390 FOUNDATIONS OF BOTANY 
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 succeeded in establishing themselves 
in so many cases, because they were superior in vitality 
and in their power of reproduction to our native plants. 
This may not improbably be due to the fact that the Euro- 
pean and western Asiatic vegetation, much of it consisting 
from very early times of plants growing in comparatively 
treeless plains, has for ages been habituated to flourish in 
cultivated ground and to contend with the crops which 
are tilled there. 
458. Plant Life maintained under Difficulties. — Plants 
usually have to encounter many obstacles even to their 
bare existence. For every plant which succeeds in reach- 
ing maturity and producing a crop of spores or of seeds 
there are hundreds or thousands of failures, as it is easy 
to show by calculation. The morning-glory (Jpomea pur- 
purea) is only a moderately prolific plant, producing, in 
an ordinary soil, somewhat more than three thousand — 
seeds.1 If all these seeds were planted and grew, there 
would be three thousand 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 : 
9,000,000 plants the third year. 
27,000,000,000 plants the fourth year. 
~ 81,000,000,000,000 plants the fifth year. 
243,000,000,000,000,000 plants the sixth year. 
729,000,000,000,000,000,000 plants the seventh year. 
1 Rather more than three thousand two hundred by actual count and 
estimation. 
_ 
