I04 Journal of Agricultural Research voi. iii. no. » 



required for the proper development and "filling" of the seed after the 

 flower stalks are produced. Climatic conditions usually become rather 

 severe in the latter part of August, however, and flowers fertilized after 

 the first week in August have not sufficient time to develop and mature 

 their seed. Plants with low vitality are likely to produce flower stalks 

 so late that the seed has no time to mature. A few species which repro- 

 duce vegetatively seem to have an inherent tendency toward irregular 

 and late flower-stalk production, and such delay should not be confused 

 with that due to low plant vigor. Of these species, pine-grass and yar- 

 row, or wild tansy, require the longest time to produce flower stalks. 

 On the lowest border of the Transition zone these species actually send up 

 their flower stalks as early as July lo, and on the higher areas continue 

 until inclement weather — about September 15 — stops their activities. 



PERIOD OF SEED MATURITY 



Under the most favorable conditions the time required for the devel- 

 opment of seed is about 25 days. The period varies slightly with the 

 length of the growing season and is longest at the lower elevations. It 

 also varies among different species, and the time given for ideal condi- 

 tions should be considered as approximate. 



Naturally the same factors which promote or retard the growth- 

 resumption period and flower-stalk production also determine the time 

 of seed maturity, though the last named fluctuates least, since toward 

 the end of the growing season physical conditions, especially soil mois- 

 ture and air and soil temperature, tend to become uniform throughout 

 the range. 



The length of the seed-maturing period varies widely from vear to 

 year as the result of the presence or absence of killing autumn frosts. 

 In 1907 practically no seed of the more valuable perennial herbaceous 

 species were matured in the Hudsonian zone until August 20, while in 

 1908 the ripening period came at least five days earlier. From this it 

 might appear that the seeds were matured more slov.Iy in 1908 than in 

 1907. This was not the case, however, the apparent difference being 

 explained by the fact that in 1907 weather conditions after the first 

 week in September were so unfavorable that the seeds which had not 

 matured by that time were destroyed. In 1908, on the other hand, 

 heavy frosts and low temperatures did not appear until September 20, 

 and practically all the seed matured. 



In 1908 seeds of mountain bunch-grass and other important species 

 began to ripen on August 15, and by August 25 fully two-thirds of the 

 crop had matured. After September 5 practically no immature seed 

 were to be found, even on the cool north slopes at the higher altitudinal 

 limits of the species. The secondary grazing plants, almost without 

 exception, had matured their entire seed crop by September 10. 



