202 WILLIAM S. COOPER 



and various shore plants. Another depression, probably having 

 once contained a pool, was found completely filled with a mat of 

 Aulacomnium and Polytrichum strictum Banks, bound together 

 in the same manner as the last. Other species found inhabiting 

 the margins of rock pools were Amblyslegium Kochii B. & S., 

 A. fluviatile (Sw.) B. & S., and Campylium stellatum (Schreb.) 

 Bryhn. In certain wet depressions and particularly along 

 crevices containing intermittent small streams Bryum Muhlen- 

 beckii was frequent. 



In the formation of the heath mat (which must not be under- 

 stood to be often continuous over extensive areas) the three sub- 

 successions unite. Many of the early arrivals among the mosses 

 still persist, and numerous others, including those peculiarly 

 characteristic of the climax forest, make their appearance. Most 

 important at this stage are Thuidium abietinum and Calliergon 

 Schreberi (Willd.) Grout, which are frequently abundant in a 

 stunted condition among the tangles of Juniperus horizontalis, 

 J. communis var. depressa and Arctostaphylos Uva-ursi, which 

 form so important a part of the heath mat. Besides contributing 

 to the supply of humus the mosses now come to aid in the conser- 

 vation of soil moisture. Upon areas where crevices in the under- 

 lying rock are rare the large cladonias (C. rangiferina, C. sylvatica, 

 C. alpestris) form extensive dense growths during the late stages 

 of the Rock Surface Subsuccession, which frequently remain 

 intact until entirely surrounded by the climax forest. Calliergon 

 Schreberi is apparently the only species that is able to supersede 

 the cladonias in such places. The young plants germinate 

 among the branches of the lichen clumps and gradually spread 

 over their surfaces, cutting off light and air and thus bringing 

 about their death. An excellent example of this is shown in 

 figure 2. If the cladonia areas remain uninvaded until the forest 

 surrounds them they are termed "rock openings" and become 

 very pleasing features of the woodland scenery. In addition v to 

 Calliergon and Thuidium, the following forest species commonly 

 make their first appearance with the heath mat: Dicranum 

 undulalum Ehrh., Hylocomium proliferum (L.) Lindb., Hypnum 

 crista-castrensis L., Hylocomium triquetrum, which has already 



