aie ‘ pains ss 
vincing evidence that J] dealt all the time with one and the same 
si species, varying according to its different stations of life. 
The gradual sinking of the water level during dry summers 
has a tendency to produce an abundance of var. dimorpha and 
var. monomorpha, just as the gradual rise of the water level in 
wet summers causes an increase in the numbers of var. folymor- 
pha and var. cuneata. 
Typical specimens of var. folymorpha and var. cuneata, 
with the gradual lowering of the water level and the disappear- 
ance of the water, are transformed into typical specimens of var. 
dimorpha and var. monomorpha. Here the same process is ob- 
served as when Alisma arcuatum angustissimum is changed to 
A. arcuatum lanceolatum (vide J. Lunell, The Genus Alisma in 
North Dakota, in Botanical Gazette, 43: 213. 1907). But with 
the rise of the water, the reverse does not occur more than to a 
certain extent. Var. amorpha and var. monomorpha then be- 
come more slender, but the leaves are not changed to phyllodes. 
The analogy between Alisma arcuatum and Sagittaria arifolia 
in their habits and through their whole processes of life is very 
striking and wonderful indeed. 
A rise and a fall in the water level, emersion or submersion, 
or rooting in the terra firma, wet or dry summers, etc., are fac- 
tors to be earnestly considered when an effort is made with a 
view of explaining the different formations of these most vari- 
able plants. 
My investigations warrant the following conclusions: 
1. When the plant is submersed in the water, or a few 
leaves and the scape are reaching the surface of the water, a pure 
var. cuneata will be the result. 
2. When the plant grows in terra firma or in veiy shallow 
water, allowing the leaves and the scape immediately to rise 
into the air, a pure var. monomorpha is the inevitable result. 
3. When part of the plant is emersed, and part subinersed, 
a formation with mixed characters is the result—(when the con- 
ditions are more terrestrial) var. dimorpha, or (when the condi- 
tions are more aquatic) var. polymorpha. 
