66 
IOWA ACADEMY OF SCIENCES. 
this number. This suggests the question, whether or not 
this indicates a shifting of the species from a lower to a 
higher number of ray flowers. It was also noticed 
that the plants of Anthemis growing in rich soil near barns 
had heads of greater variability in the number of ray flow- 
ers, than those in poorer soil; showing the theory to be 
true in this case, that more food causes greater variability. 
The ray flowers of 1,160 heads of Yarrow {Achillea mille- 
folium L.) were next counted, in four different localities, 
three localities in the Bitter Root mountains of western 
Montana about fifteen miles apart, and one at Ames, Iowa, 
just south of the College campus. In Achillea, heads con- 
taining five ray flowers greatly predominated, the entire 
variation being from two to seven. With one slight ex- 
ception it will be noticed that the curves are steeper on 
the side above five than below that number. The curves 
for the three localities of the Bitter Root mountains are 
very similar in form and amount of variation. The ray 
flowers of these localities were much larger and purer white 
in color than those of Iowa. 
In addition to the Mayweed and Yarrow, heads of three 
other species in the Bitter Root mountains were counted; 
Senecio triangularis Hook, Aster adscendens Lindl., and 
Erigeron salsuginosus Gray. In the curves for Aster adscen- 
dens it will be noticed that in nearly every case the line falls 
for odd numbers of ray flowers and rises again for even 
numbers. In this plant, as also in Erigeron salsuginosus, 
there seems to be no predominating number of rays as in 
Anthemis and Achillea. 
The time element has not been taken into consideration 
in this study except in the case of Anthemis for Ames, 
where tables and curves are compared as between August 
and September. This time element according to Schull 
(1904) is a very important factor in variation. He says, in 
speaking of Asters, “ There is a continuous and more or 
less regular change in the variable characters from day to 
day throughout the season.’’ 
