320 Annals of the Carnegie Museum. 



dissemination must be ascribed, most of the territory included each 

 year by the plants is that taken in by the radiate enlargement of the 

 parent plant. Isolated plants, families, or even communities, are to 

 be found out on the sand-plain in advance of the heath, and have evi- 

 dently been distributed by birds, but such occurrences are compara- 

 tively rare. Such isolated occurrences of the plant are necessary, 

 however, for the advance of the formation sufficient to keep up with 

 the forward march of the other formations. The vegetative method 

 of dissemination would, at the most, advance the plant not more than 

 two or three feet in a season. 



With the red cedar {Junipenis idrghiiana) the conditions of dis- 

 semination are quite different. Occasional isolated specimens of this 

 species occur far out on the sand-plain in such positions that they 

 plainly indicate dispersal of the seeds by birds. On the sand-plain 

 such specimens almost invariably stand on the leeward side (east ) of a 

 Cottonwood trunk (see Plate XXX). The most abundant reproduc- 

 tion of the red cedar, however, occurs in the Arctostaphylos mats 

 where it apparently finds conditions better suited for its ecesis. As 

 conditions are at present in the heath, the relative predominance of 

 Arctostaphylos and Jiiniperus, as to the number of individuals and as to 

 the area occupied, is decidedly in favor of the former species in the 

 younger stages of the formation and of the latter species in the older 

 stages. At the climax of the formation the Juniperus occupies about 

 80 per cent of the total area. 



The reaction of the heath upon the edaphic conditions of the habitat 

 is, as compared with such effects in most formations, very rapid. This 

 reaction consists mainly in the accumulation of humus. Arctostaphy- 

 los forms over the surface of the sand an entangled mat which very 

 effectually catches and retains its own leaves when shed, as well as 

 leaves which are blown over from the forest to the west or southwest. 

 The soil being protected, both by the Arctostaphylos mat and by the 

 dense compact Juniperus acting as a windbreak, humification rather 

 than eremacausis becomes the rule, and there is an annual accumula- 

 tion of humus and fine sand, blown in by northerly and northeasterly 

 gales, finally resulting in a layer above the sand of a fine sandy loam. 

 As compared with the soil of the almost bare sand-plain we have in the 

 soil of the heath much better conditions of available moisture for plants 

 (capillary and hygroscopic moisture), and excessive aeration is pre- 

 vented by the vegetational covering, as well as by the fine sandy loam. 



