Like all the flowering plants, trees reproduce and spread naturally by seeds. Some species and garden varieties also produce suckers or shoots directly from the roots, e.g. aspen and some of the elms. Seeds are the sexual means of propagation, formed by the fusion of pollen grains (male cells) and ovules (egg or female cells). 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. Each pollen grain germinates on the stigma. Sending clown a tube into the ovary. This tube carries the contents of the pollen grain (nucleus) to an ovule, with which it fuses. After the male and female cells have fused, a seed begins to develop and the ovary eventually becomes the fruit. In conifers the carpels do not fuse to form a pistil and the egg-cells are not enclosed in an ovary but are carried on an ovuliferous scale; trees of this type are called gynuiosperms.
Flowers containing both stamens and pistils are termed bisexual. Such flowers are to be found in the lime, cherry, false acacia, etc. Flowers containing only pistils, or only stamens, are termed unisexual. These are to be found in all conifers and in most broad-leaved trees that are wind-pollinated (birch, alder, oak, poplar, etc.). Species with both male and female flowers growing on the same tree (spruce, birch, alder) are termed monoecious. 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).
Tree flowers are rarely borne singly. As a rule they grow in clusters (inflorescences), which can contain many flowers or just a few. The commonest types are as follows:
Spike, including Catkin — petalless flowers attached directly to an unbranched stem either erect or pendulous (alder, birch). Raceme — similar to spike, but each flower having a separate stalk or pedicel (bird cherry). Panicle — a branched flowering stem bearing several flowers (horse-chestnut). Umbel — several stalked flowers arising together and forming a flattened or convex head (cornelian cherry). Dichasium — a branched inflorescence, the branches of which terminate in a flower and two lateral stems, each one of which may also terminate in a flower and further lateral branches. Corymb — similar to a raceme, but the lower flowers have longer pedicels (stalks) bringing all the flowers up together to form a flattened or convex head (lime).
In addition to these there are various compound flower arrangements comprising several of the various simple types of inflorescence.
The flowers of most trees are much less conspicuous and less brightly coloured than those of shrubs and herbaceous plants. Furthermore, adapted to pollination by wind-borne pollen, they frequently lack, or possess only rudimentary, petals, whose bright colours otherwise serve to attract insects.
The mode of pollination largely determines the period of (lowering of the various species. Wind-pollinated trees such as the poplar, aspen, alder and hornbeam blossom early in spring, before the crown is in leaf, when the pollen can be distributed much more easily. Insect-pollinated trees such as the lime, black locust or cherry blossom later when the crown comes into leaf.
Wind-pollinated plants produce vast amounts of pollen because most of it falls by the way and does not rea.-h the (lower 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 (lowers on separate trees, another, the blossoming of male and female (lowers on the same tree at different periods.
The quantity of seeds produced depends not only on the number of (lowers but also on the weather conditions dining the period of (lowering 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 o( seeds every year, for this requires a large quantity of reserve food supplies which the tree must build up over a period of lime. 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.
Individual trees start to produce seeds on attaining a certain age, usually one that is fairly advanced. Trees growing in forests usually start some ten to fifteen years later than those growing solitarily. The seed-bearing age of alders, birches and larches growing solitarily is about ten years, thai of some maples, the Douglas fir and spruces twenty to thirty, and that of the silver fir and beech thirty to fifty years.
The quantity of seeds in a good year is enormous. One hectare of forest planted with five to fifteen thousand young seedlings will yield up to three million seeds for a pine stand, live million for spruce, three to five million for beech and up to a hundred million for birch. Simultaneously, however, there are great losses both of the seeds and the young plants. Large quantities of seeds are eaten by birds and animals and many fall in places unsuitable for growth where they either do not germinate at all or die shortly after germinating, having used Up the store of food in the seed. Similarly, many young trees are destroyed in their first years by drought, frost, invading grass or other plants, or by animals that feed on them. Of the huge crop of seeds, all that usually remains within a lew years is less than one per cent per hectare.
To ensure that they fall in a suitable open space and do not merely drop beside the parent tree, where [he prospects for their further growth are poor, the seeds are adapted for dissemination to longer or shorter distances. Most are equipped for dispersal by the wind and are either covered with down (willow. Poplar) or have membranous wings (birch, elm, pine, spruce) or thick wings (maple, lime, hornbeam). The seeds of another »roiip of woody plants are dispersed by animals, mainly birds. In general these are trees with pulpy, bright coloured fruits which serve as food for the birds, the seeds then being disseminated over a wide area in their excrement (mountain ash, cherry, yew). A third group is formed by seeds which are themselves food for birds and mammals (oak, beech, walnut) and even though most are eaten, some are hidden in a concealed spot as a food store and then forgotten, or else dropped on the way. One would think that seed dispersal by animals is less effective than by wind, but the history of tree migration in the wake of the retreating ice sheet in the period following the Ice Age gives evidence to the contrary. Birch and pine, the first pioneers, were followed by the rapid northward spread of the oak and hazel, whose heavy seeds fall only a few metres from the parent tree. Acorns, however, are a favourite food of jays and pigeons, while hazelnuts are eaten by nutcrackers and woodpeckers and are sometimes carried great distances in their beaks. Naturally, some would drop to the ground while being carried; and thus the offspring of a given tree might take root several hundred metres or even kilometres from the parent. In this way even these trees migrated hundreds of kilometres to the north within a fairly short space of lime.
Dispersal of tree seeds by water, as in the case of certain water plants, occurs less frequently. Of the European trees the alder is one whose seeds are dispersed by water as well as by the wind, and of I he tropical species the coconut palm is a noteworthy example. The small seed of the alder is equipped with air sacs that keep it afloat on the water surface for weeks until the spring floods carry it far afield. The coconut palm, now found on the shores of continents as well as tropical islands, was brought to these places by sea currents and the regular swell of the ocean. But over long distances man also played a part. Fruits that fall on the shore are carried to the sea by the receding tide, then borne great distances by the currents to be thrown up again by the incoming lide on (he shores of other islands and continents.
A closer look at the type of seed, its method of dispersal, and the biological characteristics of a given tree, will reveal that all are closely linked with an efficiency found over and over again in nature. For example, pioneer trees such as the birch, aspen, alder and pine, which are the first to occupy wide, treeless expanses, produce a vast quantity of seeds which are very light and adapted for flights of hundreds of metres or even kilometres. That is how young trees may spring up in places where no other tree is to be found. These trees are also well adapted to the climatic conditions of such localities. They are able to withstand both heat and frost, and have moderate soil requirements, while their rapid early growth enables them to win 111«-battle against surrounding grasses and other vegetation. They begin bearing seeds at an early age and thus rapidly fill the surrounding area with their offspring. On the other hand, shade-tolerant trees such as beech and oak, whose offspring can thrive in shade and appreciate the protection of the surrounding forest in their first years, have heavier seeds that do not fall far.