4 INSECTICIDES, FUNGICIDES, AND WEED KILLEES. 



biotic associations the lichens must be regarded as the most typical and 

 the most perfect example. It is the most complete association known, 

 where the elements, fungus, and alga are so closely associated that 

 henceforth it is impossible for them to live apart. The most interesting 

 cases are those of evident symbiosis between certain fungi and trees 

 such as Fagus (beech), Corylus (hazel), Castanea (chestnut), and 

 several species of Conifercz. In this symbiosis, known as Mycorhiza, 

 the mycelium of the fungus invades the roots of the tree, and, whilst 

 borrownng the necessary elements of life from the tissues of the plant, 

 cedes to it the nutritive elements favourable to its growth. Other 

 symbioses of great importance to agriculture exist between certain 

 cultivated plants and certain microbes. We have, on the one 

 hand, the Bacteriorhiza of Hiltner and Stromer, in which bacteria, 

 whilst drawing their nutriment from the cells of the root epidermis 

 of Beta (beet) and Pisum (pea), without injuring the latter pre- 

 vent injurious fungi, such as Phoma and others, from invading and 

 destroying these plants. Again, microbes and Leguminosce are as- 

 sociated together : Bhizobium leguniinosarum (Frank) and Bacillus 

 radicola (Beijerinck). These bacteria live on the excrescences, the 

 characteristic nodosities which they produce on the roots of the 

 LeguminoscB. If they draw the necessary elements of life from the 

 cellular tissue of the root, they as a matter of compensation cede to 

 the nursing plant nitrogen of the air in an assimilable form. These 

 symbioses are favourable to the development of the plant, and the 

 rupture of this association necessarily creates a less prosperous state, 

 more akin to a grave pathological condition ; the more complete the 

 symbiosis the more the health of the plant depends on the nutritive 

 elemeats with which these associates are capable of supplying it. In 

 the disinfection of the soil by chemical agents it sometimes happens 

 that by destroying noxious parasites, the mutualistic parasites are des- 

 troyed at the same time, and that in curing one disease another is 

 created. Pure cultures of these useful bacteria, or of the mycelium of 

 mutualistic fungi, must, therefore, be spread on the fields some time 

 after disinfection. Pare cultures of these bacteria are on the market, 

 under the name of " Nitragine," etc. The very contradictory results 

 obtained on the large scale by the use of pure bacteria arise from the 

 fact that these should be appUed, not upon any soil, but on a soil dis- 

 infected by carbon disulphide. The soil is like a wort, and there is a 

 great analogy between selected leavens (yeasts) and soil bacteria. The 

 selected leavens cannot yield a result except on wort previously freed 

 — -by sterilization, by means of heat — from any other leaven which 

 would hinder their development. In the same way, the soil which it 

 is desired to sow with mutualistic bacteria should be freed, by previous 

 disinfection, from the elements which can, by their presence, oppose 

 the normal evolution of these useful bacteria. Each plant possesses 

 its own bacteria, and in each particular case requires the aid of a pure, 

 selected, and well-defined culture of microbes. 



2. Antagonistic Symbiosis. — The most common result of the an- 

 tagonism between auimated beings is the parasitic disease — general or 

 local — to which the plant, badly equipped, or in bad condition to resist 



