ECE8I8. 71 



dry mold or dust increases the distance roots have to go. Shreve (1909: 289) 

 has found that the seedling mortality of Parkinsonia in the deserts of Arizona 

 was 70 per cent during the first year and 97 per cent by the end of the third 

 year. 



Growth. — If the seedling establishes itself it is fairly sure to develop. This 

 seems to be the rule with herbaceous plants, though it suffers some exceptions 

 in the case of trees and shrubs. Even though conditions become more extreme, 

 the old plant is usually better able to resist them. With increasing size of 

 individuals the demands increase correspondingly. Hence, growth causes an 

 increasing competition. Out of this competition some species emerge as 

 dominants, reacting upon the habitat in a controlling way and determining 

 the conditions for all other species ia the community. Others represent an 

 adaptation to conditions caused by the dominance and play always a subor- 

 dinate part. A third behavior is shown by those species or individuals ordi- 

 narily capable of becoming dominant, whenever they appear tardily, or repro- 

 duce under unfavorable Kght intensities. The growth is diminished and the 

 plant becomes suppressed. In forest and thicket suppression is progressive, 

 and usually results in death, either through insufficient nutrition or in conse- 

 quence of the attacks of insects and fimgi. While suppression occurs in all 

 degrees, its most important effect lies in inhibiting reproduction, and it would 

 be well if the term were restricted to this sense. 



Beproduction. — ^The invasion of a bare area is made possible by reproduction 

 or seed-production in the neighboring communities. The development of each 

 stage in the resulting sere is the consequence of the excess of reproduction over 

 immigration. Reproduction is in consequence the final measure of the success 

 of ecesis. In terms of succession at least, ecesis occurs only when a species 

 reproduces itself, and thus maintains its position throughout the stage to which 

 it belongs. In changes of vegetation the total period of ecesis may be much 

 shorter; in fact, annuals may appear and disappear finally in a single season. 

 In the case of annuals it is evident that there is no ecesis without reproduction. 

 With perennials it is less clear, but there are few species that can maintain 

 themselves in an area by vegetative propagation alone. Since bare areas are 

 rarely invaded in this way, complete ecesis in them must rest upon reproduc- 

 tion. 



Ecesis in bare areas. — The selective action of bare areas upon the germules 

 brought into them is exerted by ecesis. It has repeatedly been pointed out 

 that the essential nature of such areas is f oimd in the water relations, and that 

 it can best be expressed in the amount of departure from the climatic mean. 

 The two extremes, water and rock, are the extremes for ecesis, the one impos- 

 sible for plants whose leaves live in the air and the light, the other for those 

 whose roots must reach water. The plants which can ecize ia such extremes 

 are necessarily restricted in number and specialized in character, but they are 

 of the widest distribution, since the habitats which produced them are uni- 

 versal. From the standpoint of ecesis, succession is a process which brings the 

 habitat nearer the optimum for germination and growth, and thus permits the 

 invasion of an increasingly larger population. The fundamental reason why 

 primary succession is long in comparison with secondary lies in the fact that 

 the physical conditions are for a long time too severe for the vast majority of 

 migrants, as well as too severe for the rapid increase of the pioneers. Second- 



