No. 634] STENOTHERMY AND ZONE-INVASION 391 



under different temperatures. One constituent of the 

 marine flora of the northern hemisphere which has inter- 

 ested me very much indeed, is the common eel-grass, 

 Zostera marina, a marine spermatophyte. As commonly 

 regarded as to specific limits, this plant extends from 

 the northern coasts of Europe down along the western 

 coast and enters the Mediterranean Sea, occurring spot- 

 wise in the northwestern portion of it and being repre- 

 sented also in the northern Adriatic. Zostera marina is 

 represented in one or two localities in southwestern 

 Greenland and, reappearing at the Strait of Belleisle, it 

 seems fairly continuous in its distribution thence down 

 to the coast of North Carolina, at least, and is reported 

 from West Florida and the Bermuda Islands, although 

 I can not make certain as to whether it actually grows in 

 either of the last mentioned stations. Zostera marina is 

 reported from both the North American and the Asiatic 

 coasts of the North Pacific, but the exact limits of its oc- 

 cupancy of these shores is in doubt. The greatest range 

 of temperature experienced by the Zostera is that on the 

 Atlantic coast of North America, where it extends from 

 waters of a mean maximum of 0° C. to those of a mean 

 maximum of somewhat over 25° C. It is also found in 

 localities where the seasonal temperature range of the 

 surface waters is from somewhat below 0° C. to 15° C. 

 It ranges through all the temperature zones of surface 

 waters from the Upper Boreal (0°-10° C.) to the Trop- 

 ical (25°-30° C), i.e., five zones in all. I shall discuss 

 the reasons for this wide extension of the range of Zos- 

 tera marina later, but desire to call attention here to the 

 facts that we are dealing with a perennial plant with un- 

 usually effective methods of vegetative multiplication 

 and devices for wide dispersal. 



Another eurythermal marine species is Ascophi/lhun 

 nodosum, one of the bladder-bearing Fucaceae or Rock- 

 weeds. On the Atlantic coast of North America, this 

 species is found in some abundance from well up on the 

 west coast of Greenland down to the northeastern coast 

 of New Jersey, or on coasts having a range of mean 



