EXPERIMENTAL EVOLUTION 159 
the mud form was kept covered with a thin film of water; the leaves of the 
form in shallow water were kept floating on the surface, and those of the 
last form submerged just below the surface. The water in which the sub- 
merged form grew was aerated by means of a spigot near the bottom of the 
barrel. From time to 
time water-content de- 
terminations were made 
of the soil in the pots 
until it was definitely as- 
certained that the holard 
was practically constant. 
The nine new forms ob- 
tained by adaptation 
showed striking differ- 
ences in vigor. and 
growth, as may be seen 
from the figures. In all 
cases, these were ac- 
companied by distinct 
and often striking dif- 
ferences in the number 
_and position of the sto- 
mata, the amount of 
sponge and palisade tis- 
sues, and the develop- 
ment of air passages. 
Photographs were made 
of a typical plant of r j 
. ae Fig. 49. Floating form of Ranunculus sceleratus 
each form, and the dif- grown under control. 
ferent leaf structures _ 
were preserved in permanent mounts. The xerophytic and the submerged 
form were unable to produce flowers, and it was necessary to develop them 
anew in each generation. The other forms fruited abundantly, and the 
succeeding generations of each form were produced from plants which had 
grown the year before in the same conditions. In addition to the develop- 
ment of a series of new water-content forms, this experiment was begun 
in the hope of determining whether the modifications of a plastic species 
tend to become fixed if each new form is grown constantly under the same 
conditions. A period of four years is too short, however, to throw much 
light upon this problem. 
