Chapter 2 - Reproduction
Contents - Class Homepage
Flowering Fruit Development and Anatomy Seed Structure and Dormancy
Seed Germination Asexual Reproduction

When you are done with this chapter, you should be able to:

1. Know the stages of sexual reproduction in trees from flower bud initiation through seed germination.

2. Know differences in reproductive habits between trees.

3. Know terminology associated with sexual and asexual reproduction in trees.

4. Know factors associated with reproductive success and failure in tree species.

5. Know the practices used in the management of sexual and asexual reproduction in commercial forestry and horticulture.

Back to top FLOWERING
Generally, trees have a juvenile stage during which they do not flower. This period prior to flowering varies greatly in length, but typically lasts for 10 - 20 years. However, several species flower very quickly. Alders produce cones in 3 years, apples may flower in three years, and several species of pine may flower in seven years.

Once trees begin to flower, they may not flower every year. Several factors such as frost or drought may reduce or preclude flowering from year to year. Some species such as Ailanthus actually quit flowering as they get older. Other trees may flower more if they are under a great deal of stress or if they are close to death. Flowering can be encouraged... trees that are open-grown will flower earlier and more consistently. Apple orchards and pine seed orchards take advantage of this.

Tree species may or may not have male and female flowers on the same tree. If a tree has both male and female flowers on the same tree, the tree is said to be monoecious. This means that both sexes are in "one house". Most trees are monoecious. If a tree has only one type of flowers on a given tree, that tree is said to be dioecious - the sexes reside in "two houses". Boxelder, willow, persimmon, holly and ginkgo are all dioecious.

Tree species may be further divided by the presence or absence of flower parts. A complete flower has all of its parts, including the corolla (petals), sepals, stamens, and the pistil. Incomplete flowers are missing one or more of these parts. There is one further subdivision... flowers may be perfect or imperfect. Perfect flowers have functioning male and female flower parts (may be complete or incomplete). Imperfect flowers lack either a stamen or a pistil and are therefore always incomplete. Flowers that have a stamen only are referred to as staminate. Flowers that have a pistil only are pistillate.

This pawpaw (Asimina triloba ) flower is perfect - it has a corolla, sepals, stamens and a pistil.

Red maple trees can be monoecious or dioecious. Red maple flowers are commonly imperfect, with separate male and female flowers, but trees may have perfect flowers. Males have obvious stamens (pollen-bearing anther and filament combinations). Females have paired pistils (stigmas, styles, and ovaries)... that eventully develop into 2-winged samaras with paired fruits. Can you tell which is male and which is female?

In coniferous species, flowers are referred to as male and female strobili.

Individual male sex cells are referred to as pollen.  Individual female sex cells are contained in the ovules.  Tree species are typically either wind or insect pollinated.  In wind pollinated species, pollen "clouds" can often be seen rising from trees where it can then travel for great distances.

Pollination is the arrival of pollen at or near the surface of a receptive ovule. Fertilization is the fusing of pollen and ovule nuclei leading to the formation of a seed.  Controlled pollination is often done to produce specific genotypes.  Female flowers are covered to prevent unknown pollen from entering and pollen from a known male flower is then injected into the bag.

In most angiosperms, seeds are formed in one growing season. A notable exception to this is the red oak group that requires two years for mature acorn development. In conifers, larches, spruces, firs, hemlocks and douglas-fir mature cones are produced in one year. Most pines, however, require two full growing seasons for mature cone development.

Seed production is a very important thing to try to predict. If you are a wildlife manager, it is an essential component in the determination of the food supply for a variety of animals. If you are a forester, knowing what species are producing seed and in what quantity are both important components in predicting regeneration.

Foresters often use seed trees and shelterwood cuts to ensure regeneration. A shelterwood opens up the canopy providing sun to the forest floor. White pine readily regenerates in a shelterwood.

