THE MANNER OF INVASION 81 



or absence of vegetation. Areas without vegetation are 

 either originally naked {areae nudae) or denuded {areae 

 denudatae), while vegetation with respect to the degree of 

 occupation is open {sporadophytia), or closed {pycnophytia). 

 Each type of area presents different conditions to invaders, 

 largely with respect to the factors determining ecesis. 

 Naked habitats, rocks, talus, gravel-slides and dunes, while 

 they offer ample opportunity for invasion on account of 

 the lack of occupation, are really invaded with the greatest 

 difficulty, not only because they contain originally few or no 

 disseminules, but also because of their xerophytic character 

 and the difficulty of obtaining a foothold, on account of the 

 extreme density or instability of the soil. Denuded habitats, 

 blowouts, sanddraws, ponds, flood plains, wastes, fields and 

 burns, usually afford maximum opportunity for invasion. 

 They invariably contain a large number of disseminules 

 ready to spring up as soon as the original vegetation is des- 

 troyed ; the surface, moreover, is usually such as to catch 

 disseminules and to offer them optimum conditions of mois- 

 ture and nutrition. Open formations are readily invaded, 

 though the increased occupation renders entrance more 

 difficult than it is in denuded areas. Closed formations, on 

 the other hand, are characterised by a minimum of in- 

 vasion, partly because invaders from different formations 

 find unfavorable conditions in them, but chiefly because the 

 occupation of the inhabitants is so complete that invaders 

 are unable to establish themselves. 



Invasion takes place by the penetration of single individ- 

 uals or groups of individuals. This will deoend in the first 

 place upon the character of the disseminule. It is evident 

 that, no matter how numerous the achenes may be, the 

 invasion of those anemochorous species with comate or 

 winged seeds or one-seeded fruits will be of the first type, 

 while all species in which the disseminule is a several or 

 many-seeded fruit or plant, as in hooked fruits, tumble- 

 weeds, etc., will tend to produce a group of invaders. 

 Occasionally of course, the accidents of migration will bring 

 together a few one-seeded disseminules into a group, or will 



