INTEODUCTION TO CEYPTOGAMIC BOTANY. 299 



with facts relative to the distribution of Phsenogams over the 

 surface of the earth. The chemical composition of the soil 

 has a great deal to do -with that distribiition. The occurrence 

 of moulds in closed cavities has been mentioned above, and the 

 extent to which the spores or other reproductive bodies insin- 

 uate themselves in the most deeply seated tissues. Dutrochet 

 professes to have seen mUk globules changed into the spores 

 of moulds, or at any rate developed into moulds. Certain it 

 is, that when milk is arrested for a long time in the udder of 

 the cow,* and forms clots there, moulds are frequently 

 found, and that they find their way into cavities which are 

 almost closed to external influences, as in the urinary bladder 

 of man, and that under more than one form. Such anomalies 

 may at first surprise us, but they may, nevertheless, admit of 

 explanation, as the presence of the larvee of Tapeworms in 

 deep-seated organs, and even in the brain, which was so long 

 a stumbling-block of science. On surfaces freely exposed to 

 the air, as the pulmonary cavity, or communicating with it 

 occasionally, as the walls of the stomach, they are not unfre- 

 quently developed, under peculiar conditions of disease. 



316. One of the most remarkable qualities possessed by 

 certain moulds is the power they have of producing or accele- 

 rating fermentation. Yeast is, in fact, nothing more than a 

 peculiar condition of a species of PenicilUwm, which is capa- 

 ble of almost endless propagation, without ever bearing perfect 

 fruit. Attempts have been made to show that the structure 

 of yeast globules is different from that of ordinary moulds, but 

 without success.f It appears that wherever exosmose and 

 endosmose take place, there is chemical action ; and thus, 

 when yeast is mixed with any saccharine matter, a multitude 

 of points are presented at which an active interchange is going 

 on between the contents of the globules and the external fluid, 

 and at which chemical action can take place. The process is 

 only accelerated by the presence of the ferment, or rather the 

 fermentation is regulated, and the putrefactive and acetous 

 fermentation which might otherwise be estabHshed, effectually 



* Turpin, Memoires du Mua. d'Hist. Nat., 1840. 

 t See Article " Yeast," in Morton's Encyclopaedia. 



