302 INTRODUCTION TO CRTPTOGAMIC BOTANY. 



they are immersed, is sufficient to displace them. With every, 

 drawback, however, they still afford much interest with ordi- 

 nary powers of manipulation. 



321. It was stated above, that moulds have a singular 

 facility of adapting themselves to particular circimistances ; but 

 not only do changes take place when they are immersed in 

 fluid, but the degree of moisture to which they are exposed 

 has a marked influence upon their development, especially 

 when accompanied by an absence of a free current of air. 

 Under circumstances where the moisture is too great for 

 normal development, two changes take place. In one the 

 mycelium, though at first apparently in its normal condi- 

 tion, soon ceases to grow externally, and forms within a com- 

 pact cellular mass, which is at length entirely exposed by the 

 gradual disappearing of the floccose threads, forming, in fact, 

 what are commonly called species of the genus Sclerotium ; in 

 the other case, not only is the mycehum developed, but the 

 fertile threads appear and produce fruit, though under a different 

 form from the normal condition of the plant, transforming, for 

 instance, the genus Penicillium into GoremiuTn. Blastotri- 

 cliiim, Confervoides, Corda, is probably a state of some Dac- 

 tyUum, as Dactylium roseum {Trichothecium roseuTn of 

 authors). At any rate, it has not the air of a normal plant, 

 and is scarcely separable as a genus, if it were from Bac- 

 tylium. 



322. Their distinctive characters depend upon differences of 

 ramification, of carbonization, and of the more or less compli- 

 cated nature of the spores, which may be either simple or 

 variously septate. There is often a distinct mycelium, called 

 an Hy^jhasma, from which arises a forest of fertile threads ; 

 but the floccose creeping mycelium may be reduced to a 

 few cells, and the fertile threads to mere points on the 

 mycelium. 



823. Traces of moulds occur occasionally in the cells of 

 fossil woods, or at least threads which are not referrible to any 

 other known phenomena of vegetable organisation ; but if the 

 nature of these is doubtful, there is no uncertainty whatever 

 about the occurrence of at least two genera in Amber, a sub- 



