24 PRINCIPLES OF PLANT CULTURE 



fort able living temperature. The seeds should be fre- 

 quently examined, and may be removed as they sprout, 

 when by substracting the number that failed to sprout 

 from the number put in, the per cent of viability may be 

 readily computed. The cloths should be placed in boiling 

 water a few minutes before using them for a second test, 

 to destroy any mold with which they may have become in- 

 fected. 



39. The time required for germination varies greatly 

 in different kinds of seeds. In lettuce seed, the tiny 

 white shoot often breaks through the seed-case within 

 twenty-four hours from planting, while celery seed re- 

 quires several days to germinate to this extent. The seeds 

 of many plants will not germinate the same season they 

 are formed, even if planted under the most favorable 

 conditions (162). 



Individual seeds of the same kind and of the same sample 

 often vary greatly in the time required for germination. 

 Some seeds of red clover will germinate promptly, while 

 others from the same lot under the same conditions will 

 remain " hard " and dormant for months or even years 

 before germinating. Even in seeds that germinate 

 soonest, as lettuce and radish, some individuals will not 

 germinate until several days after the larger number have 

 sprouted. Seeds of tobacco and purslane sometimes con- 

 tinue to germinate through several successive seasons. 

 The reasons for these variations are not known. 



THE PLANTLET 



By watching the germination of seeds, we may learn 

 some interesting facts. Viable seeds will usually germinate 

 freely on the surface of well-moistened soil or sand, if we 



