230 



PLANT SOCIOLOGY 



Pawlowski. In the Deschampsia media association on marly soil near 

 Montpellier an air content of 2 to 9 per cent by volume for three- 

 fourths of the year rose to 35 to 40 per cent at the end of the summer 

 after a dry period. The entire ecology of this association is dominated 

 by this factor (Fig. 117). 



Certain alluvial soils on the shores of the Mediterranean Sea, 

 especially near the mouths of streams, are very densely packed, finely 

 dispersed, and poor in air content. One may walk on these sparsely 

 vegetated places without sinking into the soil very much, even after 

 several days' rain. On investigation of the recently inundated soil 



Jan. Feb. Mar Apr. May June July Aug. Sept Oci Nov Dec 



Fig. 117. — Seasonal range of water {A) and air {B) in the soil of a Deschampsia media 

 association. (After Braun-Blanquet and Pawlowski.) 



(water 5 to 10 cm. deep), it is found to be almost dry at a depth of 

 only 15 to 20 cm., crumbly, and entirely free from roots. 



The Salicornietum macrostachyae (Fig. 118), a pronounced halo- 

 phytic association, is characteristic of these soils. Besides the mostly 

 dominant Salicornia macrostachya and the sparsely intermixed dwarfed 

 specimens of S. fruticosa and Atriplex portulacoides (all nanophanero- 

 phytes) there are also a number of short-lived therophytes such as 

 Sphenopus gouani, Frankenia puherulenta, Sagina maritima, and 

 Hutchinsia procumbens, which shoot forth after the spring rains and 

 whose roots penetrate scarcely to a depth of a centimeter. Even the 

 shrubs have very superficial roots. The luxuriant shrubs of S. macro- 

 stachya, often more than a meter wide, resembling giant turtles, can 

 easily be lifted from the ground with their whole root system. It is 

 surprising to find that the highly developed root mat extending in all 

 directions and filling the upper layer of the soil in a dense network 



