REPRODUCTION AND DISPERSAL 927 



ferns, whose spores are readily scattered by wind. Fifteen years after 

 the eruption, fifty-three species of seed plants had reached the island, 

 and of these it was estimated that 60 per cent, chiefly shore species, were 

 brought by ocean currents, 32 per cent by wind, and 8 per cent by animals. 



The dispersal of epiphytes is of interest because of the difficulties attending the 

 lodgment of disseminules in places fit for germination. Most epiphytes have wind- 

 scattered disseminules, as in the spores of the lichens, mosses, and ferns, or the 

 seeds of the orchids and bromelias. Most such disseminules are minute, and, 

 while many are wasted, a few find lodgment in bark crevices. The seeds of some 

 epiphytes are scattered by birds, as is the case also with many of the pseudo-epi- 

 phytes of temperate climates, which occur in soil in the crotches of trees (as the 

 raspberry, gooseberry, and nightshade). Mistletoe, which is parasitic on trees, 

 is also scattered by birds; after eating the enveloping fleshy rind, the slimy seeds 

 which often stick to their bills may be wiped off upon the limbs where they are 

 perched, and hence in places suitable for germination. 



A study of the geographic distribution of plants shows that some 

 species, which are known as endemic, are confined to restricted areas, 

 and that other species, which are known as cosmopolitan, are almost 

 world-wide in distribution; the members of a third class, embracing a 

 much greater number of species, occupy relatively large but not world- 

 wide areas. It might be supposed that the size of the area occupied by 

 a species is determined by its means of dispersal, but this is not obviously 

 the case. While many mobile species {i.e. those with easily scattered 

 disseminules) are widely distributed (as in the willows and cat-tails), 

 and while some immobile species are endemic (as in Torreya), there are 

 many cases in which the reverse is true; for example, the immobile oaks 

 and beeches are among the most widely distributed trees, while the 

 wonderfully mobile orchids furnish many cases of endemism. 



In explaining the distribution of species, many factors other than the 

 mobility of disseminules are to be considered. An important element 

 in the problem is time. For example, even though the oak or beech 

 in a century might be able to migrate only a few meters, in contrast with 

 as many kilometers in the case of the willows, such a difference is of 

 little consequence in the eOns of geological time. Hence it may be 

 stated as a somewhat general truth that the rapid occupation of a new 

 area depends largely upon the mobility of plant disseminules,' but that 



• There are some cases of rapid migration, where the disseminules are not conspicu- 

 ously mobile, as in Galinsoga parviflora and Artemisia Stelleriana, two composites without 

 the usual hairlike pappus, which have spread over the world in a comparatively few years. 

 Apparently such cases are associated in some way with man, whose various means of 



