290 ANNALS OF THE MISSOURI BOTANICAL GARDEN 



[Vol. 2 



inces, or districts, but no general paper has as yet appeared 

 dealing with the distribution over the oceans in general or 

 any definite suggestions as to the factors concerned. 



The nearest approach to an attempt to account for the gen- 

 eral facts of distribution is my own attempt (cf. Setchell, 

 '93) to explain the main facts of the geographical distri- 

 bution of the Lamina riaceae. The plants of this family are 

 rather inhabitants of the colder than of the warmer waters, 

 proceeding, as it were, from the poles towards the equator, 

 but lacking in strictly tropical waters. It was found that the 

 Laminariaceae flora changed its facies with every increase 

 or decrease of 5°C. of summer temperature, thus forming 

 latitudinal zones controlled by temperature relations. This 

 idea was extended to explain the demarcations of the floras 

 of the west coast of North America by Gardner and myself 

 (cf. Setchell and Gardner, '03) with apparent adequate 

 reason. 



In attempting to discuss the more general facts of distri- 

 bution we first necessarily consider the various marine floras 

 and their subdivisions. While the term flora has been used 

 in all sorts of senses, both wider and narrower, to include any 

 aggregation of plants of any region under discussion, whether 

 larger or smaller, it generally carries a certain idea of uni- 

 formity of composition with it when used in connection with 

 the floristics of distribution. This uniformity may, however, 

 be only as regards region. It is desirable, here, to use the 

 word for the aggregation of species of marine algae found 

 in a certain region, province, or district, having a certain 

 fairly considerable percentage of species in common through- 

 out its extent, even of the more extended region. 



The world's surface, whether land or water, is usually 

 divided into zones of temperature, these in turn into regions, 

 the regions into provinces, and the provinces into districts. 

 For marine floras, the districts must be still further divided 

 into formations, and these in turn into bathymetric or littoral 

 belts. The bathymetric belts, in their turn, show different 

 algal associations. 



