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Pinus: Reproductive Morphology

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Pinus is a monoecious plant and bears male and female cones (strobilli) on different branches of same plant.

The male cone consists of microsporophylls or stamens with microsporangia and female cones consists of megasporophylls or carpels with megasporangia. Both male and female cone are distinct morphologically.

Male cone 


  • Male cones arise in clusters from the axil of scaly leaves as a dwarf shoot just below the apex of long shoot.
  • They are woody and 1-2 inches in length.
  • Each cone consists of a number of microsporophylls or stamens arranged spirally around the axis.

Microsporophylls


  • The microsporophyll is differentiated into a stalk and a terminal expansion, triangular flattened and bent upward and a microsporophyll (pollen sac) at its abaxial surface.
  • Pollen sacs are filled with number of microspore mother cells which later form microspores or pollen grains after meiosis.

Pollen grains


  • Each pollen grain or microspore is yellow, double layered unicellular and uninucleated structure. The pollen grains are winged as they are adopted for anemophilous pollination.
  • The pollen grains are bounded by outer thicker layer exine, middle exo- intine and innermost thin intine.
  • The exine is extended as two wings on each side of pollen grains.
  • The microsporangium when matures dehisces by a longitudinal silt to release the microspores which are yellowish in colour and appear like sulphur powder.
  • After the release of pollen grains the male cone wither and fall off.

Female cone 


  • Female cone arise solitary or in a whorl of 2-4 cones, each in the axil of scaly leaves.
  • Since Pinus is protandrous the development of female cone takes place much later than male cone.
  • Young female cones are smaller, reddish, green and soft while the mature ones are large brown woody and hard.
  • Each female cone consists of bract scales or carpellary scales or megasporophylls.

Bract scales (Megasporophylls)



  • The bract scales are small, thin, dry and brownish, slightly fringed at upper part. They are inconspicuous in matured cone.
  • On the upper surface of each bract scales are ovuliferous scales which are bigger,stout woody and somewhat triangular in shape.
  • At the base of each ovuliferous scale, there are two sessile ovules lying on the upper surface.

Ovules


  • Ovules in Pinus are sessile and anatropous (inverted)
  • Each ovule consists of a central mass of parenchyma like tissue called nucellus.
  •  Nucellus is surrounded by a protective covering called integument that arises from base of the ovule and covers it except the top making a narrow passage called micropyle.
  • The integument is three layered- outer and inner fleshy layers are sarcotesta and the middle stony layer called scelerotesta.
  • When matures, the ovule develops a cup like chamber on the top of nucellus called as pollen chamber which is meant for the deposition of pollen grains after pollination.
  • In the nucellus towards the micropyle, a single, large and dense cell gets differentiated called as diploid megaspore mother cells. 
  • The mother cell undergoes meiosis to form a linear tetrad of four haploid megaspores of which the upper three degenerates and basal one becomes functional and it is called embryo sac cell.

Pollination

The pollination in Pinus is anemophilous type, i.e.the microspores are transferred from the pollen sacs to ovules by the means of wind. Each ovule oozes a small drop of colourless liquid through the micropyle called pollen drop. This pollen drops attract pollen which are then deposited in the nucellus in the pollen chamber.