SPECIAL ASPECTS OF NUTRITION OF PLANTS 12$ 



available form for the forest trees. It is supposed that the fungus 

 mycelium obtains some benefit from this association with the roots 

 of trees, but this is not well understood. These mycorhizae are 

 shorter, stouter and also branched more than the 

 normal roots. This difference in form, as well as 

 their more complex structure, renders the name 

 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 commercial value 

 in southern France and in Italy, is supposed to have 

 a similar relation to the roots of certain forest trees. 

 206. Symbiosis. This living together in close 

 physiological relation of two different organisms is 

 called symbiosis. In the case of the root tubercles Fi - M. 



, . . i i r Beech root in wood 



or leguminous plants the relation is one or mutual humus freed from 



.... . fungus, root hairs h. 



benefit, each partner in the symbiosis (each partner (After Frank.) 

 is a symbiont) deriving some benefit from the other. The same rela- 

 tion is supposed to exist in the case of the mycorhiza of forest trees. 

 This kind of symbiosis is called mutiialistic symbiosis. Another 

 well-known example is seen in the case of the lichens (see paragraph 

 432). Another kind of symbiosis occurs in the relation of a parasite 

 to its host where the parasite living on or in the host injures 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 (Clos- 

 tridium 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 (Asper- 

 gilliis oryzcz] 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. 



