308 INTRODTJCTION TO CKYPTOGAMIC BOTANY. 



the stem and tubers, and at length makes its way either to 

 the external surface or some free cavity, where it fructifies- 

 Difficulties may occasionally arise, but nothing is more stire 

 than that the decay of the leaves and stem is induced entirely 

 by the Botrytis infestans. In a damp warm day, the progress 

 of the disease may be watched with ease, and the parasite may 

 be seen spreading rapidly in a circle, converting all in its way 

 into a mass of decay. As regards the tubers, there is often a 

 second enemy, Fusisporium Solani, which is equally de- 

 structive with the Botrytis; and according as the two are sepa- 

 rate or combined, different appearances arise. A host of other 

 Fungi, such as Acrostalagmus cinnabarinus, soon make their 

 appearance, but these are consequences of the disease, not 

 causes. The Fusispm-ium, it should be observed, puts on 

 different forms according to age and varying condition. Periola 

 pubescens is but a form of it, as is also the minute Dactyliwm, 

 which so often grows at the base of the haulm. Sometimes, 

 again, it appears as an obscure Sporotrichuin, and in some 

 cases produces an extreme degree of hardness, inducing a 

 condition like that of the mummified silkworms (p. 309). 

 Sometimes, on the contrary, it causes rapid and loathsome 

 decay, especially when in company with the Botrytis. The 

 Botrytis alone in general taints the external cells to the depth 

 of a few lines, but does not produce complete decay. On the 

 contrary, the buds of the tubers remain peculiarly active, and 

 will produce an excellent crop in the ensuing year, and one 

 which is not necessarily diseased. 



328. It is quite certain that the species agreeing in habit 

 more or less with these species of Botrytis or Peronospora, 

 referred to the genus Oidium, are not true moulds ; but there 

 is one, at least, which has not yet been reduced to any other 

 genus, which forms concentric patches on apples, pears, plums, 

 &c., producing very rapid decay, which deserves notice, viz., 

 Oiditim fructigenuTn. The concentric tufts are most remark- 

 able, showing even in these lower Fungi a habit like that of 

 larger and nobler species, which causes the formation of what 

 are called fairy rings. When once the mycelium of this species 

 has gained possession, the decay is very rapid, and this often 



