No. 634] STENOTHERMY AND ZONE-INVASION 395 



season of the year than late summer, that the farther 

 south the species grows, the earlier will be the fruiting 

 season. Unfortunately, it is impossible to obtain any 

 extensive data on this subject, but reliable testimony 

 indicates that it flowers and fruits somewhat over a 

 month earlier on the coast of New Jersey than it does on 

 the coast of northern New England. It seems therefore 

 that the critical temperature for persistence of this spe- 

 cies, at least through flowering and seeding, is the same 

 throughout its limits and that the species does not differ 

 from a typical stenothermal species from this point of 

 view. Zostera marina, however, is one of the most typi- 

 cal of the eurythermal species in that it must endure 

 extremes of both heat and cold in various portions of its 

 extensive range and in the various seasons of the year in 

 each and every portion of its habitat. It is not known 

 as to the temperature limits of the vital activity of the 

 vegetative portions of the Zostera, but it does not seem 

 possible that their separation can possibly be as wide as 

 the differences between the extreme limits of the tem- 

 peratures of endurance and probably are very much less. 

 The Zostera probably has rather narrow limits to the 

 temperature range of its vegetative activities and un- 

 doubtedly passes into a resting or hibernating condition, 

 a condition of cold-rigor or of heat-rigor as the case may 

 be, at the upper as well as at the lower portions of its 

 temperature range. The land perennials of temperate 

 zones do this and it seems safe to assume that Zostera 

 does the same. 



The case of Ascophyllum nodosum, a perennial brown 

 alga of complex structure, is a very excellent one for 

 study. This species ranges from the western coast of 

 Greenland to that of New Jersey and it has a similar 

 range on the northern and western European coast. On 

 the coast of Greenland, it fruits in summer and it fruits 

 earlier and earlier in the season as it proceeds towards 

 the south, until, in the region of Long Island Sound, it 

 fruits in late winter and early spring. The frond, or 



