134 SUCCESSION 



modified habitat are sorted by ecesis into three 

 groups: (1) those that are unable to germinate or 

 grow, and soon die; (2) those that grow normally 

 under the conditions present; (3) those that pass 

 through one or more of the earlier stages in a dor- 

 mant state to appear at a later stage of the suc- 

 cession. 



(2) Wherever ruderal vegetation is present, it con- 

 tributes a large number of the pioneer species of 

 each succession, on account of the thorough ecesis. 

 In other regions, this part is played by subruderal 

 native species. 



(3) Annuals and biennials are characteristic of the early 

 stages of secondary successions, on acount of their 

 great seed production and ready ecesis. 



(4) In layered formations, heliophytes appear before 

 sciophytes: they ultimately yield to the latter, ex- 

 cept where they are able to maintain a position in 

 the primary layer. 



(5) Excessive seed production and slight mobility lead 

 to the imperfect ecesis of individuals in dense stands, 

 and in consequence usually produce great instability. 



(6) Each pioneer produces about itself a tiny area of 

 ecesis and stabilisation for its own offspring, for the 

 disseminules of its fellows, or of invaders. 



(7) Species propagating by offshoots, or producing re- 

 latively immobile disseminules in small number, 

 usually show effective ecesis, as the offspring appear 

 within the area of the reaction of the uarent forms. 



V. Stabilisation. 



(1) Stabilisation is the universal tendency of vegetation. 



(2) The ultimate stage of a succession is determined by 

 the dominant vegetation of the region. Lichen for- 

 mations are often ultimate in polar and niveal zones : 

 grassland is the final vegetation for plains and alpine 

 stretches, and for much prairie, while forest is the 

 last stage for mesophytic midlands and lowlands, as 

 well as for subalpine regions. 



