346 SEMI-CENTENNIAL OF TORREY BOTANICAL CLUB 
of +6.6. The 221 heads from plants grown in the garden had 
an average ray-flower number of 37 and a standard deviation of 
+9.2. These differences observed were quite evidently due to 
the effect of nutrition. A point that is of interest and that Weisse 
does not call attention to is the smaller variability of the starved 
plants. Weisse states further that certain plants that were heavily 
manured had as many as 82 ray-flowers in a head and a maximum 
at 55; while heads with less than 30 ray-flowers usually were found 
on plants which were checked and retarded in growth by the 
crowding of neighboring plants. Weisse combines the data ob- 
tained from the two cultures, the one on sand and the other 
on garden soil, and points out that the resulting bimodal curve, 
having a maximum at 21 and one at 34, is a result of different 
nutritional conditions under which the population was grown. 
He believes such conditions might readily occur in nature and 
that bimodal curves that have been ascribed to the mixture of 
two races are often due to differences in nutrition occurring in 
nature. 
MacLeod ('99) published some observations on Centaurea 
atropurpurea which show the effect of nutrition on the number of 
ray- and disk-flowers. Here again there is a lower flower number 
under conditions of poor nutrition, but the number of disk- 
flowers seems to be more affected than that of ray-flowers. Mac- 
Leod gives only the averages for each culture, so that the vari- 
abilities of the cultures cannot be determined. 
De Vries (от) repeatedly states that favorable environmental 
conditions, such as optimum water-supply and manuring, tend to 
increase the size of organs (fruits of Oenothera) and the number of 
parts, as the number of rays in the umbels of Umbelliferae or the 
ray-flowers in the heads of Compositae. 
Danforth ('o8) comparing ray-flowers of the daisy growing 
in a well-drained situation with those in a drier situation, reports 
a lower mean and less variability for the latter. Koriba (708) 
made successive collections of heads of Arnica unalaschensis 
(also some other Composites) from two different localities, one 
from a valley and the other from the slope of a mountain. Those 
on the slopesweregrowing under the least favorable conditions and 
gave uniformly lower values. 
