ONE MUST KEEP in mind the place of the seed in the life of the plant if he is to understand the processes of germination.

A seed is essentially a young plant whose life activities are at a minimum. The drying out of the young seed as it ripens on the plant brings about this reduction of activities. The dry seed thus is in a condition to be held, stored, and preserved until time and place are suitable for the start of a new plant.

Many seeds, especially crop seeds, begin to germinate as soon as they are planted under moist conditions and they absorb water. Thus new corn plants appear promptly when corn grains are kept over winter and planted in moist, warm soil.

The germination of other seeds, including seeds of many flowers and weeds, does not begin until special conditions, besides moisture, are provided. Such seeds have a block or blocks to the germination processes. They do not germinate until the blocks are removed.

If a crabgrass seed (Digitaria) should germinate when it fell to the ground at maturity in late summer, cold weather would soon kill the young seedlings. Special germination requirements of the freshly ripened crabgrass seed prevent it from germinating until the next season.

Seeds with special germination requirements are called dormant (blocked). The special conditions associated with seed dormancy are considered in the chapter that follows.

The first start toward germination is the absorption of water, which allows the protoplasm of the cells to carry on active life.

The imbibition of water by seeds involves two processes.

One is much like the taking up of water by any dry material, such as a sponge.

The other involves the osmotic nature of the living cells. The osmotically active cells of the living seed have great attraction for water. Seeds absorb enough water to start to germinate in soil that is so dry that it will not support subsequent growth of the seedlings.

Each kind of seed must absorb a fairly definite proportion of water before germination will start. The amount depends on the structure and the composition of the seed. When seeds have taken up enough water for germination to start, they contain about 40 percent of water (as in corn) to about 70 percent (as in beans).

The first visible evidence of germination is the breaking of the root tip through the seed covering.

The bean is typical of most seeds in regard to the start of germination. The root tip emerges as a result of the elongation of the hypocotyl (the stem tissue between the root tip and the cotyledons). At about this time, the cells of the root tip and hypocotyl begin to divide. The continuing elongation of the newly formed cells establishes the root in the soil and pushes the hypocotyl and cotyledons into the air.

Soon after the seedling is well above the surface, elongation of cells and then cell divisions start in the plumule, the young growing point of the stem. The elongation of newly formed cells pushes the stem tip and young leaves above the cotyledons.

The very first changes leading to germination, however, are not these visible growth activities, which require the energy and building materials that they obtain from chemical activity within the cells.

A marked increase of respiration occurs before we can see any growth. This early increase of respiration releases energy from food materials already present in usable form in the cells that start growth.

The mobilization of food reserves precedes visible signs of germination by many hours. In the tiny, oil-rich seed of foxglove (Digitalis purpurea), new starch grains appear in the root cap at least 12 hours before elongation of the cells of the radicle. Sugar and protein building materials increase in the root tip and in the plumule tip at an early stage.

As growth proceeds, the increasing demand for materials for energy and for new tissues is met by the digestion of reserve foods. After the small amounts of nearby reserves are used, the abundant stored foods of the cotyledons (as in bean) or of the endosperm (as in corn) are drawn upon.

The nature of the food reserves varies with the kind of seed.

The cells of the cotyledons of the bean are filled with starch and protein. Those of the soybean usually contain no starch but are filled with oil and protein.

The "germ" of wheat and corn (the embryo, including the scutellum or single cotyledon) contains much oil and is rich in protein, but the endosperm, which is much larger, is largely starch.

Much of the stored food of the date seed and the carrot seed is in thickened walls made of hemicellulose. These reserves of starch, oil, hemicellulose, and protein are large in amount.

Many other reserves must be present in smaller amounts for active germination and normal development of the seedling. Nucleic acids are present in the cotyledons of bean and in the endosperm of wheat and are transported to the growing axis during early germination. Organic phosphorus compounds, present during germination, are extremely important for the transfer of energy for growth. Inorganic phosphorus must be present for the formation of more organic phosphates.

A wide range of enzymes must be available to digest these reserves, make energy available from them through respiration, and build new tissues. The respiratory enzymes responsible for the initial release of energy must be present in the resting seed, but perhaps some of the others are produced as germination gets underway.

THE GERMINATION requirements of the seeds of many crop plants are much the same as the conditions for continued growth of the established plant.

Corn and bean plants grow best at moderately warm temperatures, and the seeds germinate best and most rapidly at similarly warm temperatures. Wheat and pea plants develop best at cool temperatures, and the seeds germinate best at similar temperatures. For these seeds, germination is simply the resumption of growth of the young plant. It is controlled by the same factors as later plant growth.

For varying periods after harvest, the seeds of many crop plants have special requirements for the initiation of germination. These seeds later will germinate readily at a wide range of conditions. The seeds of other plants (especially seeds of weeds) may require special conditions for germination throughout the life of the seed.

It is important to know the germination requirements of different seeds as a guide to the time and conditions for planting the seed, as a guide to any necessary special treatment, and, in the case of weeds, as an aid in the control of undesired plants.

