12 THE AGARICACEAE OF MICHIGAN 



apart, are not sufficient to produce a crop outdoors. The exact com- 

 bination of temperature, time and moisture necessary is hard to cal- 

 culate with certainty even after much experience. The mycelium 

 must be sufficiently well developed before it has enough energy to 

 produce fruit-bodies and this development is often slow for reasons 

 not clear to the collector. Every field student of mushrooms knows 

 that there are '•good" collecting grounds and poor collecting places. 

 The conditions mentioned above are probably responsible in large 

 part and yet very similar fields or woods may be exceedingly unlike 

 in the number and abundance of forms which are found in them 

 Just why this is so is not understood. 



The species which grow on living trees are many. The most 

 prominent are here given : 



Armillaria melleu. (On roots of living trees.) 



Armillaria corticatus. (Hickory, maple.) 



Collybia velutipes. (Willow, birch, oak, alder, elm, poplar, etc.) 



Pholiota adiposa. (Maple, oak, ash, etc.) 



Pholiota alboerenulata. (Maple, birch and hemlock.) 



Pholiota destruens. (Yellow birch, willow.) 



Pholiota spectahilis. (Birch, oak, etc.) 



Pholiota squarrusoides. (Maple, birch, beech.) 



Pholiota squarrosa. (Birch, beech, willow, poplar, alder, etc., in 



Europe. ) 



Pleurotus applicatus. (Maple, poplar, birch, etc.) 



Pleurotus atrocuerulius. (Mountain ash, sorbus, etc.) 



Pleurotus ostreatus. (Willow, birch, basswood, beech, oak, wal- 

 nut, locust, etc.) 



Pleurotus sapid us. (Similar to ostreatus.) 



Pleurotus subareolatus. (Maple, basswood.) 



Pleurotus ulmarius. (Maple, elm, basswood, hickory, etc.) 



Volvaria boiubyeina. (Maple, beech, elm, horse-chestnut, etc.) 



These species are probably all capable of some degree of para- 

 sitism, i. e., can affect living tissue. Direct evidence as to the 

 extent of this power in each species is hard to get. The spores prob- 

 ably effect an entrance at a wound, the plant first growing 

 on the dead tissue at the wound, then pushing through the heart- 

 wood which becomes rotten as a result and finally affecting the 

 sapwood and cambium and so injuring the vitality of the tree. 

 Even if not killed by the fungus, the decayed interior is a source of 

 mechanical weakness and the tree is eventually blown down by 

 storms. 



