650 MINNESOTA BOTANICAL STUDIES. 
As the embryo departs from its primitive spherical form it is 
evident that its axis lies more or less directly across the ovular 
cavity. As the plumule develops superficially near one side of 
this flat expanse of tissue the remaining larger surface is to be 
considered as that of the young cotyledon. The cotyledon 
soon meets obstruction to its further growth in the horizontal 
plane and hence turns down into the ovular cavity. Its later 
bifurcation is undoubtedly the result of its having to adjust it- 
self to the cylindrical cavity in which it grows. That the 
shape of the cavity does to some extent affect the shape of the 
cotyledon seems to be evidenced by the fact that the cotyledon- 
ary lobes are at all times closely appressed to their enclosing in- 
vestments. The bifurcation extends along aline of mechanical 
stress. 
The mature embryo of Velwmbo can perhaps best be com- 
pared with those of the grasses, especially one having but a 
small amount of cotyledonary tissue below the junction of the 
plumule and cotyledon. The embryo of the wild rice, Zezanza 
aquatica, as figured by Kennedy (99, fl. 3. fig. 22-24) af- 
fords such an example. It has a long epicotyl and a very 
short hypocotyl which is imbedded in a small amount of coty- 
ledonary tissue. The cotyledon grows around and nearly en- 
closes the plumule with a uniform thickness of tissue. In 
/Velumbo the conditions are almost exactly similar, except that 
the cotyledon in addition to growing around the plumule has 
become divided lengthwise nearly to the base into two equal 
parts. 
THE SYSTEMATIC POSITION OF THE NYMPHAACE. 
As indicated above the one character which has led to the 
placing of the Nymphzaceze among the Dictotyledons is the 
structure of the embryo in the seed. In their other characters 
they conform more nearly to the Monocotyledons. The embryo 
of Velumbo has been shown to be monocotyledonous in its de- 
velopment. Although in the other genera there is present in 
the seed a functional endosperm and perisperm, a careful ex- 
amination of the mature embryos shows them to be in all essen- 
tial respects quite similar to that of Velumbo. Those common 
characters, then, which have previously united these plants into 
a family justify the conclusion that they agree in embryogeny. 
In order to remove all possible doubt the embryogeny of a 
