Debunga

Daripada Wikipedia, ensiklopedia bebas.
Lompat ke: pandu arah, cari
Imej SEM butir debunga daripada pelbagai pokok biasa: bunga matahari (Helianthus annuus), seri pagi (Ipomoea purpurea), prairie hollyhock (Sidalcea malviflora), lili oriental (Lilium auratum), evening primrose (Oenothera fruticosa) dan biji jarak (Ricinus communis)
Imej jarak dekat bunga kaktus dan stamennya
Lebah madu Eropah membawa debunga dalam bekas debunga kembali ke sarang
Hujung stamen tulip. Perhatikan butiran debunga
Debunga melekat pada lebah. Serangga tanpa sengaja membawa debunga dari bunga ke bunga memainkan peranan penting dalam banyak kitaran pembiakan tumbuhan
Lalat marmalad hinggap pada Cistus incanus, muka dan kakinya diselaputi debunga

Debunga (berakar daripada kata gabungan debu bunga) ialah serbuk halus hingga kasar yang mengandungi mikrogametofit benih tumbuhan, yang menghasilkan gamet jantan (sel sperma). Butiran debunga mempunyai lapisan keras yang melindngi sel sperma semasa proses pemindahan dari stamen ke pistil tumbuhan berbunga atau dari kon jantan ke kon betina tumbuhan konifer. Apabila debunga tiba di pistil serasi tumbuhan berbunga, ia bercambah dan menghasilkan tiub debunga yang memindahkan sperma kepada ovul ovari pokok tersebut. Butiran individu debunga cukup kecil untuk memerlukan pembesaran bagi melihat perincian.

Struktur debunga[sunting | sunting sumber]

Debunga sendiri bukan merupakan gamet jantan,[1] tetapi setiap bijian debunga menandungi sel (cuma satu sel dalam kebanyakan pohon berbunga dan beberapa dalam tumbuhan benih lain) vegetatif (tidak membiak) dan sel generatif (membiak) yang mengandungi dua jenis nukleus:

  • nukleus tiub yang menghasilkan tiub debunga; serta,
  • nukleus generatif yang membahagi kepada dua sel sperma. Kedua-dua sel ini dikelilingi dinding sel berselulosa yang dipanggil intina, dand dinding luaran sporopollenin yang bernama eksina.

Debunga dihasilkan di dalam microsporangium yang terletak pada stamen bunga angiosperma, biji kon pokok konifer atau yang seumpamanya. Butiran debunga datang dalam pelbagai bentuk (kebanyakannya berbentuk sfera), saiz serta permukaan yang bergantung kepada jenis spesies. Ada juga butiran debunga yang mempunyai sayap, seperti pada spesies pain. Butir debunga yang paling kecil, iaitu dari spesies Myosotis spp., mempunyai diameter sekecil 6 µm (0.006  mm) manakala butiran debunga bawaan angin boleh mencecah sebesar 90 hingga 100 µm.[2] Pengkajian terhadap jenis-jenis debunga (palinologi) begitu penting dalam sains-sains paleoekologi, paleontologi, arkeologi, and forensik.

In angiosperms, during flower development the anther is composed of a mass of cells that appear undifferentiated, except for a partially differentiated dermis. As the flower develops, four groups of sporogenous cells form with in the anther, the fertile sporogenous cells are surrounded by layers of sterile cells that grow into the wall of the pollen sac, some of the cells grow into nutritive cells that supply nutrition for the microspores that form by meiotic division from the sporogenous cells. Four haploid microspores are produced from each diploid sporogenous cell called a microsporocyte, after meiotic division. After the formation of the four microspores, which are contained by callose walls, the development of the pollen grain walls begins. The callose wall is broken down by an enzyme called callase and the freed pollen grains grow in size and develop their characteristic shape and form a resistant outer wall called the exine and an inner wall called the intine. The exine is what is preserved in the fossil record.

