1917] 
GATES—THE GENUS TRILLIUM 69 
Pursh with the statement that it is distinguished from 
T. ovatum Pursh by its obovate, white, or rose-colored petals. 
The relationships of the species of Trillium seem complex 
and confusing because of the numerous cross-relationships 
which appear. But the difficulties of interpretation are, I 
believe, considerably clarified when we realize (1) that par- 
ticular elements of the germ-plasm vary independently of 
each other and that (2) the variation of a single germinal ele- 
ment may affect the external morphology in various parts of 
the organism. The application of these two principles helps 
to clear up what may otherwise become a hazy maze of rela- 
tionships. This is particularly true of large genera, in which 
the number and diversity of species greatly exceed that of 
Trillium. 
The genus is naturally divided into two groups having 
respectively pedunculate or sessile flowers. Whether the ses- 
sile-flowered gave rise to the pedunculate group or vice versa 
is difficult to say, but it appears probable that the transition 
from one condition to the other occurred but once (presum- 
ably through a mutation) since there appear to be no cross- 
relationships from one group to the other. I mean by this 
that the members of each group may be considered to be de- 
scended from one ancestor, and e.g., none of the characters 
of the pedunculate group are such as might have been derived 
from particular members of the other group. On the con- 
trary, within each group parallel mutations have probably 
taken place, as in the dwarf origin of T. rivale and Т. mvale. 
VARIATION OF TRILLIUM 
The genus Trillium has long been known to botanists and 
horticulturists for its variability. Nearly all parts of the 
plant vary, particularly the shape of leaves and petals, and 
the color of the petals. On the other hand, the size of the 
plant and the relative length of filaments and anthers is 
usually constant within certain limits, and the latter is fre- 
quently used as a specific differential, though it too is sub- 
ject to some variation. The number of members in the whorls 
