272 PLANT PHYSIOLOGY AND ECOLOGY 



The seeds of two or more species which are intermingled or 

 adjacent are frequently carried into a new area at the same time. 

 This process of mass invasion is characteristic of new habitats 

 and of the transition areas between formations or regions. The 

 best examples of it are found in valleys, where there is a definite 

 line of movement. 



289. Invasion at different levels. The invasion of a forma- 

 tion or some part of it may occur at three different levels: (1) at 

 the level of the facies, (2) below the facies, (3) above the facies. 

 This is determined by the relative height of invaders and occu- 

 pants. The level at which invasion occurs determines the whole 

 future of the formation, as well as the structural modification 

 of the plants concerned. The entrance of invaders of the same 

 general height as the facies regularly produces a mixed formation. 

 Facies and invaders are rarely so equally matched that they 

 remain in permanent equilibrium. Any slight advantage of the 

 one over the other tells sooner or later, and invader slowly yields 

 to occupant, or the reverse. 



When invasion takes place below the facies, the invaders are 

 either slowly adopted into the formation, or gradually disappear. 

 In either case there is usually little change in the formation, and 

 its structure is modified slightly if at all. If the invaders over- 

 top the facies in any considerable number, the entire formation 

 is more or less modified, or it disappears entirely, as is the rule 

 in succession. 



290. Kinds of invasion. Continuous invasion occurs between 

 formations or areas more or less alike, and is usually mutual. 

 There is an annual movement from each one into the other, and 

 often a forward movement through each, resulting from the in- 

 vaders of the previous year. Most invasion is of this sort. Tran- 

 sition areas are good examples of continuous invasion, which 

 is usually mutual also. Intermittent invasion commonly arises 

 through migration to a distance. The movement is more or 

 less accidental, and may never recur. Such invasion is relatively 

 infrequent, but it is often striking, owing to the fact that the 

 invader often wanders far from the original home. 



Invasion is complete when the number of invaders is so great 

 that the original occupants finally disappear. Such invasion 

 occurs in many ruderal or weed formations, and is typical of 

 many successions. It is ordinarily the result of continuous in- 



