396 THE AMERICAN NATURALIST [Vol. LIV 



vegetative portion, of Ascophyllum does not seem at all 

 vigorous during the summer of the southern portion of 

 its range. It seems perfectly evident that Ascophyllum 

 is normal to the Upper Boreal Zone and invades the 

 zones to the south because it finds, even on the north- 

 eastern coast of New Jersey, seasonal temperatures of 

 proper duration below 10° C. The isocryme, or winter 

 isotherm, of 5° C. touches the coast of New Jersey at 

 about the point that marks the southern limit of the 

 range of Ascophyllum nodosum. We have in this spe- 

 cies, then, a eurythermal species whose critical tempera- 

 ture and amplitude for persistence range from 0° to 10° 

 C. and which undoubtedly passes into a condition of heat 

 rigor during the hotter months of the year in the south- 

 ern portion of its range. It differs from the last ex- 

 ample, in that its course of invasion is in one direction, 

 viz., to the south. 



Rhodochorton Rothii is a very delicate, filamentous, 

 perennial red alga of very lowly stature. It has a range 

 very similar to that of the last species and, in the south- 

 ern portions of its range, fruits only in winter. The 

 same things may be said of this species as were said of 

 AscopltylluH). RliodochortoH, however, is a shade or cave 

 plant in the more southern portions of its range, seek- 

 ing the cooler portions of the warmer districts. This is 

 doubtless its only opportunity of surviving the heat and 

 is of great benefit to its delicate structure. 



Monostroma Grevillei and Polysiphonia urceolata are 

 annuals, with about the same- range as the last two. 

 They are summer annuals in the waters of Greenland, 

 but are winter and early spring annuals of the southern 

 portions of their range. With the exception, then, of the 

 temperatures endured by their resting spores, they are 

 confined to the temperature range of the Upper Boreal 

 Zone and are practically stenothermal. 



The last example quoted is Grinnelia americana, an 

 annual red alga, apparently normal to the Long Island 

 Sound district and therefore of the North Subtropical 



