6 H. O. HALVORSON 



activity is from the seeds themselves or from a slow metabolism 

 carried on by fungi, found on the seeds, which increase their 

 rate of respiration when the moisture content is raised above the 

 very minimum levels^"^. Certainly when such seeds are kept in 

 environments where the moisture content is maintained below 

 the critical levels of from 6 — 10% (depending upon the type 

 and species), there is very little, if any, metabolism going on 

 within the embryo of the seed itself. If this is not a true crypto- 

 biotic state, it certainly is not far removed from one. 



The seeds of many weeds, flowering plants, vegetable plants, 

 fruits, and legumes usually require special trigger mechanisms 

 to germinate, in addition to the imbibition of water. In these, 

 there may be a period of after-ripening or other special require- 

 ments necessary. Some of these special requirements are the 

 removal of inhibitors, the absorbtion of inducers, the breaking 

 or damaging of hard seed coats, and the need for exposure to 

 light to permit photoinduction. Whatever the mechanism — and 

 it is often different for different species of seeds— many may 

 remain dormant for many months or even years in an environ- 

 ment where one would normally expect them to germinate^i. 

 Those of you who have lived in the northern half of the United 

 States of America, or in other areas that have similar climate, 

 will know what I mean when I remind you of the difficulty of 

 getting rid of crab grass in your lawn. Year after year one can 

 remove new shoots in order to prevent the formation of any 

 new seeds, and yet have the weed continue to come up year after 

 year. Here a photoactivation is undoubtedly involved, but the 

 biochemical mechanism that is triggered by this means is 

 certainly not understood. In other cases, contact with oxygen 

 through an exposure to normal atmosphere is necessary. Thus, 

 many farmers have experienced the phenomenon of seeing new 

 types of weeds appear in fields following deep plowing, weeds 

 that had not been seen in those fields for many years. Here the 

 deep plowing presumably brought deeply buried seeds to the 

 surface where they could become exposed to oxygen and 

 perhaps light, or both. 



