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Uploaded byDr. Renu Jangid
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Origin & Evolution of seed habit in gymnosperm.pptx

The document discusses the evolution of ovules and seeds in plants. It describes 6 key evolutionary events in ovule evolution, from heterospory to the formation of integuments. These events led to the retention of the megaspore within the nucellus and the development of endosporic megagametophytes. The evolution of ovules also involved a reduction in megaspores from multiple to a single functional one per megasporangium. The document then discusses theories on the evolution of integuments and seeds, including the nucellar modification concept where integuments developed from the nucellus through vascularization and lobing.

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Dr. Renu Jangid
 Stewart and Rothwell(1993) proposed six evolutionary events that took place in ovule evolution after the onset of heterospory: 1. Degeneration of three megaspore and formation of a single functional megaspore in megasporangium. 2. Retention of the functional megaspore in the megasporangium (nucellus) until embryo development. 3. Formation of endosporic megagametophytes within an indehiscent megasporangium. 4. Elaboration of the apical part of nucellus to receive microspores or pollen grains. 5. Formation of an integument which delimited a micropyle. 6. Formation of pollen tube or pollen tube like structure from endosporic microgametophyte.
 In geological time, the evolution from homospory to heterospory. During Late silurian & Early Devonian, only homosporous plants were present. Middle devonian saw the advent of heterospory.  Two kinds of spores were produced in different sporangia. The microspors were the more numerous and remained approximately the same size, wherease the megaspores numbered less and ultimately to one per megasporangium.
 Most pteridophytes are homosporous and monoecious (the spores produced by any particular individual are all of equal size and develop into gametophytes bearing both archegonia and antheridia). With such an arrangement, the probability of self-fertilization is relatively high and evolutionary progress is correspondingly slow.  It may be prevented, or made less probable, if the sex organs on a prothallus ripen at different times(e.g. in fern prothalli, the antheridia commonly ripen before the archegonia), or if there is some incompatibility mechanism: but, in any case, cross fertilization can only occur if there is sufficient water for the spermatozoids to swim from one prothallus to another.
 Some pteridophytes have escaped from this gametophytic limitation by becoming endosporic, i.e. the prothallus is retained within the swpore wall, scarcely protruding from it, even at maturity when the spore cracks open. The prothalli of such pteridophytes are invariably dioecious, i.e. the antheridia and archegonia are on different prothalli, thus completely preventing self- fertilization. Such pteridophytes are also heterosporous, i.e. the spores are of two sizes megaspore and microspore.
 Seed is a ripened or fertilised ovule. A seed (ovule) may be defined as an integumentted indehiscent megasporangium.
 The ovule of gymnosperm consists of three basic parts: (a)Outer protective layer-Integument, (b)Middle protective layer- Nucellus, and (c)Innermost female gametophyte in which female gamete and later embryo develops.  The evolution of ovule, from free sporing, heterosporous condition involved:- 1.Reduction in number of fertile(functional) megaspores(where only one is functional). 2.Elaborat of nucellar tissue (apex of the microsporangium) to receive male gametophyte. 3.Differentiation of additional protective layer intequment.
 Two theories are known which use fossil evidence to show how seed and integument evolved.  1. Nucellar Modification Concept (Andrews,1961):- According this theory the Lagenostomalean integument is derivable from the nucellus or sporangium wall by its vascularisation and growth and loobing of the apex. These concept makes use of coenopterid fern megasporangium which exhibitstwo megaspore in the upper half of the sporangium. The lower half is sterile and is supplied by a single vascular strand.Andrews opines that a further reduction in the megaspore(from two to one) took place. The surviving megaspore increase in size and sinks towards the basal sterile part.
 The proliferation of the sporangial wall took place and was accompanied by the division of the basal vascular strand into two, one each going on the two sides of megaspore in the nucellur tissue. The fused nucellus and integument of lyginopterid seeds gives support to this theory.
 The first stage of development is represented by Rhynia type branching telome, some of them being fertile bearing terminal sporngia.  Gradual reduction of some axes and corymbose clustering around one terminal sporangium gave rise to a central sporangium encircled by a ring of vascularized sterile telomes.  Fusion of encircled telomes at the base would cover the sporangium with vscularized integument.  Concurrent to these changes, the tip of the sporangium is modified for pollen reception and reduction of number of megaspores from many to one would give rise to an evolved seed.  The integument may fuse with the nucellus (as in lyginopterids) or may remain separate.
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