MIGRATION. 65 



surface of the disseminule. This is particularly true of seeds and fruits carried 

 by wind and water. Man and animals distribute fruits for so many reasons 

 and in so many ways that the only test of mobility in many cases is the actual 

 movement. This is especially clear in the case of many weeds of cultivated 

 fields, which owe their migration wholly to their association. Mobility is 

 also directly affected by the amount of seed produced. It is increased by 

 large seed-production, both on account of the large number of seeds or fruits 

 and the correspondingly smaller size. 



The relation of mobility to succession is obvious. In bare land areas, and 

 especially in denuded ones, the order of appearance of species is largely a 

 matter of the size and modification of the disseminule. The earhest pioneers — 

 lichens, liverworts, and mosses — usually have microscopic germules, whether 

 spores, soredia, or gemmae. The early herbaceous pioneers are grasses and 

 herbs with small seeds and fruits, well adapted for wind-carriage, as in fire- 

 grass (Agrostis hiemalis) and fire-weed (Chamaenerium angustifolium) , or 

 mobile by virtue of association, as in Brassica, Lepidium, Chenopodium, etc. 

 The sequence of shrubby species is determined partly by mobility, as is true 

 of Rubus in burns, Salix in lowlands, and Cercocarpus in grassland. The 

 same relation is shown in trees by the fact that Populus and Betula are every- 

 where woodland pioneers. Trees constitute the climax life-form, howev6r, 

 and their successional relation is chiefly due to other factors. 



Seed production. — The absolute seed-production of a species bears a general 

 relation to its power of invasion. The latter is expressed more exactly by 

 the efficient seed-production, which is the total number of fertile seeds left 

 after the usual action of destructive agents. The number of seeds produced 

 by a tree of Pinus flexilis is large, but the efficiency is almost nil. The toll 

 taken by nut-crackers, jays, and squirrels is so complete that no viable seed 

 has yet been found in hundreds of mature cones examined. The fertility of 

 seeds is greatest in typical polyanthous species which produce but one seed per 

 flower, such as grasses, composites, and other achene-bearing families. This 

 is shown by the large number of successful invaders, i. e., weeds, produced by 

 these groups. Fertility is often low in polyspermous plants, due to the lack 

 of fertiUzation or to competition between the ovules. The number of seeds is 

 often correlated with size, but the exceptions are too numerous to permit the 

 recognition of a general rule. The periodic variations in the total seed-pro- 

 duction is a factor of much importance, especially in trees and shrubs. This 

 is due to the fact that birds and rodents consume practically the entire crop 

 in the case of conifers, oaks, etc., during poor seed-years. The efficient pro- 

 duction is high only during good years, and the invasion of such species is 

 largely dependent upon the occurrence of good seed-years. 



The influence of seed-production is felt in mobility, in ecesis, and in domi- 

 nance. Its effect can only be estimated at the present, owing to the lack of 

 exact study. It is probable that the quantitative investigation of the seed- 

 production of dominant and characteristic species will go far towards reveal- 

 ing the real nature of dominance. 



Influence of the organ used. — ^When runners, stolons, and rhizomes carry 

 buds several to many feet from the parent plant, the result may well be regarded 

 as migration rather than aggregation. Such migration plays a small part in 

 the colonization of new areas. It is almost negligible in comparison with the 



