Wind-pollinated plants produce vast amounts of pollen because most of it falls by the way and does not reach the flower and pistil. When the pine or spruce are in bloom huge clouds of pollen are carried by the wind and a layer of yellow dust covers the surface of nearby puddles and ponds. The pollen grains of most wind-pollinated trees have air sacs which make it possible for them to be carried as much as ten to twenty kilometres.
In the case of trees without air sacs (larch,Douglas fir, walnut) the pollen grains can be carried only several tens of metres and the trees produce a large number of barren seeds. Trees pollinated by insects produce a far smaller quantity of pollen, as more of the grains manage to reach their intended destination on the body of the insect.
Most plants are protected against being pollinated by their own pollen, for such in-breeding can result in less rugged individuals that are unable to hold their own in the struggle for survival. One such means of protection is the occurrence of male and female flowers on separate trees, another, the blossoming of male and female flowers on the same tree at different periods.
The quantity of seeds produced depends not only on the number of flowers but also on the weather conditions during the period of flowering and seed maturation. Frosts or rainy weather can prevent pollination or fertilization so that few or no seeds are set. Furthermore, some trees do not bear a good crop of seeds every year, for this requires a large quantity of reserve food supplies which the tree must build up over a period of time.
Trees producing large seeds and thus requiring larger food reserves (oak, beech, walnut) may only bear them at two to four year intervals. Again, in harsher climates, e.g. in high mountain regions or in the north, where a longer time is required to accumulate the necessary food stores, the seed-bearing intervals may be longer.
Species with both male and female flowers growing on the same tree (spruce, birch, alder) are termed itionoecious. Those where either only the male or only the female flowers occur on a single tree (yew, juniper, willow, poplar) are termed dioecious. If both bisexual and unisexual flowers occur, the tree is termed polygamous (ash).
In the case of trees without air sacs (larch,Douglas fir, walnut) the pollen grains can be carried only several tens of metres and the trees produce a large number of barren seeds. Trees pollinated by insects produce a far smaller quantity of pollen, as more of the grains manage to reach their intended destination on the body of the insect.
Most plants are protected against being pollinated by their own pollen, for such in-breeding can result in less rugged individuals that are unable to hold their own in the struggle for survival. One such means of protection is the occurrence of male and female flowers on separate trees, another, the blossoming of male and female flowers on the same tree at different periods.
The quantity of seeds produced depends not only on the number of flowers but also on the weather conditions during the period of flowering and seed maturation. Frosts or rainy weather can prevent pollination or fertilization so that few or no seeds are set. Furthermore, some trees do not bear a good crop of seeds every year, for this requires a large quantity of reserve food supplies which the tree must build up over a period of time.
Trees producing large seeds and thus requiring larger food reserves (oak, beech, walnut) may only bear them at two to four year intervals. Again, in harsher climates, e.g. in high mountain regions or in the north, where a longer time is required to accumulate the necessary food stores, the seed-bearing intervals may be longer.
Species with both male and female flowers growing on the same tree (spruce, birch, alder) are termed itionoecious. Those where either only the male or only the female flowers occur on a single tree (yew, juniper, willow, poplar) are termed dioecious. If both bisexual and unisexual flowers occur, the tree is termed polygamous (ash).
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Wood garden planning is an ideal material for blending a house garden into a natural setting, especially one surrounded by tree.
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