132 GROWTH OF PLANTS 



Summary 



We have seen the advantages to plants of delayed and time-distributed 

 germination of seeds in furthering the persistence of species, and the 

 advantages to man of at least a temporary dormant period in seeds in 

 furnishing him seeds for food and for propagation of plants. We have seen 

 how delayed germination presents difficulties in propagation and in fight- 

 ing weeds on the farm and in the garden. We have seen that delayed ger- 

 mination of seeds in nature is secured by a variety of means: by hard coats 

 that allow no water to enter until decay or other factors corrode the coats; 

 by requiring light or darkness for germination so that the first seeds are 

 prevented from germinating if they are covered Avith soil and the latter 

 if they are not; by coats, very thin in the upper seed of the cocklebur, 

 that reduce the oxygen supply below that needed for germmation; by 

 dormancy in embryos that gives them sluggishness in early growth and 

 manifests itself by dwarfishness in the later growth of the seedling, unless 

 previously given a low-temperature period in the soil which after-ripens 

 the embryos and perhaps aids in coat destruction; by dormancy of the 

 epicotyl that is after-ripened by a period of low-temperature exposure 

 only after the root has started; by demanding a high-temperature period 

 in the soil for decomposition of coats by soil agents, especially organisms, 

 followed by a low-temperature period for after-ripening the embryos; by 

 demanding a low-temperature period to after-ripen the seed so that the 

 root can grow, followed by a high-temperature period for root growth, and 

 in some seeds for epicotyl elongation and rupture of cotyledonary sheath, 

 followed again by a low-temperature period for epicotyl after-ripenmg, 

 and a high-temperature period for growth of the epicotyl. 



At every phase of this study we have seen the great significance of seed 

 and fruit coats, non-livmg or in the main non-living, in securing this delay. 

 We have seen that two big factors in after-ripening temperate zone seeds 

 and preparing them for germination are periods of low-temperature moist 

 exposure and periods of dry storage; and hardly less important is the 

 coat-dismtegrating action of soil agents, especially organisms which act 

 best at gro^^•ing temperatures. We have discussed various methods for 

 quick vitality tests of seeds that require long periods for germination, the 

 best of which seems to be isolating or partially isolating the embryos and 

 placing them on moist filter paper. Fmally, the mam problems in delayed 

 germination and dormancy of seeds are yet to be solved. The work to 

 date has thrown much light on the way dormant seeds of various types 

 behave m nature; it has put into the hands of commercial and amateur 

 growers methods of growing many kinds of seeds that could not be grown 

 before; and it has done much to define the big physiological and biochemi- 

 cal problems still to be solved. No doubt further research A\ill shorten the 

 time needed to get complete germination of the seeds that are the most 

 difficult to handle. 



