108 PLANT-LIFE ON LAND [ch. 



square inch of its surface we arrive at the output of 

 1^ millions of conidia as the result of a few days' 

 growth over that limited area. 



As a last example, taken this time from the 

 Bacteria, it has been stated by Cohn that in a suitable 

 medium at a temperature of 35° C. a cell of Bacillus 

 subtilis will take about 20 minutes to divide into two. 

 If this process be repeated continuously, and the 

 resulting cells all retain their vitality, the product of 

 a single germ would in a period of nine hours (i.e. in 

 a single night) amount to over 134 millions ! 



Kerner has illustrated the results which would 

 follow if such numerical increase as that we have 

 been contemplating were actually carried out un- 

 checked, in the case of certain Seed-Plants. He 

 points out that if a Henbane plant developed 10,000 

 seeds in one year, and 10,000 plants sprang from 

 those seeds next year, and themselves produced 

 10,000 seeds each, by the end of five years ten 

 thousand billions of Henbane plants would have 

 come into existence. As the entire area of the dry 

 land of the earth is approximately one hundred and 

 thirty-six billion square metres, and there is room for 

 about 73 Henbane plants on one square metre, if all 

 the seeds referred to ripened and germinated, the 

 whole of the dry land would at the end of five years 

 be occupied. Or, in the case of the Flixweed 

 (Sisymbrium Sophia), the normal multiplication from 



