io6 YEARBOOK OF AGRICULTURE 1961 anisms within the seed. They must be removed before the germination can occur. The terms "rest" and "rest period" Afterrípeníng, Rest Period, also have been used to describe seeds, and buds that are inactive because of these internal blocks. and Dormancy One should be aware of this confu- sion in terminology when he reads, about seeds and germination, but he BRUCE M. POLLOCK AND need not think the dual terminology is VIVIAN KEARNS TOOLE an exercise in scientific semantics. Our scientific terminology has to be precise. Often in writings it is difficult to tell SEED MATURITY and seed germination whether the investigator was working follow in direct sequence in the life of a with seeds that were "dormant" be- plant, but normally they are separated cause they were dry or cold, or with in time and space. seeds that were "dormant" because of The interval may be a few hours or blocks. It is simple to write on paper a many years. It may be a few inches or definition of "dormancy" or "rest," thousands of miles. but it is more difficult to apply the The function of the seed is to carry definitions to a seed or a seed lot. its embryo plant through the hazards An example illustrates the problem. of time and space to a time and place The seed of silver maple (Acer sacchari- where the new plant can grow, flower, num) can germinate as soon as it falls and in its turn produce seeds. from the parent tree. You are familiar It is of advantage to the seed to re- with the appearance of seedlings in main in an inactive condition until it early summer under the trees. This reaches a favorable time and place for seed is "inactive dormant" at the time, germination: A young plant is vulnera- of maturity, but it is not "resting" or ble to lack of water and extremes of "blocked dormant," because it germi- heat and cold—hazards that the em- nates as soon as it reaches the water bryo plant within the seed is adapted supply in the soil. to withstand. In the nongrowing con- At the other extreme, the seed of the dition, the water content of the tissue is apple tree {Pyrus mains) is "resting" or relatively low, the protoplasm of the "blocked dormant" at maturity. It will cells is protected from damage, and not grow even under good conditions the metabolic rate is low. Thus the for germination until it has undergone seed can survive on its nutrient re- changes, known as afterripening, to serves for a long period. remove the germination blocks. If one defines these two extremes as DELAYED GERMINATION is not acci- "dormant" and "resting" and then at- dental. It is the result of physiological tempts to apply these definitions to mechanisms that keep the seed in a other seeds, difficulties arise. Take the nongerminating state. lettuce seed {Lactuca sativa). It germi- The term ''dormancy" is used to de- nates promptly in total darkness if it is scribe two inactive conditions. One re- planted in moist soil at 57° F. It does sults from an unfavorable environ- not contain a germination block. If the ment. The other is due to internally same seed is planted at 84°, however, imposed blocks. For example, germi- it remains inactive. If, following 84° nation may be delayed by inadequate for a few days, the temperature drops water supply or unfavorable tempera- to 57^, the seed still cannot germinate. ture. In some seeds, however, germina- Exposure of the imbibed seed to the tion is prevented by blocking mech- high temperature induced the forma- AFTERRÍPENING, REST PERIOD, AND DORMANCY ÏO7 tion of a, block that did not exist pre- The result of the interaction of ge- viously. This block may be removed by netic and environmental factors is ex- an exposure to red light. treme variability in the rate at which Can one then easily apply a rigid germination of different kinds of seeds definition of dormancy or rest to de- and different seeds of one kind begins. scribe a lettuce or similar seed? The germination of seeds has a con- Obviously not, without qualifying tinuous range from prompt growth the definition by listing carefully the over a wide range of environmental conditions under which germination conditions to sluggish growth over a was attempted. The variety of the let- narrow range of environmental condi- tuce and the previous history of the tions. Most farmers, scientific workers, seed also are important—not all vari- seedsmen, and gardeners recognize eties behave in the way we described, this variability. It is the variability of nor do all lots of one variety. Nature. A species survives because of blocks GERMINATION BLOCKS are relative, that delay germination. They tend to not absolute. spread germination over a period of Close examination of some seeds, years. One