HISTORICAL 149 



Each zone was named from ifcs facies, giving the series, 

 caricetum, phragmitetum, scirpetum, nupharetum, potamogetone- 

 tum, and chmetum. He discussed the influence of the 

 general physical conditions of the lakes and of the structure 

 of the various species upon zonation, but in such general 

 terms that he vras unable to give them definite expression. 

 MacMillan (1896:500) studied the zonation of the sphagnum 

 moors of Minnesota, and expressed his results in an impor- 

 tant generalisation. "It would seem that there are two 

 principal types of plant arrangement in their habitats. 

 These are: (1) Zonal and (2) Azonal. The first is connected 

 either with environmental conditions as a principal factor, 

 as in the case of the zonal distribution upon dome-shaped 

 islands, upon roches moutonndes, and on a larger scale upon 

 mountain peaks and isolated ranges, along lake or ocean 

 strand, and surrounding moors, or it may depend more 

 particularly upon the charactet- and habits of life of the 

 plants themselves, as, for instance, in the case of the 

 "fairy-ring" fungus. The matter may be summed up in a 

 sentence. GeneraUy, when there is well-marked radial symmetry 

 in the topographic feature upon which a group of plants is dis- 

 tributed, zonal arrangement is the response of the plant population 

 to these symmetric physiognomic conditions, but when the topo- 

 graphic feature is devoid of such well-marked radial symmetry 

 the plants dispose themselves according to the azonal type. 

 Talus-heaps, flat extended meadows, highly irregular hills, 

 shallow marshy ponds, and other such localities may serve 

 as illustrations of asymmetrical habitats. A variety of 

 conditions determines whether the distribution upon a given 

 area be zonal or azonal. And it is worthy of note that the 

 same formation may in one case arise by zonal, in another 

 by azonal distribution". In a later paper, (1897:949), Mac- 

 Millan classified an extensive series of shore formations as 

 zonal and azonal, including in the latter talus, boulder, rock 

 and humus shores, in which there is often a more or less 

 rudimentary zonation. Clements (1897:968) described the 

 lateral zones of subruderal plants, which border the old 

 trails of the Great Plains. Poand and Clements (1898:220) 