We discuss germination requirements in relation to temperature, moisture, aeration, light, and the interaction of these factors.

MUCH ATTENTION used to be given to "cardinal" temperatures—the minimum, optimum, and maximum temperatures for germination. Early studies were limited to seeds that do not have special requirements as to temperature.

For many kinds of seeds, the rate of germination—the rate of growth of the seedling—increases with a rise of temperature until near the upper temperature limit for growth, when the rate of germination slows down.

In the early records, the temperature for the maximum rate of germination often was taken as the optimum temperature. Many individual seeds, however, may not germinate at all at the temperature of most rapid germination of other seeds.

One should consider therefore a compromise between highest percentage of germination and fastest rate of germination.

Most seeds germinate slowly at low temperatures. The reported minimum temperatures for germination therefore often depend on the patience of the observer. Several observations of seeds germinating on cakes of ice have been recorded.

Scientists now give emphasis to understanding the ways in which temperatures affect the germinating seeds rather than to establishing rigid limits of the overall effect of temperature.

The seeds of many plants will not start germination at high temperatures even though the seedlings will grow normally at high temperatures. In them, some step leading to the start of germination is blocked at higher temperatures but can proceed at lower temperatures. The temperature that keeps them from germinating varies with the kind of seed and the conditions under which the seeds mature.

Generally, the critical temperature is lowest just after harvest and gradually gets higher until the special temperature requirement disappears after a variable period of storage. Storing the seeds under very dry conditions and at very low temperatures delays such a change. Wheat often will not germinate at temperatures above 59° F. for 1 or 2 months after harvest. This condition varies with variety and with the weather at the time of maturity.

Most samples of lettuce seed will not germinate in soil that stays at high temperatures (76° to 86°). The critical temperature becomes higher as the seed ages in storage, but very few samples of lettuce seed will germinate at 86° even though seedlings will develop at that temperature.

Seeds of lettuce and many other plants that are held in darkness for several days at a temperature too high for germination become especially dormant and then will not grow when transferred to a lower temperature that previously would have favored germination.

The seeds of some plants have an opposite response to temperature. Seeds of alyceclover (Alysicarpus vaginalis), a semitropical plant, will germinate only at a temperature of 85° or higher when planted soon after harvest, but a year later they will germinate readily at lower temperatures.

The germination of seeds of coffee and the best growth of the young seedlings are restricted to a narrow temperature range (near 80° to 85°). The coffee tree thrives best at somewhat lower temperatures. The nurseries for young coffee plants therefore are usually at a lower elevation than the plantings of bearing trees.

The seeds of many grasses and flowers and of some vegetables germinate poorly at any temperature that is constantly maintained at a uniform level, but they germinate well if the temperature is alternated between a lower and a higher temperature.

Bluegrass seed will germinate well if the temperature is kept at about 60° for 16 hours and at about 75° for 8 hours each day or even at a daily alternation between 68° and 86°. These seeds usually germinate in nature in spring when the night and day temperatures vary sharply.

A satisfactory explanation of the physiological changes involved in this requirement for daily changes of temperature has not yet been given.

SEEDS need to absorb a fairly definite amount of water before germination will start. Different kinds of seeds vary in their response to surrounding moisture during germination. This probably is associated with the influence of the surrounding moisture on the aeration of the seeds.

Rice will germinate under water and with a low supply of oxygen.

The seeds of cattail (Typha latifolia) and probably those of other water plants, instead of being sensitive to a lack of oxygen, germinate only when the supply of oxygen is reduced.

Many other seeds, including clovers, will germinate under water. Others, such as cabbage, do not germinate if even a film of water surrounds the seed.

Spinach is especially sensitive to excess moisture; the spongy covering that surrounds the seed can become filled with water and so reduce aeration and prevent germination.

LIGHT does not influence the germination of many kinds of seeds, but the germination of others is controlled by the presence or absence of light.

When fully moist seeds of certain varieties of lettuce are held in total darkness at 68°, few of the seeds will germinate. If the seeds are exposed briefly to light, all the seeds are stimulated to germinate. The flash from a photographic flashlamp, in fact, is enough to cause them to germinate.

The promotion of seed germination is brought about by red light of a comparatively narrow range of wavelengths.

If moist lettuce seeds are promoted by exposure to red light and are then exposed to far-red light (just at the limit of visibility), the promoting effect of the red light is reversed, and the seeds will not germinate. This promotion and inhibition can be repeated many times, and whether germination occurs or not depends on the band of light that is given last.

THIS REVERSIBLE photoreaction that controls seed germination has been demonstrated in at least 20 kinds of seeds. Undoubtedly it occurs in many more.

It is interesting that the same mechanism that controls the germination of some kinds of seeds is also responsible for the photoperiodic control of flowering, for the control of the elongation of seedlings, for the coloring of seedlings and of certain fruits, and for the control of other phases of the development of plants.

