Michael Klement, age 13, of Coplay, Pennsylvania, for his question:
What causes pine cones to close?
On the same pine tree bough, there is likely to be an assortment of cones of different sexes and at different stages of development. The male pollen bearing cones look somewhat like bunches of catkins. The young female cones point upwards, with their scales open. The year old cones axe sealed shut. Two year old cones are open and pointing downward.
The scales of a pine cone are specialized leaves arranged in a neat spiral. Their purpose is to produce and ripen the seeds. In some conifers, this takes but a few months. In other species, the seeds take one or two years to ripen and the sturdy old cones stay on the tree even after the seeds have been shed.
The reproductive cycle begins in the spring, when male and female cones appear near the ends of new foliage. The male cones, looking like fists of pale catkins, produce scads of pollen. When the pollen is ripe, it is scattered by the winds and for a few spring days a pine forest is half lost in a cloud of golden dust.
Meantime, the female cones have developed egg cells, down inside at the base of their woody scales. Success depends on fertilizing the egg cells with grains of pollen. The young cones open their scales and a few of the zillions of pollen grains drift down inside. The fertilized egg cells, called macrosperms, now need time, lots of time to ripen and mature. And it is the pine cone's duty to protect them through the months or even years ahead.
Soon after the macrosperms are fertilized, the young pine cone closes its scales. In some cases, the tree exudes a resinous secretion that seals the cracks between the scales. This closing of the scales provides a weatherproof, shelter for the maturing seeds. It also makes it harder for
When at last the seeds are ripe, the cone prepares to release their. It turns downward and its woody scales begin to open with enough force to crack their resinous seals. The little brown ripe seeds come loose and fall onto the ground. If you come upon a cone at this stage of development, you can shake the seeds out into your hand.
The almost 500 conifer species produce neither fruit nor pods around their bare seeds. They are called gymnosperms, or naked seed plants and their ancestors were among the first plants to bear seeds of any sort. Pines and conifers have been bearing their seeds inside scaly cones for at least 300 million years.