The pollen wall protects the sperm nucleus while the pollen grain is moving from the anther to the stigma, it protects the vital genetic material from drying out and solar radiation. The pollen grain surface is covered with waxes and proteins, which are held in place by structures called sculpture elements on the surface of the grain. The outer pollen wall prevents the pollen grain from shrinking and crushing the genetic material during desiccation and it is composed of two layers. These two layers are the tectum and the foot layer, which is just above the intine. The tectum and foot layer are separated by a region called the columella, which is composed of strengthening rods. The outer wall is constructed with a resistant biopolymer called sporopollenin. The pollen tube passes through the wall by way of structures called apertures.[3]

Pollen apertures are any modification of the wall of the pollen grain. These modifications include thinning, ridges and pores, they serve as an exit for the pollen contents and allow shrinking and swelling of the grain caused by changes in moisture content. Furrows in the pollen grain are called colpi, which along with pores, are a chief criteria for the identifying pollen classes.[4]

Zarah-zarah debunga barangkalinya mempunyai alur di mana orientasinya boleh membahagikan jenis debunga kepada kolpat atau sulkus. Bilangan alur atau pora yang ada pada permukaan zarah membantu para pengkaji mengklasifikasikan jenis bunga yang menghasilkan sampel debunga contoh - debunga dari tumbuhan eudikot mempunyai tiga kolpat, manakala kumpulan-kumpulan tumbuhan yang lain hanya mempunyai satu sulkus sahaja.[5][6]

Except in the case of some submerged aquatic plants, the mature pollen-grain has a double wall, a thin delicate wall of unaltered cellulose (the endospore or intine) and a tough outer cuticularized exospore or exine. The exine often bears spines or warts, or is variously sculptured, and the character of the markings is often of value for identifying genus, species, or even cultivar or individual. In some flowering plants, germination of the pollen grain often begins before it leaves the microsporangium, with the generative cell forming the two sperm cells.

Pendebungaan[sunting | sunting sumber]

Rencana utama: Pendebungaan

Peredaran debunga dari organ jantan tumbuhan kepada pasangan betinanya dikenali sebagai pendebungaan. Proses ini dapat dilakukan sama ada melalui tiupan angin, di mana debunga yang terpilih cenderungnya ringan dan berkuantiti banyak, mahupun serangga, di mana debunga yang terambil cenderungnya berat, melekit dan kaya dengan protein.

Apabila sebutir zarah debunga diletakkan di atas stigma bunga dengan keadaan yang mengizinkan, butir debunga tersebut akan melepaskan tiub debunga yang menumbuh ke dalam tisu seakan struktur ovari melalui plasenta tumbuhan tersebut hinggalah ia sampai ke mikropil ovul. Sementara itu pula, nukleus generatif turut menyusul sambil ia membahagikan diri menjadi dua sel sperma yang akan tiba di destinasi mereka di hujung tiub yang terbina itu.

Debunga sebagai pembawa maklumat ekologi tumbuhan[sunting | sunting sumber]

A Russian theoretical biologist, Vigen Geodakyan (Geodakian), has suggested that the quantity of pollen reaching a pistillate flower can transmit ecological information and also regulate evolutionary plasticity in cross-pollinating plants. Plentiful pollen indicates optimum environmental conditions (for example a plant that is situated at the center of its natural range, in ideal growing conditions, with a large number of male plants nearby, and favorable weather conditions), whereas a small amount of pollen indicates extreme conditions (at the borders of its range, with a deficiency of male plants, and adverse weather conditions). Geodakian believes that the quantity of pollen reaching a pistillate flower defines the sex ratio, dispersion and sexual dimorphism of a plant population. High pollen quantity leads to a reduction of these characteristics and stabilization of a population. Small quantity leads to their increase and destabilization of a population.[7]

Dependence of the secondary sex ratio on the amount of fertilizing pollen was confirmed on four dioecious plant species from three families — Rumex acetosa (Polygonaceae),[8][9] Melandrium album (Cariophyllaceae),[10][11] Cannabis sativa[12] and Humulus japonicus (Cannabinaceae).[13] (see summary of all these data in review article[14]).

