REST-PERIOD 



REST-PERIOD 



2919 



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 green- 

 house. 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 dis- 

 tinct 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 dis- 

 tinct 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 



Elace, it does not necessarily prevent the rest-period 

 om coming to an end and thus leaving the trees ready 

 to grow as soon as surrounding conditions are favora- 

 ble. 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 w r ater in a 

 warm moist room. It is very necessary to have the air 



185 



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 herba- 

 ceous 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 organ- 

 ism 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 circu- 

 lation. 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 accumu- 

 lated 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 mam 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 car- 

 bohydrates 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 begin- 

 ning 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. (See 

 Research Bulletins Nos. 1, 15, 16, 17, 21, Missouri 

 Agricultural Experiment Station.) \y. L. HOWARD. 



