CONTINUING CAUSES OF SUCCESSION 209 



ginning. There is no dormant period and, therefore, no germination , 

 properly speaking. Some seeds, such as those of cottonwood and 

 willow, can remain dormant only a few days or, at most, a few weeks 

 and if they do not find conditions favorable for germination during 

 this time they permanently lose their ability to germinate. Some 

 seeds, such as those of cultivated oats, will germinate as soon as 

 produced, under favorable conditions, but will remain dormant for 

 months if artificially stored and then germinate readily when placed 

 under favorable conditions. Wild oats, on the other hand, normally 

 lie dormant until the following spring after their production no 

 matter what the conditions of their environment may be. Seeds 

 of certain mustards and plantains have been known to lie dormant 

 in the soil for twenty to forty years before finally germinating and 

 the reason why some farmers believe that under certain conditions 

 wheat will "turn to chess" is that the seeds of chess, or cheat, may 

 lie dormant in the soil for years and then, when conditions are favor- 

 able for chess but not for wheat, may germinate and grow in a field 

 where wheat was planted. 



The most common factors of the habitat that cause dormancy of 

 seeds are unfavorable temperature and lack of sufficient water. 

 There are other factors, however, such as enzymes, light, and certain 

 nitrogenous compounds, that may be important in some cases. 

 Some seeds, however, such as those of certain mints and legumes, 

 have seed coats that are so impermeable to water or to ox^'gen that 

 they can never germinate until the seed coat is made permeable 

 either bv natural deterioration or bv some means of artificial scari- 

 fication. The latter can usually be done very quickly by the use of 

 acids or by some mechanical means while the former may require a 

 period of several years. 



After a seed has germinated and the embryo has developed sufii- 

 ciently to free itself from the seed coat the plant is said to have 

 reached the seedling stage and this is likely to be the most critical 

 period in its life cycle. Seedling mortality is often exceedingly high. 

 Seedlings may be destroyed by insects, bu-ds, rodents, or larger ani- 

 mals; they may be destroyed by excessive competition; or they may 

 be killed by drought or by low temperatures. Often the entire crop 

 of seedlings of woody plants may be killed by desiccation and this 

 phenomenon may be repeated several years in succession. This is 

 attested to by the fact that where young trees are developing it is 

 often found that they are all of the same age or that only a few age 

 14 



