THE ORIGIN AND GROWTH OF PLANT COMMUNITIES 589 



Concerning the latter, it is interesting to note that they are mostly 

 concentrated in the tree layer. (Farther south and west, in the oak- 

 hickory region, the barochores replace them!) The pogonochores, on 

 the other hand, rank highest in open fields. 



Ecosystematic control. Many of the phytosociological investiga- 

 tions that concern themselves primarily with the items listed above 

 under "floristic composition" ( and to a lesser degree under "structure" ) 

 also oflFer some indication on site conditions, such as exposure, slope, 

 drainage, and, even more frequently, soil. The latter is sometimes ana- 

 lyzed in considerable physical and chemical detail. In any given stand 

 where it is intended to make a truly exhaustive study, such painstaking 

 analyses are indeed a most vital part of the record. However, the phy- 

 tosociologist whose main interest is in the composition (and structure?) 

 of the association, and who must lay down a large number of quadrats 

 in order to sample a good number of stands, cannot always resort to 

 such minute observation of site qualities. ( May I note in passing that 

 the taxonomist, the biosystematist, and the geneticist usually stop even 

 shorter of this mark?) The most commonly used allusive descriptions 

 refer to soil moisture and to shade. Thus the "habitat" is described as 

 "xerophytic," "mesophytic," "hygrophytic," or as "rich woods," "open 

 fields," "sandy plain," etc. 



Again, the best description of an individual site is a record of the 

 physical conditions actually observed there, as in the case of structure. 

 An actual representation of the existing layering is "truer" than a dia- 

 gram which forcibly fits the actual stratification into pre-established 

 height classes. Nevertheless, in both cases, for purposes of comparison 

 a standard is useful. Such a standard must always be based, and as 

 narrowly as possible, on criteria known to be significant. Some years 

 ago I convinced myself that Huguet del Villar ( 1929 ) had provided the 

 most acceptable framework for a recognition of the main habitats. In 

 1952 I translated his scheme from the Spanish and only slightly modi- 

 fied it and have applied it in many later contributions (see especially 

 1952a, 1956a, 1957a). 



Although it is recognized that single factors do not control the site 

 in an absolute way, del Villar's prime assumption is that deficient or 

 excessive elements exert a major influence. He has therefore aligned the 

 factors actually known to induce some kind of limitation upon plant 

 species in a hierarchic order. The hmitations thus undergone go from 

 exclusion through various forms of suboptimal reaction (e.g., lack of 

 flowering) to complete fitness. The latter, in each of the cases defined, 

 is nothing less than a physiological regimen. The demands made upon 

 a plant living in excessively acid or warm and dry or rocky substrata 

 are such as to elicit responses of a pecuHar kind ( which the morphol- 

 ogy of the plant may or may not betray). Therefore, when del Villar 



