42 



/ 

 fLANT SOCIOLOGY 



On the other hand, the lower layers are less exposed to variations in 

 heat, humidity, and wind movement. They are also dependent upon 

 the indirect physicochemical influences of the plants composing the 

 upper layers (root activities, type of decomposition, and chemical 

 nature of the leaf mold). 



Fusion of Layers. — Two or more layers of vegetation are often 

 closely united and always appear together in certain societies, e.g., the 

 herb and moss layers in the Centunculo-Anthoceretum, the moss and 

 shrub layers in certain types of high moor. An individual lower layer 

 may either fuse with two or more higher layers or stand out distinctly 

 by itself. In the first case we have united, in the second overlapping, 

 layers. An example of overlapping layers is seen in the Rhodoreto- 



= Co^er 

 Upper free layer 



Lower free layer 



Shrub layer 

 Herb layer 

 Moss layer 



Fig. 22. — Layering in a spruce stand in the Black Forest. July, 1926. 



Vaccinietum of the Alps (in the Pinus cembra-Picea excelsa and Larix 

 decidua forest), the Rosmarinus-Lithospennum fruticosum association 

 of southern France (either with or without an upper layer of Pinus 

 halepensis). A complete layer of Calluna or of Thymus vulgaris may 

 appear in entirely different associations. 



Among closely united layers are found such as are dependent upon 

 and conditioned by certain other definite layers. As such dependent 

 layers may be cited the highly developed broad sclerophyll shrub layer 

 in the Quercus ilex virgin forest of the Atlas mountains (Braun- 

 Blanquet and Maire, 1924) and the lower layers in the Alnus swamps of 

 Wangerin (1926, p. 189). 



Subterranean Layers. — Layering is also found in the underground 

 parts of plants, as Woodhead (1906) has pointed out. In North 

 America Sherff (1912) has shown that in swamp communities the 

 subterranean stems may lie at different depths in the soil and that their 

 roots may be produced al different depths, causing a complementary 

 rather than a competitive relation between the underground organs of 

 tho different species. The height of the water table was hero the most 



