xvin PILEUS AND LAMELLA 191 



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 1 08 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, /), on the upper or distal end of which is borne 

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

 end of the stalk is in connection with an underground 

 mycelium (my), from which it springs. 



On the underside of the pileus are numerous radiating 

 vertical plates or lamella (I) extending a part or the whole 

 of the distance from the circumference of the pileus to the 

 stalk. In the common edible mushroom (Agaricus cam- 



