48 



THE SOIL AND THE SAP 



Now the effect of all these animals pushing and shoving 

 through the soil is to open up passages for the air to enter, 

 and the soil becomes aerated, and when rain comes these 

 passages further convey water to the deeper layers of the 

 earth. We have said that the soil is as teeming with life as a 

 great city, and as regards the unicellular inhabitants there is 

 the same sort of rhythm. The fall and rise in their number is 

 comparable to that which a great city experiences when the 

 number of the population is decreased by a Bank Holiday, 

 or in the richer and more aristocratic quarters by the fatal 

 habit of "week-ending." So numerous are these micro- 

 organisms that it has been stated that "a single salt-spoonful 

 of soil will contain millions of organisms, some active, others 

 in repose, and many in the form of spores." 



If we could only see a mass of soil under a microscope, we 

 should find it as porous as a sponge. From 30 to 50 per cent, 

 is empty spaces — pore-spaces — filled with air or with water, 

 and the water contains plant-food in solution. 



The Sap 



In a moist soil each particle which helps to compose it 



is enveloped in a film of water, and 



when there is plenty of water about 



that water accumulates in the spaces 



which exist in the porous earth. This 



water is taken up into the roots by 



the root-hairs. The process by which 



the thin watery fluid of the soil with 



certain salts in solution is drawn 



through the organic cell wall and the 



protoplasm lining of the root-hair 



and mingles with the denser cell 



sap, is called os7nosis. When certain 



chemical substances in a more or less -r^ ,, . ,. , .. 



,.,,,. ^ , Fig. 11. A portion of a section 



concentrated solution are separated through a young root, showing 



byasemi-permeable membrane from some of the superficial cells 



waterwithsalts, etc., in a more dilute g/o^ing out into root-hairs. 



' ' A thm layer of protoplasm 



solution, the latter hquid will pass (dotted) lines the cell-waU, and 



through the membrane and mingle encloses the cell-sap. 