BOTH RED AND FAR-RED light are present in daylight and in most artificial light, but the red light has the strongest effect on many light-sensitive seeds like lettuce. The germination of such seeds is stimulated by unfiltered light, but seeds of henbit (Lamium amplexicaule) are so sensitive to far-red light that their germination is prevented by long exposure to daylight or incandescent light.

Germination of the seeds of many plants, like those of lettuce, is promoted by a single brief exposure to red light, but the seeds of many grasses require repeated light exposures over a period of several days to stimulate the germination of all the seeds.

Not all of the seeds of some kinds of plants, such as loblolly pine (Pinus taeda) and white pine (Pinus strobus), are ready to respond to the light stimulus at the same time. After the seeds are held at a low temperature in darkness for 2 weeks, all are ready to respond to a single exposure to red light.

LIGHT and temperature and other factors that influence germination arc interdependent.

Seeds of some samples of tobacco will germinate very little at temperatures held constant at any temperature from 59° to 86° unless the moist seeds have been exposed briefly to light. At daily alternations of temperature between 59° and 77° or between 68° and 86° however, germination is complete either in light or in darkness.

The seeds of peppergrass (Lepidium virginicum) germinate only after the moist seeds have been exposed to light, but even after full promotion by red light only a part of the seeds germinate at constant temperatures, such as 59° or 68°.

If seeds of peppergrass that have been allowed to absorb moisture in darkness at about 70° for 24 hours are exposed to light and then returned to 70° in darkness, only about one-third of the seeds germinate. If, however, at the time the seeds are exposed to light, the temperature is raised to 95° for 2 hours, all of the seeds will respond to the light stimulus and germinate at 70°.

The short period at the high temperature removes some block that prevented the seeds from responding to the light stimulus. This short-time, high-temperature treatment is effective with a number of kinds of seeds in increasing the response to treatment with light.

When lettuce seeds of varieties that ordinarily germinate completely in darkness are held fully moist at a high temperature (86° to 95°) for 1 to 2 days in darkness, they will not germinate if placed at a low temperature (59° to 65°) that originally would have been very favorable for germination.

These dormant seeds, however, will germinate if they are exposed briefly to red light and are placed at the low temperature. The prolonged treatment at the high temperature changed the seeds from light insensitive to light requiring.

These examples of the interaction of the different germination requirements indicate that one should not consider any of these requirements separately. They all must be considered together to understand how to obtain the best germination of seeds.

THE VARIOUS requirements of different kinds of seeds have been learned from laboratory experiments, but this knowledge helps us to understand the behavior of seeds in the garden and in nature.

It is customary to give little or no soil covering when sowing many kinds of small seeds. Often it is stated that the seeds are planted near the surface because the seedlings are not strong enough to come up through much soil. The real explanation for most seeds is that they need to be exposed to light in order to germinate.

Weeds often appear in meadows where they had not grown for many years. They always appear where soil is brought to the surface by wheel tracks or other disturbance of the surface.

A SIMPLE experiment illustrates what takes place.

Seeds of peppergrass are planted on the surface of moist soil in three flowerpots. The seeds in two of the pots are quickly covered with about one-fourth inch of moist soil. The seeds in the other pot are left uncovered. Drying out is prevented by covering all the pots with a pane of glass. Abundant peppergrass seedlings appear in a few days from the seeds that were not covered. No seedlings appear in the other pots. If one then draws a pencil through the soil covering the seeds in one pot, seedlings appear in the disturbed area after a few days.

THE SEEDLING after germination must establish itself in the soil. The young plant is not fully independent. It is dependent on the reserve foods of the seed for further development of root, stem, and leaves until it can become established and can manufacture enough food for all its needs.

This means that the enzyme system of the germinating seed must continue to digest the starch and oil and protein of the cotyledons or endosperm. These digested materials must be moved to the growing regions. Here other parts of the enzyme system must release and make energy available.

The temperature, moisture, and other requirements for the establishment of the seedlings in general are the same as for the future development of that particular kind of plant. The structure of the seedling must be complete, however, to allow for the development of a normal, useful plant.

If some part of the seed has been injured by handling or by poor conditions of storage, the seedlings may be incapable of developing into a useful plant—it is an abnormal seedling.

Equal vigor is not manifested in all seedlings, even the ones that will produce normal plants. When seeds are stored under unfavorable conditions, the first evidence of deterioration is slower germination and slower growth of the seedling. Seeds that have been harvested before fully mature may germinate, but the seedlings often lack normal vigor.

A person with experience in the germination of seeds can detect many of the seed lots that will grow into seedlings that lack normal vigor. A dependable method of determining the relative vigor of seed lots has not been developed.

OUR present knowledge of the details of the changes that take place in the developing seed and during germination is not sufficient for us to know what determines seedling vigor.

There is much interest at present in learning how to insure that seeds that are produced will be of good vigor and how to detect, in the seedling stage, which seedlings will produce the most vigorous plants.

Alfred Stefferud

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