128 GROWTH OF PLANTS 



will germinate at lower and lower temperatures, until after several months 

 of dry storage they germinate, although slowly, at 10° C (50° F). We 

 have already mentioned the fact that the intact upper seed of the cocklebur 

 requires about 33° C (91° F) for prompt and complete germination, and 

 that an excised embryo has a minimum germination temperature of 18° C 

 (64° F) . It is probable that high-temperature requirements for germina- 

 tion determine the late appearance of crab grass, Panicum sanguinale,^^^ 

 Portulaca oleracea, and other weeds ^^ rather late in the gro\ving season in 

 this latitude. From what has just been said it is evident that many seeds 

 remain dormant in a germinator because the temperature is too low or too 

 high or lacks variation. 



Quick Vitality Tests for Dormant Seeds 



Seeds of farm and garden plants are tested for viability before they are 

 put on the market. This is relatively easy to do by germination, for some 

 of these seeds will germinate fully within four or five days under the proper 

 conditions; and even the slower ones, like the blue grasses, will germinate 

 within 28 days under the good conditions provided in the seed-testing 

 laboratories. The testing of farm and garden seeds for viability and purity 

 has returned many-fold the expense of maintaining thoroughly equipped 

 state, government, and private laboratories. No such adequate methods 

 and equipment have been available for comparable tests of forest and 

 horticultural seeds that show great delays. For many of them, ordinary 

 germination tests are not available, because it takes so long to after-ripen 

 and germinate the sample that there is not enough time left to after-ripen 

 the main part of the seeds for early spring planting. In two ways it is 

 more important to be sure of high viability in these seeds than it is for 

 farm and garden seeds; considerable effort and expense must be put on 

 stratification, and failure of a crop due to poor seeds cannot be recouped 

 to any degree the same year by replanting. On the other hand, the total 

 crop value from farm and garden seeds is many times that from dormant 

 forest and horticultural seeds. 



Probably due to its work on dormant seeds, the Institute in the early 

 thirties began to receive samples of dormant seeds from nurseries for test- 

 ing. If the samples came in as late as January it was impossible to stratify 

 the seeds and later germinate them so that the viability tests could be 

 furnished to the grower in time for him to stratify the seeds and have 

 them ready for early spring planting. Flemion had been excising embryos 

 from dormant seeds and growing them on moist filter papers to determine 

 whether the embryos were the seat of dormancy and how the dormancy 

 of the embryo expressed itself in the later gro\vth of the seedling, as de- 

 scribed in a previous section. This led to her method of quick vitality 

 tests for dormant seeds, which consists of growing the excised or partially 

 excised embryo at room temperature on moist filter paper. 



