WORK OF THE ROOT 57 



In order to show the importance of the various con- 

 stituents, plants should be grown in the following incom- 

 plete culture solutions and the results compared : (1) Cul- 

 ture solution without potassium nitrate (Fig. 25, a), (2) 

 without magnesium sulphate, (3) without calcium phos- 

 phate, and (4) without ferric chloride (Fig 25, c). 



It will be interesting to carry out such experiments, but 

 as the results are often very variable and contradictory, it 

 is well to try several of each and express the average results 

 by means of curves. The jars containing the food solu- 

 tions should be covered with opaque paper in order to 

 exclude light, and the solutions should be renewed at least 

 once a fortnight and the vessels thoroughly cleaned and 

 sterilized with boiling water, as Algae and other organisms 

 are liable to develop in them. The plant may be sup- 

 ported by a split cork (d), as shown in the figure. Keep the 

 cork and the part of the stem passing through it dry, 

 otherwise it may be attacked by Fungi and decay. 



Analyses of plants show that a number of substances 

 are commonly present which water-culture experiments 

 prove to be non-essential. Silica, which is present in large 

 quantities in Grasses, Horsetails, &c, is one of these. 

 Chlorine also is necessary to only a very few species. On 

 the other hand, if potassium compounds or nitrates are 

 omitted, the plant suffers. If iron is omitted, chlorophyll 

 is not developed, and the leaves are sickly yellow in colour 

 (Fig. 25, c). By means of water-culture experiments we 

 learn that the food of a green plant must contain the 

 following elements : oxygen, hydrogen, nitrogen, calcium, 

 magnesium, potassium, phosphorus, sulphur, and iron ; all 

 of which a plant obtains in solution from the soil. One 

 important element, carbon, forms about half the dry weight 

 of a plant, yet is not present in a culture solution, nor is the 

 plant able to obtain it from the soil. The question there- 

 fore arises How does a plant obtain the carbon which 



