INTRODUCTION TO CBYPTOGAMIC BOTANY. 259 



and subject to decomposition. Moulds will not appear on 

 substances placed out of the possibility of communication 

 with the external air, or where the spores are obliged to 

 traverse sulphuric acid, or any other corrosive matter, before 

 they can alight upon the substance. 



257. As regards their propagation within closed cells or 

 cavities, there is no doubt considerable difficulty. In the 

 potato disease, we have seen Botrytis infestans make its 

 appearance in a few hours on the surface of a cut tuber ; 

 but, at the same time, microscopic examination has shown 

 the mycelium of the Fungus traversing the cells. But this 

 is far less astonishing than that the same species should grow 

 and fructify in the cavities of the fruit of the Tomato, when 

 there is no apparent lesion of the walls. It is quite certain, 

 from the propagation of bunt in wheat, that the mycelium, or 

 some subtle form of it, must have the power of penetrating to 

 the remotest parts, and of retaining life in those uninjured 

 for some months, tdl the tissue of the seed is sufficiently 

 advanced for its full development. The occurrence of Tricho- 

 thecium rosewm, within the cavity of ripe nuts, is perfectly 

 analogous, but, perhaps, some subtle aperture may let in the 

 spores or spermatia,* though it may be too small to escape 

 our notice. The propagation of moulds within closed eggs 

 appears to present no greater difficulty. It is very possible 

 that a spore might be attached to the egg before the deposition 

 of the calcareous coat, or, as in the case of the nut, there may 

 be an accidental pore in the shell, too small to be detected. The 

 marvellously narrow cracks in drain tiles, through which roots 

 will penetrate, and when they have once penetrated the enor- 

 mous extent to which they are often multiplied, may make 

 us less ready to see anything beyond our powers of conception, 

 in the occurrence of moulds in such localities, without calling 

 in spontaneous generation to the solution of the difficulty. 

 Fries has remarked, that in Rhytisma acerinum, the sporidia 



* The spermatia of this species which, according to Hoffmann, ger- 

 minate, though, as far aa he has seen, abortively, are extremely small, 

 when compared with the true fruit. Bot. Zeit, vol. xii., tab. 8, fig. 2. 

 17* 



