GERMINATION 933 



seed in Xanthium may be made to germinate early by exposing it to 

 higii temperatures (32" C. to 34" C), probably because the absorp- 

 tion of oxygen and water is thus facilitated; if the testa is removed 

 from the upper seed, it germinates as readily as does the lower seed, 

 and at as low a temperature (22° C. to 24° C). 



It has been seen elsewhere that the testa is chiefly responsible for 

 prolonged vitality in seeds, and it is here seen to be responsible for most 

 cases of delayed germination. Longevity obviously is advantageous, 

 and to a certain extent delayed germination also may be advantageous, 

 especially in annuals. There is reason to believe, however, that some 

 seeds, especially among xerophytes, are overprotected, the pericarp or 

 testa being so impermeable that death is likely to occur before the water 

 and the oxygen necessary for germination have an opportunity to enter. 



The relation of external factors to germination . — It is a matter of com- 

 mon observation that water and moderately high temperatures are nec- 

 essary for the germination of seeds, and very simple experiments show 

 the equal necessity of oxygen. Nor are any one or two of these factors 

 sufficient. Seeds on a dry shelf never germinate, in spite of favorable 

 temperatures, nor will they germinate in an atmosphere without oxygen 

 or at low temperatures, whatever the other conditions. Oxygen and 

 water appear to be directly necessary for germination,' the oxygen being 

 necessary to combine with the accumulated foods, thus making energy 

 for further activity available, and the water being necessary to give the 

 requisite dilution to the cell contents to permit of growth. High tem- 

 peratures, however, probably are of value only as they facilitate the 

 absorption of water and of oxygen. 



The vigorous respiration of developing seedlings is in striking contrast to the 

 weak respiration of seeds, germination soon ceasing in closed chambers from lack 

 of oxygen. The favoring influence of high temperatures is well shown in the date, 

 whose seeds germinate in a few days in a hot greenhouse, otherwise requiring weeks 

 or even months. Until recently it had been supposed that germination takes place 

 equally well in light and in darkness. Probably it is true that many seeds are indif- 

 ferent to the presence or absence of light, but a few seeds require light for germina- 

 tion (as in Viscum and in several species of Rhododendron) ; a number of seeds ger- 

 minate better in light than in darkness (as in Poa pratensis and Veronica peregrina). 

 On the other hand, there are some seeds whose germination is retarded by light 

 (as in Phacelia tanacetifolia). In certain mycophytes and parasites, as previously 

 seen, there is a fourth condition necessary for germination, namely, contact with a 



' In a few cases, as in rice and in the water hyacinth, no oxygen is required for ger- 

 mination. 



