Lisa Weeks, age 10, of Winston Salem, N.C., for her question:

WHY DO PINE CONES OPEN AND CLOSE?

Pine cones may cling to their boughs for more than two years. The young ones soon open up their woody scales and then close them. They remain closed for a long time and then reopen. All these changes are needed to do their work, which is to produce seeds for future pine trees. When this is finished, the old cones finally fall and lie on the ground with their woody brown scales open and curved backward.

Every spring, when the pine tree sprouts new bristles of green twigs, it also produces two kinds of cones  male and female. The new cones appear on the new twigs, and at first it is hard to tell the nubbly little green males from the nubbly little green females. But the females contain the egg cells, and they soon outgrow the pollen bearing male cells. Come May or June, the male cones are ready to puff their pollen into the air.

    It is time now for all the young cones to open their A few grains of dusty pollen get blown into the open scales of the female cones. Now the work of male cones is finished. They dry up and soon fall to the ground.

The work of the larger female cones has barely begun. After the pollen blowing season, they close their scales and often seal them shut with weatherproof resin. Through the first year, the pollen and egg cells develop separately between the folded scales.

Then the pollen grains grow hollow tubes, and the male cells move down to join the waiting egg cells. The pairs merge and form fertilized seed cells. During the next year, they grow secretly into woody brown seeds, each with a woody brown wing.

In late fall or early spring, when the cones are 2 1/2 years old, they open their woody scales for the second time. The ripe seeds fall out, and a few lucky ones get blown to suitable soil where they sprout and become new pine trees. The old cones may fall to the ground right away or stay on their twigs several years. But their dry, woody brown scales are curved backward and remain wide.

It takes at least two and a half years for a crop of seeds to develop inside the tight fisted cones. Meantime, each spring the tree produces batches of young male and female cones. Several generations of cones are on the tree at the same time. Those with closed scales are hatching seeds. Those with open scales have shed their seeds and are waiting to fall.