132 



GROWTH AND WORK OF PLANTS 



Fig. 96. 



Beech root in wood 

 humus freed from 

 fungus, root hairs h. 

 (After Frank.) 



mycorhiza appropriate and useful. In some cases these fungus 



threads are the spawn of certain mushrooms and puff balls. 



The mycelium of the truffle, an edible fungus of great com- 

 mercial value in southern France and in Italy, 

 is supposed to have a similar relation to the 

 roots of certain forest trees. 



207. Symbiosis. This living together in 

 close physiological relation of two different 

 organisms is called symbiosis. In the case of 

 the root tubercles of leguminous plants the 

 relation is one of mutual benefit, each partner 

 in the symbiosis (each partner is a symbionf) 

 deriving some benefit from the other. The 

 same relation is supposed to exist in the case 

 of the mycorhiza of forest trees. This kind 

 of symbiosis is called mutualistic symbiosis since 

 the two organisms mutually support one 



another. Another well-known example is seen in the case of 



the lichens (see paragraph 447). 



Another kind of symbiosis occurs in the relation of a parasite to its host 

 where the parasite living on or in the host injuries or kills it but the host 

 receives no benefit. This is antagonistic symbiosis. So there is contact 

 symbiosis where two organisms living side by side, work together, each one 

 supplying the other with some product of its work. An example of this is 

 seen in the case of the bacterium (Clostridium pasteurianum) which lives in 

 the soil in conjunction with two green algae. The algae supply the bacterium 

 with carbohydrates and it is then able to fix free nitrogen and this combined 

 nitrogen can be used by the algae. Related to this, but a step farther, are 

 the many cases of metabiosis where successive organisms digest or ferment 

 the product of previous ones. Example, a common mold (Aspergillus oryzce) 

 growing on rice converts the starch into sugar, then yeasts ferment the sugar 

 to alcohol, and then the acetic acid bacteria ferment the alcohol to acetic 

 acid. 



FERMENTS OR ENZYMES. 



208. Ferments or enzymes are substances produced in dif- 

 ferent parts of plants and animals which act on starch and other 

 substances in such a way as to digest or dissolve them (example 



