School of Sciences, Lautoka Campus BIO509 Botany Lecture 16: Reproduction in Conifers

Reproduction in Conifers Conifers form separate male and female reproductive structures called sporangia Usually on the same plant. Heterosporous, with two kinds of microsporangia Female sporangia are called megasporangia (= female cone). Male sporangia are called microsporangia (= male or pollen cone)

Female cone Male cone

Megasporangia Megasporangia borne in pairs at the base of the scales (sporophylls) of the cone. Comprises megasporocyte surrounded by nutritive tissue (the nucellus). Ovule comprises the megasporangium and nucellus surrounded by the seed coat (integument). Ovule = megasporangium (megasporocyte + nucellus) + integument, all diploid like the tree itself.

Diploid megasporocyte (=megaspore mother cell) undergoes meiosis to produce 4 haploid megaspores. One forms the female gametophyte, the other three disintegrate. Mature megagametophyte has 2–6 archegonia (1 egg each). Diploid nucellus and integument surround haploid megagametophyte. Micropyle (pore) opens to pollen chamber

Microsporangia Male cone is much smaller than female cone. Microsporangia are borne in pairs at the base of papery scales. Contain many diploid microsporocytes (=microspore mother cells) that form the pollen.

Microsporocyte (2n) undergoes meiosis to produce 4 microspores (n). Microspores undergo mitosis to produce a generative cell and a tube cell, and a pair of wing-like air sacs develop on the surface, forming the pollen grain. Pollen is often produced in astronomical amounts

POLLEN GRAINS Pollen grain is the microgametophyte and consists of 2 cells. Facilitates transport of sperm to egg and eliminates need for water.

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Pollination and Fertilization Pollen grain (mature microgametophyte) is carried by the wind to the female cone (sticks to fluid on micropyle). After pollination, generative cell divides producing the spermatogenous cell and a sterile cell.

Spermatogenous cell divides to produce the two sperms, while the sterile cell disintegrate. Tube nucleus directs growth of pollen tube. One sperm nucleus fuses with egg to produce the zygote. Sometimes more than one zygote can occur – polyembryony.

Pollination vs. Fertilization Pollination is the transfer of pollen from the microsporangium (pollen cone/anther) to the megagametophyte (female cone/carpel). Fertilization is the union of the haploid sperm nucleus with the haploid female egg to produce the diploid zygote.

Seed and Sporophyte Production Embryo develops within megagametophyte, and the diploid integument forms the seed coat. A wing forms from the tree’s sporophyll tissue to aid the pine seed in dispersal. Seed germinates and grows into mature tree (sporophyte).

Seed coat Nucellus (surrounds embryo) Micropyle Several (often 8) embryonic leaves 5, 6, 7) three parts of embryo that will from stem and roots after germination.

Pine lifecycle is extended over 2 years It takes many years before Conifers reach reproductive age. Megaspore develops one month after pollination. Fertilization occurs 15 months after pollination. Pollination in pines occurs in spring, but development of mature seeds do not happen for 2 years afterwards. In most other conifers reproduction takes only a year.

Other conifers Some conifers do not produce cones with conspicuous scales. Yews (Taxus spp.) produce solitary ovules at the shoot tips, surrounded by a fleshy red covering (aril).

Other conifers Podocarps (Podocarpus spp.) also produce fleshy-coated seeds, but these are not open at the end. Fleshy coverings arise from integuments

Other conifers Cypress and redwood seed cones are flat at the tips and narrow at the base. Cypress cones Red wood cones

California species of redwoods are reowned for their size, height and longevity. Redwoods (Sequoia sempervirens) includes the tallest trees on earth, reaching up to 115m. Sequoia sempervirens

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