BUTTERCUP EVOLUTION 



Water Buttercup (Ranimculiis aquatilis) — a species 

 that has developed aquatic habits. Probably at 

 some remote period in buttercup history the land 

 on which some species grew developed a tendency 

 to become marshy and, as this tendency increased, 

 only those buttercups would survive that adapted 

 their structure to meet the altered conditions of 

 their surroundings. In the water buttercup we 

 have the survival of the fittest — a species that has 

 so far modified its structure that it has become 

 completely aquatic ; now it flourishes in streams 

 and ponds. 



For a terrestrial plant to become aquatic 

 means much adaptation of its structure, for the 

 function of the leaves of land plants is essentially 

 to absorb gaseous food from the atmosphere, and 

 the submergence of their leaves would eventually 

 mean starvation. In Fig. 103 (Plate 74) a branch 

 of the water buttercup is shown, and it is there 

 seen that it possesses two distinct kinds of leaf. 

 Below they are divided into fine thread-like seg- 

 ments, and this form constitutes the submerged 

 leaves. Above they are of a rounded type with 

 cut and lobed edges, and these are the floating 

 leaves. 



Now the submerged leaves of habitual water 

 plants are almost always small and narrow, often 

 quite thread-hke, because in this form they can 

 best move with the currents and disturbances of 

 the water, and are thus enabled more freely to 

 search for and absorb the small quantities of car- 

 bonic acid and oxygen dissolved in the water. 