It is very difficult to predict how much seed will be available. At least there are a few good rules of thumb to predict seeding:
-good seed years are not the same for all species
-production for a species varies between areas from year to year
-it is rare for all species in an area to have a bad seed year
-some individuals are particularly good seed producers
-good seed years can not be predicted until flower buds are formed

After flowers are observed, we usually have some idea as to what kind of a seed year to expect. Most trees produce fruit during the same growing season as the flowers. Notable exceptions are the red oaks and most pines, which mature over two growing seasons. Time is not the only factor important in seed production. Excessively high or low temperatures may lead to seed losses, as would drought or excessive precipitation. Wind may blow the developing seed from the trees. Insects always eat some percentage of the seed crop... you may have found worms in chestnuts or acorns or heard of cone worms eating pine cones. Birds and mammals may have either a positive or a negative effect. Fungi and bacteria may destroy a whole crop. (Have you ever tried to grow plums without spraying?)

Beetle larve are feeding on this Douglas-fir cone (photo courtesy USDA Forest Service, Cone and Seed Insects of North American Conifers, 1980).

The life of a seed is a complex series of events that culminates in the breaking of seed dormancy and the sprouting of a new plant. In this subchapter, we will focus on seed anatomy, dormancy and how to use this knowledge to better manage the harvesting and use of seeds.

Parts of a seed:

radicle- Root of the embryonic or newly emerged seedling

seed coat- outer, protective layer of a seed

hypocotyl- portion of the embryonic or developing seedling between cotyledons and radicle. Can also be thought of as an embryonic stem.

The developing part of the embryo above the cotyledons. The epicotyl will be the first leaves and stem.

cotyledons- leaves of the embryonic plant which will become first leaves following emergence

from the seed coat. Stores food and is the site of photosynthesis once exposed to light.

endosperm- seed tissue containing stored food including carbohydrates, fats and proteins.

After seeds are produced, does that automatically mean that those seeds will grow? Many species of trees in our area may germinate rapidly after they fall to the ground - silver maple and white oaks are excellent examples. However, the seeds of most species do not germinate immediately after falling on the ground. Most species exhibit some kind of dormancy - they won't germinate without some change in the condition of their surroundings. This usually prevents a seedling from starting to grow and than being killed by frost. Dormancy is not always broken by a chilling period. Some species may remain dormant for several years. Black cherry can remain dormant for three or more years. Fire cherry may remain dormant for 50 to 150 years! Some desert plants may remain dormant until a rain washes germination inhibitors from the seedcoat.

Dormancy may be caused by many things. Seed coat dormancy results from a seed coat that is impermeable to water or oxygen, such as the locust seed coat. Embryo dormancy usually involves an immature embryo that requires afterripening, as is the case with maple seed. Seed coats may offer mechanical resistance to the germinating embryo. There may be chemical inhibitors that need to be broken down before the embryo can germinate (a red oak acorn will germinate if the pericarp is removed). To break dormancy of those seeds with tough seedcoats, acids or mechanical abrasion are useful. This process is called scarification. Usually, these are the seeds that pass through animals. Other types of dormancy may be broken with prolonged chilling under moist conditions (2°C for 30 - 60 days) followed by warm, long days and cool nights. This process is called stratification.

These raspberry seeds found in animal scat are now ready to germinate in an obviously rich germination environment.

What can we expect after dormancy is broken? Seed viability (proportion of seed physiologically capable of producing a healthy seedling) varies by species and year, and can have a tremendous range. Loblolly pine is between 15 and 100%. Oaks are between 10 and 80%. Red maple averages 80% and Yellow-poplar averages 5%.

Just before a seed germinates it becomes physiologically active. Oxygen and water are taken into the seed, as are minerals (if they are available). At this time, carbon dioxide is omitted, as a the embryo's metabolism kicks into high gear. To get the energy boost that it needs, insoluble sugar, starch and fat in the endosperm are converted to soluble sugar and translocated to the embryo. Likewise, proteins in the endosperm are converted to amino acids and translocated. When the seed coat splits, usually the radicle emerges first followed by the cotyledons. The cotyledons quickly enlarge and begin photosynthesis.

