Double Adaptations 433 



choice is final, and a further change does not oc- 

 cur in the normal course of things. 



The most curious and most suggestive in- 

 stance of such an alternation is the case of the 

 water-persicaria or Polygonum amphibium. It 

 is known to occur ia two forms, one aquatic and 

 the other terrestrial. These are recorded in 

 systematic works as varieties, and are descrihed 

 under the names of P. amphibium var. natans 

 Moench, and P. amphibium var. terrestre Leers 

 or P. amphibium var. terrestris Moench. Such 

 authorities as Koch in his German flora, and 

 Grenier and Godron in their French flora agree 

 iu the conception of the two forms as varieties. 



Notwithstanding this, the two varieties may 

 often he observed to sport into one another. 

 They are only branches of the same plant, 

 grown under different conditions. The aquatic 

 form has floating or submerged stems with ob- 

 long or elliptic leaves, which are glabrous and 

 have long petioles. The terrestrial plants are 

 erect, nearly simple, more or less hispid 

 throughout, with lanceolate leaves and short pe- 

 tioles, often nearly sessile. The aquatic form 

 flowers regularly, producing its peduncle at 

 right angles from the floating stems, but the 

 terrestrial specimens are ordinarily seen with- 

 out flower-spikes, which are but rarely met with, 

 at least as far as my own experience goes. In- 



