Origins of heterospory are phylogenetically independent except for the two families of hydropteridalean ferns and some progymnosperm orders, plus their descendants, the gymnosperms. The condition of the production of two kinds of dissimilar spores, differing in shape and size in the same species, is termed Heterospory. What are the advantages of heterosporous condition over homospory? In this paper, we consider the suite of characters that define the seed habit, and discuss the probable selective pressures that produced each character. Germination of megaspore inside megasporangium. There are two hypotheses about the evolution of seed plants. The zygote develops into the future sporophyte.
Heterospory Advanced to the Seed Habitat in Land Plants: One of the most important advantages of heterospory is the formation of seed habitat. It thus forms better starting point for the new embryo, than an independent green prothallus which has to manufacture its own food. This retention and germination of the megaspore within the megasporangium ensures proper development of the zygote. The morphology of the structure precludes assignment to any known group of plants in the absence of additional information regarding the parent plant. Lycopsids do not cooccur with Megaloxylon. This in turn reflected increased speciation rates, thereby completing a well defined negative feedback loop in the coevolution of phenotypic and ecological differentiation. From the above observations it is concluded that the life history of Selaginella approaches towards seed habit because of the following features: 1.
Permanent retention of megaspore within the megasporangium 5. The considerably fused cupule of Calycosperma suggests an earlier differentiation of multi-ovulate cupules in Famennian. The following traditional classification of the lycopsids into seven orders is intended to provide a framework for discussion of this group of plants. The wall ultrastructure of the functional megaspore was interpreted as being of the progymnosperm-spermatophyte type Marshall and Hemsley 2003. Another very important difference to gymnosperms is the angiosperm double fertilization. The female gametophyte megagametophyte is hidden in the ovule. The offspring sporophyte represented by the embryo.
What is the evolutionary significance of Heterospory? By sudden lowering of the temperature, the size of the microspores in the sporocarp of Marsilea increases by six times. Common names of the living Ginkgo are ginkgo and maidenhair tree, the latter derived from the resemblance between the bilobed leaves of ginkgo and the leaves of the maidenhair fern Adiantum, P1-7. Answer Heterospory is a phenomenon in which two kinds of spores are borne by the same plant. The foliar organs may be several centimeters in length and are vascularized by numerous dichotomizing veins that terminate near the margin. These data indicate the occurrence of two intervals of logistic diversification within the Paleozoic. Heterospory and Seed Habitat Origin of Seeds and Seed Habitat in Vascular Plants from Heterospory Heterospory is the production of two or more types of spores Heterospory is a condition of the production of more than one type usually two of spores in a single plant. Moreover, 8 of the 10 inferred origins occurred during a relatively brief period of the Late Devonian and Carboniferous ca.
The second outer integument in Angiosperm ovules may therefore have evolved after the establishment of anatropy, either from the first integument or as an overgrowth of the chalaza. Boldface entries indicate the maximum number of characters exhibited by extant members of the orders. The oldest seed-bearing seed fem Middle Devonian, 385 M had a small, radially symmetrical, integumented megasporangium. The ultimate segments pinnules of Yuania have parallel venation; stomata have not been observed. Progymnoperm fossils show vegetative morphologies to seed plants. Here, we use two case studies — the Devono-Carboniferous vascular plant radiation and the largely coeval evolution of heterosporous from homosporous life histories — to examine the interface between phylogeny and ecology.
The fossil record provides no evidence as to whether megaspore retention preceded or followed the other important changes in the evolution of the seed, such as the reduction to a single functional megaspore per sporangium, the modification of the distal end of the nucellus for pollen capture, and the evolution of integuments and accessory structures such as a cupule. Note that at the present time the issue of seed plant evolution remains unsolved. But all proaymnosperms did not have seeds or seed-like structures ovules or ore-ovules. After pollination and fertilization the ovule develops into the seed. Ferns are rare and are preserved only as fragments of permineralized rachises from two species.
If pollination occurs in early April, fertilization takes place in September, embryo development is completed during spring, and seed germinates the following May. Lower Spermatophytes Gymnosperms : 1. Development of pollination mechanism 9. Therefore, integuments cannot explain the evolutionary success of seed plants. Operculate spores assigned to Discinispora were discovered in petrified Discinites sinensis cones, and these are reminiscent of spore morphology in certain sphenophyte cones J.
In addition to tracing modern pollination to the earlier Cenozoic and later Mesozoic, there is a resurgence in understanding the evolutionary history of earlier palynivore taxa spore, prepollen and pollen consumers , which led toward pollination as a mutualism Scott et al. The cones dichotomize once at the base and consist of helically arranged sporophylls. Sometimes they reflect extrinsic controls on diversification, but in other cases they appear to be a consequence of intrinsic rates of origination and extinction. Results suggest that seed plants are a monophyletic group, and Lyginopteris is resolved as the sister taxon to all other seed plants considered. The complex ecological structure of insect pollinators and their host plants is a central focus within the ever-expanding discipline of plant-insect interactions. Moreover, there is no evidence to prove the development of seed habitat from homosporous condition. In number of species one , this is the smallest plant phylum.
A progymnospermous affinity is unlikely. Several of these, including true leaves, wood, and seeds, have obvious implications for human use and economic applications. . Cupules are goblet-shaped with the basal two-third fused and the distal one-third dissected into 16 linear tips. The characters of the variations comprise a complex of the following features: 1 the number of initials, 2 the sequence of their divisions and the mode of cell wall initiation at the first developmental stages, and 3 the participation of the initials in the formation of the various layers.