Dependence of offspring phenotype variety on amount of pollen was observed by Ter-Avanesyan in 1949. All three studied species of plants (cotton plant, black-eyed pea, and wheat) showed dependence in the direction forecast by the theory — fertilization with a small amount of pollen resulted in an increase in the diversity of the offspring. Ter-Avanesian writes that as a result of a limited pollination “instead of homogenous sorts we get populations”.[15][16]

Debunga dalam rekod fosil[sunting | sunting sumber]

Pollen's sporopollenin outer sheath affords it some resistance to the rigours of the fossilisation process that destroy weaker objects; it is also produced in huge quantities. As such, there is an extensive fossil record of pollen grains, often disassociated from their parent plant. The discipline of palynology is devoted to the study of pollen, which can be used both for biostratigraphy and to gain information about the abundance and variety of plants alive — which can itself yield important information about paleoclimates. Pollen is first found in the fossil record in the late Devonian period[perlu rujukan] and increases in abundance until the present day.

Demam jerami[sunting | sunting sumber]

Rencana utama: Demam jerami

Lihat juga[sunting | sunting sumber]

Rujukan[sunting | sunting sumber]

  1. ^ Johnstone, Adam. Biology: facts & practice for A level. Oxford University Press. m/s. 95. ISBN 0-19-914766-3. 
  2. ^ Pleasants, Jm; Hellmich, Rl; Dively, Gp; Sears, Mk; Stanley-horn, De; Mattila, Hr; Foster, Je; Clark, P; Jones, Gd (2001). "Corn pollen deposition on milkweeds in and near cornfields" (Free full text). Proceedings of the National Academy of Sciences of the United States of America. 98 (21): 11919–24. PMC 59743Boleh diakses secara bebas. PMID 11559840. doi:10.1073/pnas.211287498.  Parameter |month= tidak diketahui diabaikan (bantuan)
  3. ^ http://www.geo.arizona.edu/palynology/polkey.html#key
  4. ^ http://www.geo.arizona.edu/palynology/ppapertr.html
  5. ^ Kenneth R. Sporne (1972). "Some Observations on the Evolution of Pollen Types in Dicotyledons". New Phytologist. 71 (1): 181–185. doi:10.1111/j.1469-8137.1972.tb04826.x. 
  6. ^ Walter S. Judd and Richard G. Olmstead (2004). "A survey of tricolpate (eudicot) phylogenetic relationships". American Journal of Botany. 91: 1627–1644. doi:10.3732/ajb.91.10.1627.  (full text)
  7. ^ Geodakyan V. A. (1977). The Amount of Pollen as a Regulator of Evolutionary Plasticity of Cross-Pollinating Plants. “Doklady Biological Sciences” 234 N 1-6, 193–196.
  8. ^ Соrrеns С. (1922) Geschlechtsbestimmung und Zahlenverhaltnis der Geschlechter beim Sauerampfer (Rumex acetosa). “Biol. Zbl.” 42, 465-480.
  9. ^ Rychlewski J., Kazlmierez Z. (1975) Sex ratio in seeds of Rumex acetosa L. as a result of sparse or abundant pollination. “Acta Biol. Cracov” Scr. Bot., 18, 101-114.
  10. ^ Correns C. (1928) Bestimmung, Vererbung und Verteilung des Geschlechter bei den hoheren Pflanzen. Handb. Vererbungswiss., 2, 1-138.
  11. ^ Mulcahy D. L. (1967) Optimal sex ratio in Silene alba. “Heredity” 22 № 3, 41.
  12. ^ Riede W. (1925) Beitrage zum Geschlechts- und Anpassungs-problem. “Flora” 18/19
  13. ^ Kihara H., Hirayoshi J. (1932) Die Geschlechtschromosomen von Humulus japonicus. Sieb. et. Zuce. In: 8th Congr. Jap. Ass. Adv. Sci., p. 363—367 (cit.: Plant Breeding Abstr., 1934, 5, № 3, p. 248, ref. № 768).
  14. ^ Geodakyan, V.A. & Geodakyan, S.V., (1985) Is there a negative feedback in sex determination? “Zurnal obschej biol.” 46 201-216 (in Russian). ).
  15. ^ Ter-Avanesyan D. V. (1949). Tr. Prikl. Bot, Genet, Selekt., 28 119.
  16. ^ Ter-Avanesian D. V. (1978) Significance of pollen amount for fertilization. “Bull. Torrey Bot. Club.” 105 N 1, 2–8.

Pautan luar[sunting | sunting sumber]

Clumps of yellow pollen on a flower head.

Rencana ini menggabungkan teks daripada Ensiklopedia Britannica Edisi Ke-11, sebuah terbitan yang kini di domain awam.

Templat:Botani