10 EXPERIMENTAL PLANT PHYSIOLOGY 



Having now proved that a root absorbs water, it 

 must next be determined whether other substances are 

 absorbed with the water ; and, if so, what kind of sub- 

 stances can be absorbed. 



Experiment 4 



Aim. — To determine the class of substances that a 

 root can absorb. 



Method. — Two coloured substances are chosen so 

 that any absorption that takes place may readily be 

 seen by the change of colour that the plant thereby 

 undergoes. Powdered eosin and powdered carmine 

 are suitable for the purpose, as they are both bright 

 red in colour. A small quantity of these substances is 

 put respectively into two test-tubes and well shaken up 

 with water. The eosin dissolves. The carmine simply 

 remains suspended in the water; it is a very finely divided 

 powder and does not, therefore, quickly settle at the 

 bottom. 



A broad-bean seedling is then placed in each test- 

 tube so that its roots are in the liquid. 



Observations. — The seedling whose roots are in eosin 

 soon becomes red all over; the colour of the other 

 seedling remains unaltered. 



Inferences. — The root can absorb substances that are 

 soluble, but it cannot absorb insoluble substances. 



{Note. — This experiment can be amplified by the use 

 of other coloured substances of each class. It should 

 be noticed that no account is taken here of the absorp- 

 tion of gases, which is reserved for another place.) 



The broad deductions drawn from Experiment 4 

 appear, on further thought, to need some amplifica- 

 tion. 



It has been proved that a plant cannot absorb a 

 substance unless it is dissolved, yet it is well known 

 that marble is often corroded by the growth of roots 

 upon it, and marble is not soluble in water. Does the 

 root exude something which dissolves the marble ? It 



