XVIII PILEUS AND LAMELL.1-: 191 



with the poorest food, which would be too bad for higher 

 fungi. It lives in the human ear ; it does not shun cast-off 

 clothes, damp boots, or dried-up ink. Sometimes it contents 

 itself with a solution of sugar with a very little [nitrogenous] 

 organic matter, at other times it appears as if it preferred the 

 purest solution of a salt with only a trace of organic matter. 

 It will even tolerate the hurtful influence of poisonous 

 solutions of copper and arsenious acid." It flourishes best 

 in a solution of peptones and sugar. 



This eclecticism in matters of diet is one obvious ex- 

 planation of the universal occurrence of Penicillium ; another 

 is the extraordinary vitality of the spores. They will ger- 

 minate at any temperature between 1-5° and 43° C, the 

 optimum being about 22° C. They are not killed-by a dry 

 heat of 108° C, and some will even survive a temperature 

 of 120°. And lastly, they will germinate after being kept 

 for two years. 



We have seen that the form of a Penicillium growth is ir- 

 regular, and is determined by the surface on which it grows. 

 There are, however, certain fungi which are quite constant 

 and determinate both in form and size, and are yet found 

 on analysis to be formed exclusively of interlaced hyphae, 

 that is, to belong to the type of linear aggregates. Among 

 the most striking of these are the mushrooms and toad- 

 stools. 



A mushroom {Agaricus) consists of a stout vertical stalk 

 (Fig. 41, A, St), on the upper or distal end of which is borne 

 an umbrella-like disc ox pileus {/). The lower or proximal 

 end of the stalk is in connection with an underground 

 mycelium iiny), from which it springs. 



On the under side of the pileus are numerous radiating 

 vertical plates or latnella (/) extending a part or the whole 



