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Frequently Asked Questions (FAQs)
Q1.
Give a concise introductory account of gymnosperms?
Ans.Gymnosperms, commonly known as naked seeded plants,
form vast forests that dominated the landscape for more than
200 million years. There are four divisions of gymnosperms;
Cycadophyta (commonly known as the cycads), Ginkgophyta
(represented by Ginkgo biloba), Gnetophyta (with some
angiosperm affinities), and Coniferophyta (the conifers). The
largest division (Coniferophyta) includes almost all large trees,
such as pines, firs, spruce, junipers, and redwoods. As against
angiosperms with approximately >250,000 species, there are
only about 590 species of conifers, despite which they dominate
vast forested regions, especially in the Northern Hemisphere.
Q2. There has been a bias for studies on gymnosperms. Comment
upon the statement.
Ans.Gymnosperms comprise the longest living organisms (Pinus
longaeva, 5000 yrs), largest single organisms by volume (1540 m3),
tallest organisms (115 m) and even toughest organisms. Despite
the fact that 4 of the 5 divisions of seed plants are gymnosperms
(including Cycads, Ginkgo, Gnetales, & Conifers), yet compared
to angiosperms (Anthophyta), little is known about the patterns
of diversification and evolution, especially genome evolution in
gymnosperms. Despite low species diversity as compared to
angiosperms, conifers dominate vast forested regions, especially
in the Northern Hemisphere. The bias for gymnosperms is evident
from the fact that all complete genome sequencing projects have
focused on angiosperms – mainly cereals (rice, maize, sorghum)
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and others such as Arabidopsis, Medicago etc.
Q3. Briefly explain the evolutionary trends in gymnosperms
with particular reference to the role of seed habit.
Ans.
Gymnosperms do not constitute an evolutionary line
equivalent to the angiosperms, but represent a series of
evolutionary lines of seed-bearing plants lacking distinctive
characteristics of flowering plants. Since their first appearance
in the Late Devonian, gymnosperms and other seed plants
have come to dominate almost every terrestrial ecosystem,
encompassing a greater range of habit and habitat than any
other group of Tracheophytes. One of the fundamental reasons
of dominance of gymnosperms is the seed habit. The seed habit
differs from free-sporing heterospory fundamentally in that
it facilitated delivery of the male gametophyte directly to the
female gametophyte through pollination, thereby bypassing the
free release of the female into the environment, and thus the
environmental limitations placed on the free-sporing system
by water-mediated fertilization. In addition to seed habit which
is common in angiosperms and gymnosperms, in contrast
to angiosperms, gymnosperms have rare polyploidy, more
conserved chromosome number, long life span, low species
number, low net morphological change, wind based pollination,
mainly generalists and less variation in C-value range. The seed
habit and vegetative traits such as the production of wood by a
secondary meristem (cambium), contributed decisively to the
evolutionary success of the gymnosperms and angiosperms.
Q4. What are the specific advantages of heterosporic ancestry
and seed habit in the evolution of gymnosperms?
Ans.It is important to note that the ancestor of gymnosperms was
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heterosporous and the evolution of heterospory was a significant
development because it led to the evolution of seeds. Moreover,
seeds are better than spores because spores not only have a
short lifetime, but also are thinner walled and more vulnerable to
pathogens and damage than seeds. In contrast, seed coat provides
protection and gametophyte tissue provides nourishment for
developing embryo, which due to dormancy can wait a long time to
germinate when conditions are good. The evolutionary success of
spermatophytes generally stems from their reproductive system
involving seeds, which facilitated these plants to exploit habitats
not accessible to most lower vascular plants. The seed habit and
vegetative traits such as the production of wood by a secondary
meristem (cambium), contributed decisively to the evolutionary
success of the gymnosperms and angiosperms.
Q5.Give a concise account of the origin and radiation of
gymnosperms.
Ans.Gymnosperms, first group of plants to develop seeds, probably
evolved from a heterosporous group called the progymnosperms
in the Devonian Period (408-360 mya). First gymnosperms lacked
seeds, but had developed them by the end of the Devonian. So
far as the radiation of gymnosperms is concerned, they coexisted
with the bryophytes, ferns, and other seedless vascular plants,
but their adaptive radiation occurred during the Carboniferous
and early Permian (360-245 mya ago) when the climate became
warmer and drier. Gymnosperms were the dominant plants during
the Age of Dinosaurs (Mesozoic 245-65 mya), hence this era is
called also as Age of Gymnosperms.
Q6.
Describe the role of progymnosperms as precursors of
gymnosperms.
