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 74th of an inch each way. There are 
giant bacteria 30 « (30 x .00003937 inches) thick, but the smallest are about 
.4 p» in diameter (.00015748 inch). Errera?® in an interesting paper tries 
to show that we cannot imagine any bacteria less than .o1 @ ('000003937 
inch), for it could only contain some ten molecules of albuminoid. This is 
in a way encouraging, 
49 D 
