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Ovule
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=== Integuments, micropyle, chalaza and hilum === {{anchor|Integument|Micropyle|Chalaza}} [[File:Ovule-Gymno-Angio-en.svg|thumb|300px|Plant ovules: Gymnosperm ovule on left, angiosperm ovule (inside ovary) on right]] [[File:Modellreihe von Grundformen der Samenanlagen -Brendel Nr. 164-166-.jpg|thumb|Models of different ovules, [[Botanical Museum Greifswald]]]] An [[integument]] is a protective layer of cells surrounding the ovule. Gymnosperms typically have one integument (unitegmic) while angiosperms typically have two integuments (bitegmic). The evolutionary origin of the inner integument (which is integral to the formation of ovules from megasporangia) has been proposed to be by enclosure of a megasporangium by sterile branches (telomes).<ref>Herr, J.M. Jr., 1995. The origin of the ovule. Am. J. Bot. 82(4):547-64</ref> ''Elkinsia'', a preovulate taxon, has a lobed structure fused to the lower third of the megasporangium, with the lobes extending upwards in a ring around the megasporangium. This might, through fusion between lobes and between the structure and the megasporangium, have produced an integument.<ref name="Stewart-1993">{{cite book |first1=W.N. |last1=Stewart |first2=G.W. |last2=Rothwell |date=1993 |title=Paleobotany and the evolution of plants |publisher=Cambridge University Press |isbn=0521382947 }}</ref> The origin of the second or outer integument has been an area of active contention for some time. The cupules of some extinct taxa have been suggested as the origin of the outer integument. A few angiosperms produce vascular tissue in the outer integument, the orientation of which suggests that the outer surface is morphologically abaxial. This suggests that cupules of the kind produced by the [[Caytoniales]] or [[Glossopteris|Glossopteridales]] may have evolved into the outer integument of angiosperms.<ref>Frohlich and Chase, 2007. After a dozen years of progress, the origin of angiosperms is still a great mystery. Nature 450:1184-1189 (20 December 2007) | {{doi|10.1038/nature06393}};</ref> The integuments develop into the seed coat when the ovule matures after fertilization. The integuments do not enclose the nucellus completely but retain an opening at the apex referred to as the ''[[wiktionary:Micropyle|micropyle]]''. The micropyle opening allows the pollen (a male [[gametophyte]]) to enter the ovule for fertilization. In gymnosperms (e.g., conifers), the pollen is drawn into the ovule on a drop of fluid that exudes out of the micropyle, the so-called pollination drop mechanism.<ref name="Stewart-1993"/> Subsequently, the micropyle closes. In angiosperms, only a pollen tube enters the micropyle. During [[germination]], the [[seedling]]'s [[radicle]] emerges through the micropyle. Located opposite from the micropyle is the [[chalaza]] where the nucellus is joined to the integuments. Nutrients from the plant travel through the [[phloem]] of the vascular system to the funiculus and outer integument and from there [[apoplast]]ically and [[symplast]]ically through the chalaza to the nucellus inside the ovule. In chalazogamous plants, the pollen tubes enter the ovule through the chalaza instead of the micropyle opening.
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