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Ans.Progymnosperms (phylum Progymnospermophyta), a group
of plants that existed in the Late Paleozoic era with intermediate
characteristics between those of seedless vascular trimerophytes
and those of the seed plants. Although the progymnosperms
reproduces through freely dispersed spores, they resembled living
conifers in the production of wood (secondary xylem). Amongst
the woody Devonian plants, progymnosperms were quite unique
in that they also produced secondary phloem. It is believed
that both the progymnosperms and paleozoic ferns probably
evolved from the more ancient trimerophytes, from which they,
however, differed primarily in possessing more elaborate and
more highly differentiated branch systems and correspondingly
more complex vascular system. The most significant evolutionary
advance of progymnosperms over both Trimerophytes and
the ferns is the presence of bifacial vascular cambium, which
produces both secondary xylem and secondary phloem. This
type of vascular cambium, characteristic of seed plants, evolved
first in the progymnosperms. Notable progymnosperms include
Aneurophyton type and Archaeopteris type.
Notwithstanding that enough evidence has accumulated
over the years for seed plants having been evolved from the
progymnosperms after the appearance of seed, still many
problems remain to be solved in fully understanding the early
evolution of seed plants.
Q7. Distinguish between Aneurophyton type and Archaeopteris
type of progymnosperms.
Ans.
Aneurophyton type occurred in the Devonian period,
approximately 362 to 380 million years ago, and are
characterized by three dimensional branching pattern.
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Moreover, this type possessed a solid cylinder of vascular
tissue, or protostele. In view of resemblance of branching
system of Aneurophyton type progymnosperms to early
seed ferns, some paleobotanists consider the later as
their precursors. The best example of Aneurophyton type
progymnosperms is Triloboxylon ashlandicum.
Archaeopteris type appeared in Devonian peroid some 370 m
years ago and extended upto the Mississippian period, about
340 million yrs ago. Lateral branch system was flattened in one
plane and bore laminar structures considered to be leaves. An
evolutionary advanced type of stele, Eustele (characterized by
arrangement of vascular tissues in discrete strands around pith)
apparently evolved in this group of progymnosperms. This strong
similarity links this group with the living seed plants. Unlike most
progymnosperms which were homosporous, some species of
Archaepteris were heterosporous, a further evolutionary adavaced
step. The best example of this type is Archaeopteris macilenta.
Q.8.Give an illustrated account of the extinct gymnosperms visà-vis evolution..
Ans.Pteridospermophyta (seed ferns or pteridosperms) is a highly
diverse relatively unnatural group that range in age from the Devonian
to the Jurassic. In form they ranged from Late Devonian slender
branched plants with ovules, such as Elkinsia and Archaeosperma,
to tree like Carboniferous plants, such as Medullosa. Cordaitales
represent primitive conifer-like plants. How exactly different
groups of seed ferns are linked to living gymnosperms is still not
clearly known. Cycadeoids (or Bennettitales) – consisted of plants
with palm-like leaves, somewhat resembling the living cycads.
Bennettitales are an enigmatic group of Mesozoic gymnosperms
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that disappeared from the fossil record during the Cretaceous.
Though the exact phylogenetic position of Bennettitales is still
uncertain, some paleobotanists believe that they might have
been the members of the same evolutionary line as angiosperms.
Though Bennettitales lived at the same time as extinct cycads
and both produced almost similar leaves, they were, however,
reproductively distinct from cycads in several respects such as
the presence of flower like reproductive structures that were
bisexual in some species.
Q.9. What is meant by modern gymnosperms?
Ans.
Living gymnosperms or modern gymnosperms are
gymnosperms with living representatives. These gymnosperms
have “naked” seeds that are not enclosed in an ovary (as
angiosperm seeds are) but their ovules and seeds are exposed
on the surface of sporophylls and analogous structures. There are
four phyla of gymnosperms with living representatives, including
Cycadophyta (Cycads), Ginkophyta (Ginkgo), Coniferophyta
(Conifers), Gnetophyta (Gnetophytes).
Q.10. Give a detailed account of gymnosperms with living
representatives.
Ans.
There are four phyla of gymnosperms with living
representatives, viz.Cycadophyta (Cycads), Ginkophyta (Ginkgo),
Coniferophyta (Conifers), Gnetophyta (Gnetophytes).
Cycads, with about 140 species in 11 genera and 2 families, are
palmlike plants with trunks and sluggish growth & characterized
by large palm-like leaves and large cones. Cycads appeared at
least 250 mys ago during Permian and became so numerous in
Mesozoic era that this period is often called as age of cycads and
dinosaurs Cycada are distributed mainly in Old World tropics,
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extending away from the equator to warm tropics.
Ginkophyta is represented by just one species (Ginkgo bilobacommonly known as maidenhair tree), the only living member
of phylum Ginkophyta.This plant species is unknown in wild,
but known only in cultivation. Though G. biloba was previously
widespread, the species is currently very rare. Moreover, G.
biloba has changed very little for more than 150 m years, hence is
commonly called as Living Fossil. Its living members share features
with other gymnosperm genera of Permian era (some 270 mys
ago). Its seed coat is fleshy and t comprises a widely planted
street tree.
Conifers, with about 600 species in 7 families, comprise the largest
and most widespread phylum. Most important gymnosperms
including pine, spruce, fir, cedar, etc. dominate many plant
communities throughout the world, especially wide stretches of
north. The Coniferophytes harbour complex seed cones, needled
leaves reduce water loss.
