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 ;ust 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 

 each form, and the dif- 

 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. 



Fig. 49. Floating form of Ranunculus sceleratus 

 grown under control. 



