees known as Archaeopteris.
 Vascular cambium with unlimited growth
potential is present as well as xylem and phloem.
 Ancestors of the earliest seed plants as well as the
first true trees.
 Strong monopodial growth is exhibited.
 Some were heterosporous but others were
homosporous

6. Origin of gymnosperm stele
 The Origin of gymnosperm stele was earlier
believed (Jeffrey 1899,1902,1917) to be from a
basic protostelic pattern which gave rise to
siphonostele by enclosing extrastelar
parenchymatous cortical tissue (as pith) and
simultaneous appearance of leaf gaps.
 The presence of leaf gaps was considered by
Jeffrey (1917) as one of the characteristic
features of the siphonostele in Filicales,
gymnosperms and angiosperms.

7. Homospory
 Homospory or isospory can be defined by the
occurrence of equal sized spores within the
same sporangium, which are morphologically
similar but genetically they may be similar or
different.
 Most of the bryophytes and pteridophytes
show homospory of one kind or the other.
 All the steps towards heterospory are basically
involved around the size and production of
spores within the same or differents porangia.

8. Anisospory
 Anisospory is reported only in dioecious
mosses and hepatics (e.g., Orthotrichaceae and
Targioniaceae).
 Two size of spores (small and large) are
produced in the same tetrad in a 1:1 ratio and
they subsequently give rise to male and and
female plants, respectively (Mogensen, 1983).

9. Incipient Heterospory
 Incipient Heterospory is aphenomenon
distinguished usually in fossil plants where
spores of different sizes were found within the
same sporangium but size of spores dose not
correspond to the sexes after their germination
as in the case of living Queensland genus
Platyzoma (Bower, 1926; Tryon, 1964 ).
 During the change described above the
sporangia became sex determined and the
gametophyte became from monoecious to
dioecious.

10. Heterospory
 In geological time, the evolution from
homospory to heterospory and then seed was
accomplished in about 40 million year (within
the Devonian).
 During Late Silurian and Early Devonian, only
homosporous plants were present.
 Two theories are known which use fossil
evidence to show how seeds and integument
evolved.

11. Images of progymnosperm
archaeopteris

12. Evolution of integument
 Long (1966) described and discussed the
evolution of integument of pteridosperm
ovules by the fusion of telome trusses.
 Genomosperma kidstoni represent the most
primitive form where eight filaments (tilome
trusses) cover a centrally placed
megasporangium.