150 DIRECTION OF DEVELOPMENT. 



"After the progressive development, where regeneration plays a relatively 

 minor rdle, appears a stage in which the moor passes simultaneously into 

 heath-moor over large areas with uniform topography. (In the deeper 

 hollows, the progressive development may proceed further.) In the sequence 

 of the layers, the lower Sphagnum peat is followed by a more or less coherent 

 layer of heath peat. With the development of the heath moor begins the 

 formation of hollows, and the accumulation of regenerative peat masses, 

 commonly with great sods of Andromeda-Sphagnum peat and Schenchzeria- 

 Sphagnum itesA directly above the peat of the heath moor." 



Semander's description of the formation of hollows by the death of the 

 peat and of the consequent production of tiny pools which are invaded by 

 Sphagnum furnishes outstanding proof that the retrogressive development of 

 Nilsson and Cajander is actually the death of a plant community or a part of 

 it, and the resulting formation of a bare area for colonization. No serious 

 objection can be brought against the use of the term regeneration or regenera- 

 tive development, and it has the advantage of being in harmony with the idea 

 that succession is a reproductive process. It does, however, obscure the fact 

 that the development is nothing but the normal progressive movement typical 

 of succession. It is normally secondary, but differs from the primary progres- 

 sive development only in being shorter and in occurring in miniature in hun- 

 dreds of tiny areas. 



Moss's view. — Moss (1910 : 36) makes the following statements in regard to 

 the direction of movement: 



"Succession of associations within a formation may be either progressive 

 or retrogressive. In the salt marshes in the south of England for example, 

 a succession of progressive associations of Zostera, of Spartina, of Salicornia, 

 etc., culminates in a comparatively stable association of close turf formed of 

 Glyceria maritima. The latter association, however, may be attacked by the 

 waves and ultimately destroyed; and thus retrogressive associations are pro- 

 duced. In the case of established woods, we do not know the progressive 

 associations which culminated in the woodland associations; but we can deter- 

 mine retrogressive stages through scrub to grassland. Similarly, the retrogres- 

 sive associations which are seen in denuding peat moors are recognizable. 



"A plant formation, then, comprises the progressive associations which 

 culminate in one or more stable or chief associations, and the retrogressive 

 associations which result from the decay of the chief associations, so long as 

 these changes occur in the same habitat. 



"It sometimes happens, as in the case of the peat moors on the Pennine 

 watershed, that the original habitat is wholly denuded and a new rock or soil 

 surface laid bare. In other cases, as when sand-dimes are built up on the site 

 of a pre-existing salt marsh, a habitat may be overwhelmed by a new one. 

 In such cases the succession passes from one formation to another formation. 

 Again, a new habitat is created when an open sheet of water is choked up with 

 silt and peat. 



"Every formation has at least one chief association; it may have more; 

 and they may be regarded as equivalent to one another in their vegetational 

 rank. They are more distinct and more fixed than progressive or retrogressive 

 associations. Open progressive and retrogressive associations, however, fre- 

 quently occur in formations whose chief associations are closed. Unless, how- 

 ever, the progressive and retrogressive associations are included in the same 



