The stately Pinus thunbergii is deserving of attention any day, but last week the male cones were resplendently releasing pollen, and I thought it made for a good opportunity to discuss reproduction in the Pinaceae . It took me a while to get a good photo of the pollen grains blowing away, as I had to use one hand to brush the male cone while holding the camera steady with my other hand. I didn’t manage to get any that were in perfect focus, but at least with this photo you get the general idea.
Pinus thunbergii, or Japanese black pine, is a gymnosperm. Gymnosperms have exposed ovules that become seeds (unlike the angiosperms, or flowering plants, that have ovules and ovaries that eventually become seeds and fruit). More specifically, Pinus thenbergii is classified as a conifer and, like most of its kin, it has a compounded female cone, formed from a bract and an ovule-bearing scale (the Taxaceae, also conifers, are exceptions to this rule). All conifers have a reproductive strategy that involves female cones as well as male microsporangiate strobili, which, for lack of a better common term, are often called male cones.
Today’s photo shows the male cone of a Pinus thunbergii plant. These male cones are 1-2 cm long, and grow in clumps of 15-20. They release large amounts of pollen grains that have two bladder-like wings which help the pollen reach a female cone. Unlike angiosperms, gymnosperms have not developed relationships with pollinators that allow them to move pollen accurately. Pinus thunbergii plants must release vast amounts of pollen into the wind, so that a lucky few microgametophytes will land on one of the tiny, red female cones, or macrosporangiate strobili. The female cones produce a sticky fluid called a pollination drop, which traps the pollen and, as it dries, draws it into the cone. The pollen is stored within the cone, dormant, for a period of one year while the female gametophytes (megagametophytes) develop, two to a cone scale.
During the second year, sperm from the pollen fertilizes the egg in the megagametophytes. By the beginning of the first year, the female cones are greenish-black and tightly closed, as shown in this photo from a previous BPotD post. Paired winged seeds begin to form at the base of each cone scale, and the 7cm long cones begin to turn brown. At the end of the second year, the cone scales will have opened and the mature winged seeds will have blown away, in the hopes of starting the cycle anew.
For more information, the Ohio Plants website has an excellent post on conifer reproduction, which covers Pinus as well as many of the other conifer taxa found in Ohio.