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Rest-Period in Plants. The rest-period of plants is that period or stage when the part or parts at inaction are incapable of responding to favorable growing conditions. A rest-period of some length, either short or long, appears to be universal with all perennial plant forms. So far as known, none has a continuous growth. Trees and shrubs growing in the open, both in cold and warm climates of the temperate zone, may begin to enter the rest-period in some of their branches as early as midsummer. This is indicated by cessation of length growth and formation of terminal buds. Some may be ready to grow again in a few weeks, while others are unable to grow for five or six months. Herbaceous perennials begin their rest after dying down in summer or fall. The death of the aerial parts is accompanied by the formation of strong crown-buds. They are usually ready to resume growth within a few days or weeks, although there are notable exceptions. Bulbs begin their rest when the tops die down after flowering. Many will not grow again for several months. Many seeds germinate poorly, or not at all, if planted immediately or shortly after ripening. Failure to grow is usually due to the influence of a rest- period. However, aside from the effects of a rest-period, seeds may fail to germinate promptly or at all on account of a hard or tough seed-coat which is more or less impervious to water. In garden practice, seeds are stored for a few weeks or months before planting in order that they may pass through a period of "after- ripening." Those with hard or tough seed-coats are stored by stratifying in moist sand.

Investigation has shown that practically all woody plants native to the temperate zone have a rest-period. Many of these are unable to grow in late fall or early winter, even when transferred to a warm moist greenhouse. In trees and shrubs the rest-period begins to set in as early as midsummer. The length of the rest varies greatly. In some cases it may last only a few days or weeks, while in others it may persist from three to six months. Red raspberries, mountain-ash, and spireas are almost free from a rest-period, while white oak, tulip tree, and American beech commonly rest for six or seven months. Among herbaceous perennials, the lily-of-the-valley is conspicuous for its rather long rest-period which usually lasts for two or three months. Asparagus, rhubarb, hardy chrysanthemum, golden marguerite, calliopsis, and a host of others are ready to grow again in a week or two after the old plants die down. All spring-flowering bulbs appear to have a distinct rest-period lasting from three to five months. From 50 to 75 per cent of the seeds of herbaceous plants, annual and perennial, have a rest-period, while the resting phase is noticeable and generally very distinct in fully 90 per cent of the seeds of trees and shrubs.

The rest-period is commonly looked upon as the period of dormancy in plants and seeds, but this view is incorrect. The rest-period of trees, for example, begins and ends entirely independent of the winter season. While cold weather may prevent growth from taking place, it does not necessarily prevent the rest-period from coming to an end and thus leaving the trees ready to grow as soon as surrounding conditions are favorable. In mild climates the rest begins and ends with great regularity. The same is true in cold climates, but in the case of trees, at least, the true condition of affairs is not apparent on account of the interference of the cold which prevents growth. In trees the rest sets in gradually twig by twig and is not complete until all the growing points have ceased elongating and formed terminal buds. In like manner the trees pass out of their resting state gradually, some twigs often being ready to grow much earlier than others. These phenomena are very noticeable when trees are grown under glass.

The rest-period of many trees and shrubs may be broken or greatly shortened by special treatments. Those with a long rest-period are very difficult to arouse during the early stages of their rest. The greater part can be forced with comparative ease during the middle period of their rest and to some extent during the latter phases, although at this time the treatments must be less severe or the plants may be killed. The agents most generally used for forcing growth are ether, drying, and freezing. Submerging in warm water will sometimes start growth, particularly in dormant twigs. Twigs of flowering shrubs, such as lilac, deutzia, spirea, and golden-bell, may be caused to burst into bloom in early or midwinter by placing them in a tight vessel and pouring in ether at the rate of one teaspoonful to each two gallons of space. Allow the plants to remain in the ether vapor for twenty-four hours, when they should be taken out and stood in vessels of water in a warm moist room. It is very necessary to have the air moist to keep the buds from drying out before they begin to grow. Potted plants of all kinds, both woody and herbaceous, may be similarly treated, provided the surface of the soil in the pots is dry, as moist or wet soil will absorb too much of the ether. Many herbaceous plants do not respond to the ether treatment and some are killed. The best treatment for forcing hardy herbaceous perennials is to lift them in the fall, subject them to a temperature a few degrees below freezing for twelve to twenty-four hours, and then place them in a mild hotbed. By mid- or late winter most plants may be forced with heat alone. There are perhaps thirty or forty different agents that have been successfully used for forcing plants.

Of just what value the rest-period is to the different plants and plant parts is not known. It is highly probable, however, that the period of inactivity is utilized for the conservation of energy through the special work the enzymes are able to perform while the growing parts are dormant, that they could not do in the presence of active growth.

The cause of the rest-period and the specific effects of the rest-period-breaking agents on the plant organism have been carefully studied only in connection with woody plants. It has been thought that the rest-period in trees sets in on account of the inhibition of enzyme activity due to an over-accumulation of the products of their work. The early phases of the rest-period occur and are passed through while the plants are in full leaf and often while some of the parts are still making active growth. The parts to enter the resting state first, in fruit-trees particularly, are the so-called spurs which are short lateral outgrowths usually arising from wood that grew the previous year. These cease growing early in the season, probably because of imperfect sap circulation. The crude sap from the roots rises most rapidly in those branches and twigs that are nearest in a straight line upward from the ground. It would appear then that the spurs, on account of their position, are deprived of water very early in the growing season. Assimilation goes forward rapidly in those parts that first approach a state of maturity. With a decreased water-supply and a greatly increased deposition or accumulation of carbohydrates, enzyme activity is soon checked if not entirely inhibited. As the season advances, the approach of cool weather may hasten the period of dormancy, but the trees would go dormant just the same, although later, in a mild climate or under glass. It is thus seen that the rest-period of trees is gradual, beginning in the spurs and gradually involving all of the branches.

Briefly, the rest-period perhaps begins to set in on account of the inhibition of the enzymes by the over- accumulation of the products of their work. This is the early rest of mid- or late summer. In the fall, excess supplies of carbohydrates continue to be accumulated and, with trees growing in the open, the further inhibition of the enzymes is actively aided by the approach of cool weather. These factors acting together bring about the main or middle state of rest. However, since dormant trees are never completely at rest, respiration continuing all the time, and doubtless enzyme activity too, the over-accumulation of carbohydrates is gradually reduced. Toward the last of this period occurs the after-rest, which, as the enzymes become more and more active, gives place to the beginning of growth.

Research has shown that diastatic, proteolytic, fat- splitting, and oxidizing enzymes all play a prominent part in bringing about, as well as ending, the rest- period in woody plants. Any forces or agents that will reduce the activity of these ferments will be the means of causing the rest-period to set in and, similarly, any agent that will stimulate the enzymes into activity will be the means of bringing the rest-period to an end.
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