70 GROWTH OF PLANTS 



growth, as shown by Pope and Brown. If seeds of all cultivated plants 

 failed to have a dormant period as in this illustration, man and domestic 

 animals would be without any food in the form of dry seeds and grains, 

 which form a large proportion of the food of both. Such a failure of dor- 

 mancy would also bring about a great disturbance in plant propagation 

 by making difficult or eliminating propagation by seeds. 



It is a common experience that during rainy weather grains germinate 

 in the shock. Different varieties and strains of cereals vary a good deal in 

 the length of dormancy of the grains i^, as. ^Iso the weather conditions 

 during ripening produce considerable variation in length of dormancy. 

 Length of dormancy of the grains ®- is one of the factors being studied in 

 breeding desirable new varieties of wheat. Grain dormancy will become 

 less and less significant as the combine more and more displaces the binder 

 in harvesting, for the grains seldom retain enough water to germinate 

 while still standing unharvested in the field. Recently the swather has 

 been introduced. It leaves the grain in a swath to dry and is followed 

 after drying by the pick-up combine. Here again a period of dormancy 

 will be important in preventing germination in the swath in case of rain. 



Seed Dormancy Inconveniences Man 



Dormancy in seeds causes man a great deal of inconvenience in at least 

 three ways. (1) Seeds of species of plants that are persistently dormant 

 need pretreatment to induce after-ripening, so that they will come up 

 promptly and all at the same time when planted. As we shall see later, it 

 takes a long tune to after-ripen some seeds. In some cases two or more 

 very different sets of conditions must be used for after-ripening, and in 

 others two long-time, low-temperature treatments must be applied to 

 after-ripen the seed and later to after-ripen the epicotyl. Until one knows 

 the conditions necessary for after-ripening any particular sort of dormant 

 seed, he may not be able to produce any seedlings; and after he knows 

 how, he may have a two-, three-, or four-phase job in producing seedlings. 

 These difficulties are met in their most complicated form when one under- 

 takes to grow certain wild flowers. 



(2) We have seen that some weed seeds lie in the ground for 60 years, 

 dormant and still capable of producing plants. Fighting weeds is one of 

 the heavy tasks of both the farmer and gardener. It is probable that if 

 a piece of land were cultivated so thoroughly that no weed seeds matured, 

 and weed seeds were prevented from coming in from outside, the land 

 could be freed of all weeds far short of 60 years, because cultivation stirs 

 up the dormant seeds and throws them out of dormancy. 



(3) In farm and garden operations it is customary to test seeds in 

 advance of planting to be sure that only viable and vigorous seeds are 

 sowed. Even winter cereals are sometimes sufficiently dormant to give 

 trouble in testing previous to fall sowing. Harrington '^^ found that dor- 

 mant cereal grains could be hastened in germination by various means 



