GERMINATION IxN THE C.REEXIIOrSE AND XI-RSEKV 321 



TEMPERATURE 



The efifect of temperature on germination in general is easily de- 

 scribed; but when an attempt is made to determine the specific effect 

 of certain maxima, minima, and means on a certain species, difficulties 

 are at once encountered, and it is clear that only laboratory experi- 

 ments with exactly controlled conditions can really solve these ques- 

 tions. However, we can, with some certainty, go at the problem b\- 

 using only the admittedly rough data at present available. 



It is a common experience in greenhouse testing to find germination 

 slow during cloudy weather, but rapid after clear, sunshiny days. 

 Boerker and others seem to have shown that light itself not only does 

 not increase either the speed or completeness of germination, but that 

 germination is better in the absence of light. Evidently temperature is 

 the controlling factor. 



Aiean temperature, while a convenient expression, does not apply 

 strictly to a study of germination. An examination of greenhouse ger- 

 mination shows that after two days with the same mean the germina- 

 tion increases with increase between extremes. Thus if the tempera- 

 ture is constant at 60° F. on one day and varies from 50° to 70° F., 

 with a mean of 60°, on another, the germination is highest following 

 the latter. 



The practice in testing seed in greenhouse is to hold air temperatures 

 to a minimum of 50° F., in which case soil temperature will reach an 

 average minimum of 55°, unless the air minimum is prolonged, when 

 the two will be equal. 



An examination of thermograph sheets and germination records in- 

 dicates that minimum air temperatures down to 32° F., the freezing 

 point, do not retard germitlation, and in fact appear to accelerate it 

 unless prolonged. 



Seed in the nurseries is, of course, subjected to a wider variation in 

 temperature than in the greenhouse, but it is difficult to say that this 

 results in slower germination. It is probable, though not proved, that 

 the accumulated temperatures above 40° F. control genniiiation. 



VITALITY OF SEED 



An important phase of forestation work is the age of the seed used. 

 Many (|ucsti()ns suggest themselves: How long will seed retain its 

 vitality? What deterioration is there from year to year? Is it safe to 

 use a test of one vear to apply to sowing a year or two later? Should 

 several years' supply be collected in an exceptionally good seed year, or 

 should an atlcini)! be made to secure fresh seed every year? 