Parts of a germinal:

Cotyledons are the first leaves of the embryonic plant . They store food and are the site of photosynthesis once exposed to light.

hypocotyl- portion of the embryonic or developing seedling between cotyledons and radicle. Can also be thought of as an embryonic stem.

Leaves are produced not long after germination. For more information, reach leaves through the Key Word Search

epicotyl- portion of the embryonic or developing seedling above the cotyledons.

Primary roots eventually become tap roots.

Radicle- Root of the embryonic or newly emerged seedling.

Plumule- First bud of the embryonic plant. On both types of seedlings the plumule consists of the epicotyl and the emerging leaves.

A seed is a structure that contains an embryo and stored nutrients that exists in a stress-resistant, usually dormant state.

You are about to enter the new world of vegetative reproduction - the development of a new stem or whole tree by asexual means. Seed production is not the only means by which plants may pass on a copy of their genes. Sprouting, suckering, layering, growing from cuttings, grafting, and tissue culturing are all means of asexual reproduction. (Granted, the last two require the help of people.)

Sprouting comes in many forms and is a common response to the death of the main stem, and is a common form of regrowth after harvesting. Stump sprouts arise from an intact root system from dormant or adventitious buds when the previously dominant shoot is removed or severely damaged. If young seedlings die back or are cut off, the resulting regrowth is termed a seedling sprout.

Stump sprouts are one of the main forms of angiosperm regeneration in several environments, and is an important consideration in the management of natural and plantation hardwood forest. Some interesting things to consider: The roots of oak trees can be several hundred years older than the stems. Stump sprouts have an early growth advantage over seed stock, and can allow regeneration even where deer populations are very high.

Without stump sprouting, the American chestnut would exist, but only in very limited numbers. However, stump sprouts are not without their problems. Stump sprout stems are more susceptible to rot, may have poor form, are genetically identical to the parent tree, may grow at very high densities, and may have weak stem to stump unions. Sprouts are quite common in angiosperms, but only rarely are seen in gymnosperms. Exceptions include redwood, cypress, shortleaf pine, and other southern pines when they are very young.

Sometimes stump sprouting can result in later problems with tree development. In this set of northern red oak (Quercus rubra) stump sprouts one can be seen upright and dead with obvious decay organisms present. Another large sprout has recently broken and fallen over because of weak anatomical connection. This is particularly a problem when high stumps are left during harvesting operations.

Seedling sprouts are physiologically identical to stump sprouts but deserve special mention because they are so common. Due to their small size and a hostile environment, tree seedlings are often subjected to severe injury resulting in severe damage or removal of the shoot. Common causes for this are deer, rabbits, insects, fire, mowers, and weed whackers. Sometimes shoots die back for no apparent reason. Dieback and resprouting usually occurs several times in oak seedlings before they become fully established.

This black oak seedling has died and resprouted numerous times. Note the large size of the root system relative to the shoot.

Root suckers are new shoots that arise from adventitious buds developing on roots. They are often found on the root system of trees that have been injured or felled, but are produced by some species even without injury. A root growing close to the soil surface frequently gives rise to root suckers. Black locust root suckers are often seen growing in lawns, especially from roots that have been scraped by lawnmowers. Root suckering is the major form of reproduction in black locust and quaking aspen. In these species, a single root system may give rise to hundreds of root suckers. Sassafras, blackgum, and beech also produce root suckers.

Layering is the formation of roots on branches that touch the ground while still attached to the parent tree. Layering requires moist soil and therefore occurs most often in swamps or bogs. Species that commonly layer include raspberry, rhododendron, black spruce, juniper and dogwood. Layering is a viable form of mass-producing cuttings in some hard to root species and is often used in the plant propagation industry.

Some species such as black raspberry and northern white-cedar readily air layer where their branch tips contact moist soil.

