48 THE WORK OF THE ROOTS [chap. 



require from the small quantity that the water holds in 

 solution and constantly renews from the atmosphere 

 with which it is in contact. For this reason it is advan- 

 tageous to blow a little air through the culture solution 

 once a week, especially when the root development has 

 become at all extensive. The result of cutting off the 

 oxygen can be shown by replacing the culture solution 

 in one of the bottles by some water which has been 

 boiled, to free it from any dissolved air, and then cooled. 

 Fill up the bottle with this water close to the neck of the 

 plant, and then pour on it a very thin layer of olive oil, 

 which will effectually cut off the access of air to the 

 water in which the roots are distributed. In a day or 

 so the plant will begin to show signs of ill-health, and 

 will rapidly die. That the respiration of the root is 

 accompanied by the evolution of carbon dioxide, can be 

 seen by placing another plant's roots in distilled water — 

 not, however, freed from dissolved air — pouring off this 

 water on the following day, and adding to it lime water. 

 The milkiness which ensues shows that carbon dioxide 

 has been given off from the root, and has remained to 

 some extent dissolved in the water. The same demon- 

 stration may be effected by letting the plant's roots 

 themselves dip into lime water. 



In addition to the water, it is the function of the root 

 to take in the various substances required for the 

 nutrition of the plant. These are obtained from the soil, 

 and constitute the plant's ash. A simple experiment 

 can be carried out to demonstrate that any substances 

 which get into the plant must first of all be dissolved in 

 the water in contact with the root. Colour the solution 

 in one of the water-culture bottles a bright pink with 

 eosin ; to another bottle add carmine or Indian ink. The 

 eosin, being in solution,willgraduallypenetrate the young 

 barley plant, and may be seen colouring the veins ; but 



