458 



PLANT SUCCESSIONS 



On lands where cultivation is abandoned, or which are in 

 other ways suddenly exposed to invasion, weeds of many spe- 

 cies often obtain a footing and nourish for some years before the 

 truly wild native plants of the region take final possession. This 

 is, in part at least, on account of the remarkable capacity of most 

 weeds to seed themselves (Sec. 429). 



Forest or grass land is the final stage in many successions. 

 The former gains supremacy over the weedy thickets out of 

 which it rises by shading the shrubs and herbs beneath the 

 tree tops until all those not adapted to life in deep shade are 

 destroyed. Grasses have to an unsurpassed extent the power of 

 living with their roots (and sometimes also rootstocks) inter- 

 woven in a way which would prove fatal to most herbs. In 

 this way a lawn or meadow, on good ground, may be seen to 

 improve itself by choking out other plants which occur here 

 and there among the grass. Salt marshes, with a comparatively 

 scanty vegetation, are often purposely shut away from the sea, 

 so that the rains can wash the excess of salt out of the soil. In 

 four or five years they become thoroughly self-sown with the 

 seeds of cultivated grasses and are changed into highly produc- 

 tive meadows. 