Establishing or re-establishing forests of all kinds often involves the principle of asexual reproduction. Foresters and horticulturists have taken advantage of these phenomenon for a long time and the relative role of asexual propagation appears to be increasingly important relative to sexual reproduction. Vegetative reproduction is used because new individuals can be established that are genetic duplicates (clones) of the parent plants. Most of these methods involve macropropagation - asexual reproduction from intact plant organs. Cuttings are produced by cutting and burying (sticking) all but the topmost portion of the twigs or small branches from a stool (source tree). Buds then form a new shoot and roots are induced from adventitious buds or preformed primordia. This is a common way to establish willow trees and cottonwood plantations. Both of these species develop from cuttings without special treatment. Other species will grow from cuttings if treated with plant growth regulators before sticking. Cuttings are usually collected from hedges and stuck in the late winter and early spring when stem carbohydrate levels are high and moist soil results in a favorable rooting environment. Rooted cuttings are used in difficult-to root species. Cuttings are often treated with a plant growth regulator before being placed in a high humidity environment to facilitate rooting. Once roots have developed, cuttings are then transplanted into nursery beds to grow further before outplanting.

Horticulturists have used grafting to fuse the shoot (scion) of one individual onto the root system (stock) of another. This allows the grower to take advantage of the desirable characteristics of both roots and shoots. Fruit growers often graft shoots from varieties producing good fruit onto rootstocks that are hardier or more available. While the newly grafted plant will have the desirable characteristics of both individuals, offspring will have no characteristics of the stock. Similarly, any sprouts originating below the root collar will have none of the traits of the scion. In forestry, seed orchard managers graft scions from superior trees onto established rootstocks. This allows the production of more superior seeds than would be possible by relying on a single superior tree.

Micropropagation is a form of asexual reproduction using small tissue fragments tissue and bud culture. This is especially helpful in genetic engineering - which produces a unique individual by combining portions of the genetic code from two or more individuals of another species through artificial means.

This process involves reproducing trees from individual cells or other non-reproductive material. This has several advantages over conventional reproduction:
-Several clones can be produced from a singe parent.
-Shortens the amount of time needed to reproduce a superior individual.
-Prevents passing some diseases from the parent to the next generation.
-Allows long-term storage of tissue as artificial seeds.

These somatic embryos were obtained from a single black spruce embryo

A very common method of micropropogation is called somatic embryogenesis. Somatic embryogenesis is a cloning method that results in the production, from a single seed, an unlimited number of somatic embryos (clones) that grow into genetically identical trees. The term “somatic” means that the embryos are created asexually.

These laboratory technicians in very sterile conditions are removing early embryos from petri dishes and placing them onto a maturation media.

To begin the process the embryo is first carefully removed from the seed or a seed with the seed coat removed is placed on a growing media containing plant growth regulators and nutrients. Following several weeks of growth, the tissue converts into embryonic tissue containing numerous genetically identical immature embryos. At this stage the tissue can be manipulated to continue to produce (in theory) an unlimited number of these immature embryos.

These petri dishes contain large numbers of radiata pine immature embryos that are ready for transfer.

A great advantage of this procedure is that these early embryos can be held in cold storage in liquid nitrogen (cryopreservation) for long periods of time. Large numbers of clones can be held in storage until field tests identify the best growers or disease and pest resistant clones.

A cryostorage facility.

The embryos are then removed from cold storage (or directly from media) and transferred to a new growth media generally referred to as a maturation media. This media also contains growth regulators and nutrients. The exact concentrations of these substances are generally kept secret by laboratories. Following a period of time in the maturation media, the embryos are considered mature and they are transferred to a germination media where the “seed” (actually mature somatic embryo) germinates and grows large enough to eventually be transferred to a greenhouse where they are grown to the size of a plantable seedling.

Embryos beginning to grow on maturation media (l) and ready for transfer to a greenhouse (r).

Notice the extreme uniformity in these 3-year-old loblolly pine clones produced from somatic embryogenisis. The uniformity will make future management, harvesting and manufacturing much more efficient.