unfavorable growing season such as the sour cherry {Prunus cerasus), does not obliterate a species. has disclosed that growth is not com- Consider weeds. All farmers and pletely stopped, even in a blocked seed gardeners see how weeds emerge in soil at a low temperature. Cells of the root clean cultivated for many years. Some and shoot can divide, and the whole may have been introduced recently by embryonic axis grows slightly at a time animal carriers or the wind, but most when the seed cannot germinate even were already present in the soil from under good conditions. previous years. These seeds had ger- This observation and the fact that mination blocks that previously pre- something obviously does occur during vented germination. aftcrripening to permit subsequent ger- Blocked seeds are more obvious and mination show that the blocks are only more extreme in wild plants than in relative. most of our cultivated forms. One in- A blocked seed is like an automobile herited difference between seeds is the with its motor running at idling speed ability to develop germination blocks. but with the gears disengaged—there Through the years, man has tended is no motion. to select seeds that give relatively Germination blocks are variable. prompt germination. The result is that Gardeners and farmers know that all some of our cultivated plants cannot the viable seeds they plant do not ger- survive without man to protect the minate. The proportion of those that seeds by proper storage until a favor- do germinate varies with conditions of able season for germination. In a sense, germination. A major reason for the man has substituted himself ifor the variability is that all seeds are not germination blocks that probably were genetically identical. present in the ancestors of our common Conditions required for germina- cultivated plants. tion are the expression of the seed's The term '*block" is a convenient heredity as influenced by environment name for a mechanism that restricts during seed formation, maturity, and germination. Blocks act through a germination. number of different physiological We do not know, even for a single mechanisms. Some blocks are simple kind of seed, exactly what are the and well understood. Others are com- critical environmental factors, when plex and almost completely unknown. they act, or how they may be con- The end result is the same in all trolled experimentally or in commer- instances: The seed is held in a non- cial practice. growing condition. io8 YEARBOOK OF AGRICULTURE 1961 Germination is the Road a Seed Must Travel Before Becoming a Seedling Blocks placed by environmental factors must be removed or Ij,l¡lliiiii'iii«"" i||]i|j||ji|||||||!F'^^/'" liigh temperature Some seeds travel this road iiiiiiilr ^ ^ can throw the without hindrance, seed back down provided they have adequate water and the oxygen in the air. Low temperature supplies the "push" to climb over. THE MOST complete block to germi- ized. Many blocks affect the whole nation is in seeds that have secdcoats embryo. Seeds of the apple are blocked impermeable to water. These hard and germinate only after undergoing a seeds arc common in the waterlily, period of afterripening while moist at mallow (cotton, okra), and legume a temperature around 40°. Imbibed (beans, clovers) families. seeds that have been chilled for 2 to During ripening and drying of red 3 months germinate promptly and pro- and white clover seeds ( Trifolium pra- duce normal seedlings. tense and T. repens)^ the seedcoat be- The embryo in seed of the tree peony comes impermeable to water when the (Paconia stiffruiicosa) has a root that will moisture content of the seed has grow under normal germinating con- reached a low level. The impermeable ditions. The shoot, howev^er, is blocked seedcoat has a fissure along the groove and requires exposure to low tempera- of the hilum, which functions as a ture to permit growth. The chilling is hygroscopic valve. When the seeds are not effective until after the root has surrounded by dry air, the fissure grown. Seeds of the tree peony ripen in opens and permits water vapor to late summer, and the winter tempera- escape. The fissure closes in moist air. ture is too low to permit germination. Thus the seeds can dry further through The root therefore cannot grow until outward difíiision of water vapor, spring. The required chilling is re- while reentry of water is prevented. ceived during the next winter, and the Such seeds remain impermeable to shoot emerges above ground the sec- water until the seedcoat is somehow ond spring, 18 months after the seed broken. If water cannot enter, the first matures. steps toward germina don are pre- The seed of Trillium grandiflorum, a vented.