328 INTRODUCTION TO PLANT GEOGRAPHY [CHAP- 



conserve moisture, so that, in due course, forest and ultimately some 

 kind of climax can develop (see below). 



The psammosere usually proceeds much more quickly than the 

 lithosere, the initial problem being the ' binding ' of the surface sand. 

 This is often accomplished by coarse Grasses or other ' advanced ' 

 types (Fig. 93), while on gravel slides and talus slopes the pioneers 

 may be coarse herbs or even woody plants, and succession still more 

 rapid provided a reasonable degree of stability can be attained. 

 Often, and especially on damp substrata that are immediately suitable 



Fig. 93. — Psammosere a- I'l ;lii's Xcck, Maine, showing Marram Grass {Ammo- 



pJiilo orettaria) binding sand above high-tide mark. Some stabihzed dunes and 



coniferous forest are seen behind. 



for colonization by advanced types, the phases of succession may be 

 telescoped more or less completely ; but still the general tendency 

 is evident. 



It may be noted that in these seres there is a general convergence 

 of water conditions, the hydrosere becoming progressively drier and 

 the xerosere progressively moister — until a mean is reached that in 

 any given climatic region is approximately the same in the two cases. 

 Typically this mean is inhabited by mesophytes and is said to be 

 ' mesic ', though relatively xeric (dry) and hydric (damp) exceptions 

 exist. It has been suggested that ultimately this mean should be 

 the same under particular climatic conditions whatever the initial 

 situation, but although this idea may be theoretically attractive it is 



