CO-OPERATIVE NUTEITION 11 



soluble nitrogenous organic compounds, must furnish 

 the supply of nitrogen to the roots. 



The parasitic plants, already referred to, feed on the 

 nitrogenous compounds contained in the sap of the 

 host plant. Fungi attack organic matter, living or 

 dead, and obtain from it both their nitrogen and 

 carbon. 



Co-operative Nutrition.— In some cases the feeding 

 power of roots is modified to a very considerable extent 

 by their union with another vegetable organism. Thus 

 certain trees (as oak, beech, hazel, chestnut, willow 

 and pine), heaths and orchids, growing on a soil rich 

 in humus, may possess no root hairs, but have their 

 roots covered by a fungus, the hyphse of which pene- 

 trate the root. This root fungus {mycorhiza) feeds on 

 the decaying vegetable matter of the soil and nourishes 

 the tree with the material which it has prepared. 



Another remarkable instance is afforded by legumin- 

 ous plants. All species of papilionace(B have tubercles 

 on their roots, unless the plant has been grown from 

 seed in a sterilised soil. These tubercles are occasioned 

 by the invasion of an organism, having the characters 

 of a bacterium, present in the soil. When the seeds 

 of peas, lupins, or vetches are sown in sterilised sand, 

 containing the necessary ash constituents of plants, 

 but no nitrogen, only a small, dwarfed growth is 

 obtained, and the roots are not famished with tuber- 

 cles. If, however, a minute quantity of ordinary soil 

 is added, tubercles appear on the roots, and the plant 

 now grows vigorously. At the end of the experiment 

 it is found that the quantity of nitrogen in the crop is 

 far greater where tubercles have been formed than 