Gnetophytes have about 90 species in 3 living genera (Gnetum,
Ephedra & Welwitschia) having angiosperm-like features. From
an evolutionary standpoint they comprise an important link
and transition to angiosperms? Double fertilization, unique
characteristic of angiosperms, also occurs in Ephedra, suggesting
thereby common ancestory of gymnosperms and angiosperms.
Q.11. Explain how morphological and molecular data sources
have contributed to overcome the uncertainties in gymnosperm
evolution.
Ans. The four living gymnosperm orders (Coniferales,
Cycadales, Ginkgoales, and Gnetales) are morphologically so highly
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divergent from one another, that their evolutionary relationships
have been uncertain. There are at least three different views.
a. First, from morphological data, it is proposed that the
Coniferales and Ginkgoales are sister groups. Accordingly,
Page (1990) classified the extant gymnosperms into two
subdivisions: Coniferophytina, which includes Coniferales
and Ginkgoales, and Cycadophytina, which includes
Cycadales and Gnetales.
b. Second, some researchers are of different view point and
have drawn a different conclusion from the morphological
data. They regard the Coniferales and Ginkgoales as two
separate groups and consider Cycadales as the most archaic
seed plants.
c. Third, on the basis of chloroplast and nuclear DNA sequences,
it is suggested that the conifers and cycads form a clade,
representing the earliest lineage among seed plants. In
addition to uncertainty about the interordinal relationships,
the interfamilial relationships within each gymnosperm
order are also controversial.
Q.12.
Are gymnosperms paraphyletic or monophyletic?
Ans: On the basis of morphological and fossil data, many
authors have suggested that the gymnosperms are paraphyletic,
arguing that the ancestor of angiosperms was derived from a
certain gymnosperm lineage, presumably including Mesozoic
Bennettitales, Glossopteridales, or the Gnetales. The last view
was affirmed by cladistic analyses of morphological data and
parsimony analyses of partial nuclear 18s and 26s rRNAs, &CL
gene sequences, and combined morphological and molecular
analyses. However, in contrast to the above viewpoint, phylo
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genetic analyses of 5s rRNA sequences, partial fragments of the
small subunits of the nuclear and the plastid rRNAs, rbcL gene
sequences, and chloroplast intergenic spacer (cpZTS) sequences
suggested that the extant gymnosperms are monophyletic,
implying that none of the extant gymnosperm lineages is a direct
ancestor of angiosperms. On this basis it has been suggested that
the ancestral form of angiosperms should be searched for among
progymnosperms.
Q. 13. Why Ginkgo biloba is called as a living fossil?
Ans.Ginkgo biloba has changed very little for more than 150
m years, hence is commonly called as Living Fossil. Its living
members share features with other gymnosperm genera of
Permian era (some 270 mys ago). It is pertinent to mention here
that Ginkophyta is represented by just one species (Ginkgo bilobacommonly known as maidenhair tree), the only living member of
phylum. This plant species is unknown in wild, but known only
in cultivation. Though G. biloba was previously widespread, the
species is currently very rare, though has retained its original
characteristics.
Q. 14. Gnetophytes comprise a link between gymnosperms
and angiosperms. Substantiate with brief arguments.
Ans.Gnetophytes with about 90 species in 3 living genera (Gnetum,
Ephedra & Welwitschia) have distinct angiosperm-like features,
ranging from angiosperm-like leaves of Gnetum and scale-like
leaves of Ephedra, besides occurrence of vessel like and flower
like structures in some members. From an evolutionary standpoint
they comprise an important link and transition to angiosperms?
Double fertilization, unique characteristic of angiosperms, also
occurs in Ephedra, suggesting thereby common ancestory of
gymnosperms and angiosperms.
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Q. 15. Summarise the key points of gymnosperm evolution.
Ans.The key points in the gymnosperm evolution include:
• In spite of the fact that there are only about 720 species of
living gymnosperms, in comparison to >235,000 species of
angiosperms, the individual gymnosperm are often dominant
over wide areas. Gymnosperms were the dominant plants
during the Age of Dinosaurs and one of the fundamental
reasons for their dominance is the seed habit.
• Progymnosperms comprise the precursors of gymnosperms
• Pteridospermophyta (seed ferns or pteridosperms)
Cordaitales (primitive conifer-like plants) Cycadeoids (or
Bennettitales) represent the extinct gymnosperms.
• There are four phyla of gymnosperms with living
representatives
including
Cycadophyta,
Ginkophyta,
Coniferophyta, and Gnetophyta. Of these four phy;aconifers are the most widespread, Ginkgo is the living fossil
and Gnetophytes have angiosperm affinities. The four living
gymnosperm orders (Coniferales, Cycadales, Ginkgoales,
and Gnetales) are morphologically highly divergent from
one another, so that their evolutionary relationships have
been uncertain.
• On the basis of morphological and fossil data, many authors
have suggested that gymnosperms are paraphyletic, whilst
the molecular phylogenetic analyses suggested that the
extant gymnosperms are monophyletic.