Tree Flowering and Reproduction

Tree flowers are rarely borne singly. As a rule they grow in lusters (inflorescences), which can contain many flowers or just a few.

Sexual organs in woody plants are contained in the flowers, i.e. organs composed of modified leaves whose function is the production of seed. A complete flower has four different kinds of modified leaves, namely the sepals or calyx, petals or corolla, stamens and pistils. The male sexual organ is the stamen, and is comprised of an anther and filament.

When the anther is ripe it bursts and releases the pollen grains, i.e. the actual male cells which are of microscopic dimensions. The pistil is formed of an ovary, containing the ovules, and a stigma, with either a sticky or a hairy surface, to trap the pollen grains. Quite often, the stigma is attached to the ovary by a stalk or style which may be very short, or long and slender.

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).