37^ ecolog y. 



a newly made bank of soil. The species of bidens were here 

 among the first to start in the soft black mud. These are fol- 

 lowed later by grasses, by species of the arrowhead (sagittaria), 

 pickerel-weed (pontederia), etc. The loose soil becomes per- 

 meated by a mass of roots, and year by year becomes more firm. 



683. Vegetation of sand dunes. — Along the sandy beaches 

 of lakes, or of the ocean, drift piles of the fine sand are formed, 

 which often are moved onward by the wind. The surface parti- 

 cles are moved onward to the leeward of the drift, and so on. 

 The form and location of the sand dune gradually changes. 

 Such drifts sometimes slowly but surely march along over soil 

 where a rich vegetation grows, and over valuable land. Even 

 on these sand dunes there are certain plants which can gain a 

 foothold and grow. When a sufficient number obtain a foothold 

 in such places they retain the sand and prevent the movement 

 of the dune. 



684. Reforestation of lands. — When by the action of fire or 

 wind, or through the agency of man, portions of forests are 

 partially or completely destroyed, a new set of conditions is pre- 

 sented over these areas. One of the most important is that light 

 is admitted where before towering trees permitted but a limited 

 and characteristic undergrowth to remain. Hundreds of forms, 

 which for years have been dormant, are now awakened from 

 their long sleep, and new and recent importations of seeds which 

 are constantly rushing in spring into existence to fill the gap, 

 multiply their numbers, and make more sure the perpetuation 

 of their kind. 



685. The earliest to appear are not always the ones to endure 

 the longest, and a battle royal takes place during years for su- 

 premacy. The weaker ones are gradually overcome by the more 

 vigorous, and a new crop of trees, which often springs up in such 

 places, finally usurps again the domain, in the name of the same 

 or of a different species. 



686. Domestic plants protected by man occupy cultivated 

 fields. When cultivation ceases, or the crop is removed, or the 

 fields are neglected, hundreds of species of feral plants, which 



