NATURE OF PLANTS 53 



substances as the plant requires. In this way the materials iri 

 the soils are drawn through the delicate walls of the root hairs, 

 a process termed osmosis. Some of the acids in the root hairs 

 may also reach through the cell walls and so come in contact with 

 the insoluble substances, dissolving them and thus rendering 

 them capable of absorption by osmosis. Certain it is that CO2 

 is given off by the root hairs and as we have seen, this gas enter- 

 ing into the film water would enable it to dissolve calcium — a 

 fact easil)'' demonstrated by allowing the roots to grow over the 

 clean surface of oyster shells or pieces of polished marble. Deli- 

 cate grooves in the marble show where the roots have dissolved 

 the calcium in the course of their growth. Some roots show an 

 acid reaction, as is indicated by their ability to turn blue litmus 

 paper red ; but all roots do not react in this way and it is impossible 

 to state whether acids, other than carbonic acid which would 

 result from the elimination of CO2, are excreted. 



We are now prepared to understand why the root hairs are 

 developed just back of the zone of elongation. Any disturbance 

 of these delicate absorbing organs, such as the extension of that 

 part of the root to which they are attached, would result in 

 tearing and killing them. 



22. Familiar Facts About Roots. — The character of the root 

 and its relation to the physical properties of the soil explain many 

 familiar phenomena of plant life. We see why plants must be 

 repotted. The root hairs, which only live for a few days or a 

 few weeks, are constantly being formed back of the growing 

 zone and dying off behind. This would result in not only placing 

 the new root hairs in new soil but they would also be removed 

 in a measure from any harmful substances that may have arisen 

 through the activity of the older root hairs. In this connection 

 note should be made of the suggestive work that is being ac- 

 complished today tending to show that roots give off substances, 

 probably of organic nature, which are toxic in character and so 

 render the soils unfit for plant growth. These substances are 

 readily oxidized and rendered harmless. The presence of oxy- 

 gen and certain fertilizers increases the oxidizing power of the 

 roots. While this theory is far from being established it is pos- 



