45^ Bulletin 279. 



It is a matter of common observation that after timothy has reached 

 its maximum development, which under field conditions is in the second 

 year, there is a gradual decline or "running out." "While conditions 

 under which this experiment was conducted are hardly comparable 

 with ordinary field conditions, owing to the fact that the plants received 

 thorough cultivation throughout the entire period, the possibility is 

 suggested that changes in the mean values and possibly in the distri- 

 bution itself may be due in part, though not entirely, to the age of 

 the plants. This question, however, cannot be definitely settled until 

 data are obtained for a longer period of years. 



It will be seen by an inspection of the frequency polygons that, with 

 a single exception, the curves are either bimodal or multimodal. The 

 cause of multimodality has generally been attributed to the presence 

 of two or more types. Pearson (1902) has pointed out that this is 

 not necessarily true; that in the case of flowers it may indicate hetero- 

 geneity due to environment or to errors introduced in the collection of 

 the material. 



In regard to the population under discussion, the question may 

 arise as to whether the material is homogeneous since the seed from 

 which the plants were gro^^^l originally came from different sources. 

 By reference to Fig. 144, in which are shown for comparison curves 

 for height for the three years for the total population and for the 

 plants grown on the check plats, it will be seen ^-hat there is a 

 sufficiently high degree of conformity to indicate that heterogeneity has 

 not been introduced from this source. We must look then to some 

 other source for the cause. 



As has already been stated, the positions of the modes in the distribu- 

 tions for earliness and duration of bloom are dependent to a large extent 

 upon climatic conditions during the blooming period. We may also 

 expect the same conditions, extending over a longer period of time, 

 to exert similar influences on height and yield, though to a much less 

 degree. In the case of these characters, variation in soil may have been 

 a factor in bringing about multimodality. This, however, does not 

 account for the changes from year to year in modal position except 

 as different parts of the field may have been caused to vary in different 

 degrees as the result of changed climatic conditions. 



Experiments* have showm that when the same crop has been grown 

 for a period of years on a series of plats which have received the same 

 treatment, the relative productivity invariably changes from year to 

 year, thus indicating that different areas do not respond in the same 



♦Unpublished results, Nebraska Experiment Station. 



