CHAPTER XLII 



LAND-DBAINAGE 



IN the bottom of a flower-pot you will find a small 

 round hole. Over this hole it is customary to 

 lay a bit of broken tile, and on this, if the plant to 

 occupy the pot is delicate, a few small pebbles. This 

 done, the pot is filled with vegetable mold. Why 

 this hole, this bit of tile, these pebbles? That is 

 what we are now about to consider. 



"Water is absolutely indispensable to plants, since 

 it is the medium that dissolves the various nutritive 

 ingredients of the soil and thus renders them ca- 

 pable of assimilation by the roots. Accordingly the 

 soil penetrated by these roots must be constantly 

 supplied with sufficient moisture either by rainfall 

 or by artificial irrigation. But air is not less in- 

 dispensable. It disinfects the soil and by causing 

 slow combustion of the humus gives rise to a slight 

 but uninterrupted liberation of carbonic acid gas, 

 one of the nutritive substances required by vegeta- 

 tion. Should the roots be cut off from this life- 

 giving agency, they would languish and finally de- 

 cay. Thus it is that if vegetation is to thrive the 

 soil in which it grows must have at the same time 

 both air and water. But if the bottom of the flower- 

 pot has no opening, or if its opening is stopped up, 



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