Origin of heterospory
Origin of heterospory
Pteridophytes
have many unique characters in both
structural and reproductive features. Being the first true land plants
they include some bodily features as a
primordial terrestrial habitat.The existence of
heterospory and its relation to
seed habit is one of such features.
Heterospory is the phenomenon of production of
two types of spores in an individual.Among the two types of spores smaller
spores are called microspores and larger ones are called megaspores. They are
formed in microsporangia and megasporangia respectively. Only nine genera of
pteridophytes show heterospory. Some of them are Selaginella, Isoetes,
Stylites,Marsilea,Pilularia,Salvinia, Azolla. Though most of the pteridophytes
are homosporous, i.e; produce only one kind of spores.
Origin of heterospory:
It has been accepted by all the botanists that homosporous condition is more
ancient than heterospory and a detailed study of heterospory has revealed
that heterospory originated due to
reduction in number of spores in sporangia. Evidences for the origin of
heterospory may be obtained from three different types of studies:
1. Paleobotanical evidences:
As the fossil evidence indicates, heterospory is quite widespread among the
ancient pteridophytes; in some of them,heterospory is not well pronounced,
indicating transitional stage. According to Scott(1894),an indication of
hetrospory can be seen in two species Calamostachys binneyana and C.
casheana. In Calamostachys
binneyana
most of the sporangia were with large number of small spores in
tetrad but in some sporangia spores were
larger. However in C. casheana two distinct types of spora micro- and megasporangium occurred. A
similar abortion of spores was also observed in
certain species of Lepidocarpon,Calamocarpon
and Stauropteris.
The
above examples indicate that………
i.
heterospory has not evolved in living
forms but was also present in fossil plants and
ii.
It originated due to disintegration of
some spores in a sporangium.
2.
Evidences
from developmental studies:Developmental studies conducted on
living heterosporous members like Isoetes, Selaginella Marsilea etc. clearly
point out that basically micro- and megasporangia are alike and that the
differentiation starts only at the spore
mother cell stage.In a sporangium that
is to become a microsporangium many
spore mother cells survive where as in a sporangium that is to become a megsporangium only few spore mother cells
survive.
3. Evidences from experimental
studies:Some of the experimental studies indicate nutrition,
temperature, light etc. may also play a role in the manifestation of
heterospory.According to Goebel (1891) plants of selaginella grown under feeble
illumination tend to produce only microsporangia in their strobili.Shattuck
(1910) demonstrated in Marsilea that variance of temperature may induce
heterospory.By suddenly lowering the temperature by cold water spray, it is
possible to kill all nthe megaspore and a few of the microspore in Marsilea.The
surviving microspores grow to a larger size simulating the megaspores
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