Until Time and Place Are Suitable

EBEN H. TOOLE AND VIVIAN KEARNS TOOLE.

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.

A, dry seeds of beans; B, the seeds have imbibed water, the seedcoats are wrinkled; C, the seed opened to show the embryo; D, the radicle appears; E, the seedling is pushing up through the soil; F, the seedlings are up, Part of the seedcoat still adheres to the one on the right; G, the seedlings are straightening up, the primary leaves are unfolding, and the seedling on the right shows how the two leaves are fitted together; H, the primary leaves are open and the stem has elongated; I, the trifoliate leaves have appeared.

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.