434 THE ALGAE 



over four or five zones are very few indeed and their distribution is 

 usually by no means certain. In New England many of the species 

 are apparently separated by the 20° C. isothere, which approximates 

 closely to the position of Cape Cod, so that the flora to the north of 

 the Cape is essentially different to that of the south. Those species 

 limited to one zone are called stenothermal whilst the wider ranging 

 forms are termed eurythermal. The former species are particularly 

 characteristic of the warmer waters, but, even so, many apparent 

 eurythermal species are found on examination to be essentially 

 stenothermal. Monostroma grevillei and Polysiphonia urceolata are 

 summer annuals in the cold waters of Greenland, but in the south- 

 ern part of their range they develop in winter and early spring when 

 the temperature will be the same as it is in the Greenland summer. 

 With the exception, then, of the temperatures endured by the rest- 

 ing spores they are essentially stenothermal. Ascophyllum nodosum^ 

 with a temperature range from to 10° C, is another case, and in 

 the southern part of its range the plants pass into a heat rigor 

 during the hotter months. 



On the basis of these temperature studies Setchell recognized a 

 number of cHmatic zones each of which could contain species that 

 were characteristic of the zone. These zones were the Boreal, 

 Upper Boreal, North Temperate, North Sub-tropical, Tropical, 

 South Sub-tropical, South Temperate, Upper Austral and Austral. 

 This temperature factor was later recognized by Borgesen and 

 and Jonsson (1905) and Jonsson (191 2) in their studies of Arctic 

 floras. They concluded that the component species of the floras 

 could be divided into a number of elements : 



(i) The Arctic group, with its southern European border in 

 north Norway and Iceland, although in America the group 

 may extend as far south as Cape Cod. 



(2) A sub-Arctic group, the species of which are common in the 

 Arctic sea and the cold boreal area of the Atiantic as far 

 south as western France. 



(3) Boreal Arctic group. These species are common in the 

 Arctic Sea and the boreal area of the Atiantic as far south as 

 the Atiantic coast of North Africa, some perhaps penetrating 

 even farther south. 



(4) A cold boreal group which is of more Hmited distribution, 

 extending northwards from western France to south Iceland 



