io8 



PLANT BIOLOGY 



example. Lay a piece of moist bread on a plate and 

 invert a tumbler over it. In a few days it will be moldy. 

 The spores were in the air, or perhaps they had already 

 fallen on the bread but had not had opportunity to grow. 

 Most green plants are unable to make any direct use of 

 the humus or vegetable mold in the soil, for they are not 



saprophytic. The shelf- 

 fungi (Fig. 134) are sap- 

 rophytes. They are com- 

 mon on logs and trees. 

 Some of them are perhaps 

 partially parasitic, extend- 

 ing the mycelium into the 

 wood of the living tree 

 and causing it to become 

 black-hearted (Fig. 134). 

 Some parasites spring 

 from the ground, as other 

 plants do, but they are 

 parasitic 011 the roots of 

 their hosts. Some para- 

 sites may be partially 

 parasitic and partially 

 saprophytic. Many (per- 

 haps most) of these 

 ground saprophytes are 

 aided in securing their 

 food by soil fungi, which spread their delicate threads over 

 the root-like branches of the plant and act as intermedi- 

 aries between the food and the saprophyte. These fungus- 

 covered roots are known as mycorrhizas (meaning "fungus 

 root"). Mycorrhizas are not peculiar to saprophytes. 

 They are found on many wholly independent plants, as, 



Fig. 134. — Tinder Fungus (Pohforus 

 igniarius) on beech log. The external 

 part of the fungus is shown below; the 

 heart-rot injury above. 



