THE STRUGGLE FOR EXISTENCE. 199 
240. Origin of Weeds.1— By far the larger proportion of 
our weeds are not native to this country. Some have been 
brought from South America and from Asia, but most of the 
introduced kinds come from Europe. The importation of 
various kinds of grain and of garden-seeds mixed with seeds 
of European weeds will account for the presence of many of 
the latter among us. Others have been brought over in the 
ballast of vessels. Once landed, European weeds have suc- 
ceeded in establishing themselves in so many cases because 
they were superior in vitality and in their power of repro- 
duction to our native plants. This may not improbably be 
due to the fact that the vegetation of Europe and the neigh- 
boring portions of Asia, much of it consisting from very early 
times of plants of comparatively treeless plains, has for ages 
been habituated to grow in cultivated ground and to contend 
with the crops which are tilled there. 
241. Plant Life maintained under Difficulties. — Plants 
usually have to encounter many obstacles to their growth or 
even to their bare existence. For every plant which succeeds 
in reaching maturity and producing a crop of spores or of 
seeds, there are hundreds or thousands of failures. It is 
easy to show by calculation in the case of any particular 
kind of plant, how small a proportion the seeds which live 
must bear to those which are destroyed. The common 
morning-glory (Ipomcea purpurea) is only a moderately pro- 
lific plant, producing, in an ordinary soil, somewhat more 
than 5,000 seeds.” If all these seeds were planted and grew, 
there would, of course, be 3,000 plants the second summer, 
sprung from the single parent-plant. Suppose each of these 
plants to bear as the parent did, and so on. ‘Then there 
would be: 
1 See the article ‘“‘ Pertinacity and Predominance of Weeds,” in Scientific Papers 
of Asa Gray, selected by C. S. Sargent, vol. II, pp. 234-242. 
2 Rather more than 3200 by actual count and estimation. 
