The First Land Plants 



minute grottos and irregular winding fissures which 

 penetrate in and amongst these particles. The total 

 surfaces of these minute subterranean hollows must be 

 enormous. Let us take a very simple case and suppose 

 that all the earth particles are small spheroids one 

 thousandth of an inch in diameter. It has been calcu- 

 lated that with such a soil the surfaces of all the 

 particles in a cubic foot of earth would amount to at 

 least one acre. A thin film of moisture lines and 

 carpets all these internal hollows, cracks, fissures, and 

 miniature grottos ; in this moisture the reader must 

 imagine a liberal sprinkling of bacteria, for there are 

 at least 100,000 in one gramme (15.432 grains) of 

 garden earth. 



Some of these are actively squirming or cork-screwing 

 their way through the watery film, others are motion- 

 less. They are certainly small, for one could pack 

 about 1,700,000,000 of ordinary bacteria in a cubic 

 millimetre of water without inconveniently crowding 

 them.* 



Some of these germs are benevolent, and will be 

 breaking up the bodies of deceased insects, leaf mould 

 and the like ; others may be closterium or azotobacter 

 forming nitrates : but many are malignant typhoid germs, 

 or vegetable fiends. Here and there one might see 

 running through some of the caverns the exquisitely 

 divided and finely branching threads of a mould fungus ; 

 in another place a plant's root-hairs and exploring 

 rootlets which may be themselves clothed in a fringing 

 mantle of fine fungus threads. 



* A cubic millimetre measures about -j^^th of an inch each way. There are 

 giant bacteria 30 fx {30 x .00003937 inches) thick, but the smallest are about 

 .4 /i in diameter (.00015748 inch). Errera^° in an interesting paper tries 

 to show that we cannot imagine any bacteria less than .01 fi (•000003937 

 inch), for it could only contain some ten molecules of albuminoid. This is 

 in a way encouraging. 



49 D 



