190 LIVING PLANTS 



world in round numbers is fifty-one million 

 square miles, and supposing for the sake of 

 the illustration, that every foot of this ground 

 is free from all encumbrance and equall}^ 

 capable of supporting plant life, we shall 

 arrive at the astounding result that a single 

 parent plant b^^ the tenth year will have 

 given rise to enough plants to occupy ever\^ 

 foot of this great area, and over five hundred 

 thousand billions besides. This means that 

 an annual plant, increasing at a fifty-fold rate, 

 has the capacity to supply a plant for every 

 eight square inches of land surface of the 

 whole globe within one decade. 



Such marvelous results of fecundity are al- 

 most past belief, and it is worth while to 

 Weeds as inquire if any facts exist that give counten- 



examples ance to such deductions. The ubiquity and 



prolificacy of weeds are proverbial, but upon 

 closer acquaintance even the popular fancy 

 seems scarcely to reach the reality, to judge 

 from the records. Sturtevant found that a 

 plant of shepherd's purse, one of the most 

 common and most insignificant of weeds, bore 

 fully 12,000 seeds, that a plant of burdock 

 had 40,000 seeds, and that a number of other 

 common weeds were equally fertile. But the 

 list that he examined reached its climax in 

 the purslane, a plant which by its obtrusive- 

 ness and pertinacity has become the symbol 



