Germination of Forest Trees 19 



of conclusions based upon one species of plant apparently may 

 or may not hold for others. Shull's investigations (14, 15, 18) 

 are based mostly on Xanthium seeds. In his experiments he finds 

 no evidences of the diffusion of oxygen through an absolutely dry 

 seed coat. This is significant in that it shows an important role 

 of water in preceding oxygen in penetrating seed coats. Ex- 

 perimenting with Crataegus mollis Davis and Rose (16) find that 

 seeds treated dry or those placed under water do not go through 

 the process of after-ripening. Here again is evidence that both 

 water and oxygen are necessary. These investigators, working 

 on the effects of temperature upon the period of after-ripening, 

 conclude that favorable moisture conditions and temperature con- 

 ditions shorten the period. Atwood (19) confirms almost all of 

 the conclusions drawn by Eckerson although working on Avena 

 fatua. Crocker and Davis (20, 21) worked with water plants 

 and their results totally different than those described for land 

 plants need not be given. 



Unfortunately these conclusions are not based upon forest tree 

 seeds. Such investigations have not been undertaken. This 

 phenomenon will probably explain many of the cases of delayed 

 germination which are well known to foresters. It is reasonable 

 to assume that the conclusions based on Crataegus would also 

 hold for such fatty seeds like the birches, spruces, hard maples, 

 etc. It is also reasonable to suppose that most tree seeds pass 

 through this period of after-ripening during the winter months ; 

 if this is true it explains why it is often impossible to germinate 

 certain tree seeds immediately after they have been gathered. 



Method of Attacking the Problem 



There are two general methods of determining the causes in- 

 fluencing the behavior of seeds or plants growing under natural 

 conditions. These are the observational and experimental 

 methods. In the observational method we observe the kind of 

 vegetation produced in response to a certain complex of physical 

 factors and seek to find constant relations of one to the other in 

 order to draw conclusions. In the experimental method we may 



