370 INTRODUCTION TO PLANT GEOGRAPHY [CHAP. 



weather. Still, the sequence of zones is often remarkably definite, 

 and in a general way reflects not only the prevailing degree of 

 salinity but also the poor aeration accompanying the increasing soil- 

 moisture. Here again there is no certainty that succession will ever 

 proceed to any proper climatic climax in the absence of biotic 

 disturbance, especially as the tendency is for the salinity to increase 

 rather than decrease with time. Consequently it seems best to 

 consider the marginal vegetation as forming a zoned series of sub- 

 climaxes so far as the general region is concerned (or edaphically 

 limited climaxes of their own immediate areas). In any case it 

 appears likely that the local allogenic conditions will have more 

 effect upon the future of these inland saline areas than will any 

 autogenic serai tendencies. Fig. 107, B, shows an area of saline 

 marsh outside the oasis of Shithatha, in southern central Iraq. The 

 vegetation is mostly closed and sedgy-grassy, with some shrubby 

 Tamarisks, being dominated by such types as Scirpus maritimus, 

 Cvpenis distachyos, Aehiropus littoralis, and Tamarix pentandra. The 

 pH was 8-6 where tested and the area was said to remain damp 

 throughout the year, having some shallow pools of open water at 

 least in spring. Open mud ' polygons ' were bound by mixed 

 Oscillatoria, Lyngbya, and other primitive x\lgae. 



Some hitherto productive cultivated areas in arid regions requiring 

 irrigation have become too saline for the growth of crops — owing 

 to prolonged evaporation leaving the salts of the ' raw ' irrigation 

 water behind when no drainage was provided. In other cases the 

 crops are limited to facultative halophytes such as Date Palms 

 {Phoenix dactylifera) or Barley. Much of central and southern Iraq 

 is of this nature, and as Man has not yet become proficient at 

 remedying such soil salinity on a wide scale, the increasing amount 

 of irrigation is worsening the situation. 



Seral Communities 



Besides subclimax communities, various other deflected or 

 ' straight ' successional ones have been described or implied above, 

 while many others are of such limited occurrence or importance 

 (as in the cases of salt-sprayed coastal areas or the ' serules ' occurring 

 for example on fallen logs) that we can scarcely even mention them 

 in this brief outline of the main vegetational types of temperate 

 and allied lands. However, to complete our picture there remain 

 a number of (mostly seral) communities that should be cursorily 



