324 



THE MICROSCOPE AND ITS REVELATIONS. 



multiplication have a large share in the increase and extension of the 

 disorder, just as the growth of the Yeast-plant excites and accelerates 

 fermentation; while its reproduction enables this action to be indefinitely 

 extended through its instrumentality. But just as the Yeast-plant 

 will not vegetate save in afermentible fluid that is, in a solution which, 

 in addition to sugar, contains some decomposable nitrogenous matter 

 so does it seem probable, on consideration of all the phenomena of the 

 Potato and Vine diseases, that neither the Peronospora of the one nor 

 the Oidium of the other will vegetate in perfectly healthy plants; but 



JEcidium tussilaginis ; A, portion of the plant magnified: B. section of one of the concepta- 

 cles with its spores. 



that a disordered condition, induced either by forcing and therefore 

 unnatural systems of cultivation, or by unfavorable seasons, or by a 

 combination of both, is necessary as a ' predisposing ' condition. This 

 condition, in the case of the Potato-disease, is said by Prof. De Bary to 

 consist in an undue thinness of the cuticle, accompanied by excessive 

 humidity; whereby the sporules of the fungus will germinate on the 



Clavaria crispula 



portion of the mycelium magnified. 



surface of the plant, sending out processes which penetrate to its inte- 

 rior, though otherwise germinating only on cut surfaces. 



321. In those lower forms of this Class which have been now described, 

 there is not usually any complete separation between the Nutritive or 

 vegetative, and the Reproductive portions of the fabric. But such a sep- 

 aration makes itself apparent in the higher; and this in a very curious 

 mode. For the ostensible Fungi, known as Mushrooms, Toadstools, 

 Puff-balls, etc., consist, in fact, of nothing else than the organs of 

 gonidial fructification (Fig. 260), inclosing an enormous mass of non- 



