In this article we will discuss about the modes of reproduction in pinus with the help of diagrams.
Pinus is monoecious but male and female cones are borne on separate branches. The male cones are on lower and female cones are on upper branches.
The male cones which replace dwarf shoots, occur in clusters (Fig. 16.7A). The number of male cones in a cluster varies: 15 in P. wallichiana and 140 in P. roxburghii.
Each male cone has an axis with spirally arranged microsporophylls (Fig. 16.7D). Every microsporophyll bears two microsporangia on the lower surface. On elongation of male cone microsporangia are exposed and dehisce longitudinally, dispersing the pollen.
The female cones (Fig. 16.7B,C) replace long shoots and are born in pairs, but their number may exceed up to six. Each female cone has an axis bearing spirally-arranged 80-90 magasporophylls (Fig. 16.7E). These magasporophylls (or ovuliferous scales, or bracts) are axillary.
Each megasporophyll bears two ovules (ov) on the upper surface. The ovules are inverted, their micropyles face the cone axis. The bract and ovuliferous scale form a seed-scale complex. To begin with, the megasporophyll or ovuliferous scale (ovs) is much smaller than the bract scale (bs), but after pollination it overgrows the bract scale (bs). The exposed part of the ovuliferous scale in a mature cone in known as apophysis.
Pinus is wind-pollinated. Its yellow pollen are produced in abundance, the so-called ‘sulphur shower’ can be seen in a forest.
Prior to pollination the ovule at its nuclear tip secretes a sugary liquid-pollination drop, on which rest the pollen grains and germinate to from pollen tubes. A pollen tube enters the archegonium forcing its way in between the neck cells.
Enzymes secreted by the egg burst open the tube to release male nuclei. These free nuclei lie in a new cytoplasm, jointly contributed by cytoplasms of male and female cells. After fertilization, union of male and female nuclei, the zygote and embryo develop into the neocytoplasm.
