Inslitidc of France. 61 



M. Decandolle, of Montpeiier, has made known some parasite 

 mushrooms of a nen- i2;enus, which he calls rhizocione.^, or deatk 

 of the routs, beciuise they attach themselves to the roots of the 

 plants, and speedily destroy them. M. Persoon had united, 

 under the head sclerotnti;!, the fleshy fungosities in the interior 

 like truffle^, but without those veins which give to the consis- 

 tence of trufHt^ a marhled appearance. Hedwig had separated 

 from them the trys'inhcs, which exist on the surface of the leavete ; 

 biit "vve might siill obser-\'e in those which remained, characters 

 sufficient to fmnish two genera : one genus not essentially para- 

 sitic, growing on dunghills and among putrid vegetables, has 

 neither roots nor fibres : another (and to this the rhizoctones 

 belong) sends out simple or ramified filaments, grows on the roots 

 ©f livir.g plants, attacks them externally, and kills them by ab- 

 sorbing their nutrition. They are multiplied with rapidity bv 

 means of these filaments, which propagate them from one plant 

 to another, and thus occasion contagious diseases, from which 

 nianv of our luirservmen liave suffered severely. We lyere well 

 acquainted vith one species only, which produces the disease too 

 well known in the Gathicls by the name of dealli of safron. 

 Another, which M. Decandolle descrilies for the first time, ex- 

 ercises its ravages on lueern, the roats of which are lightly em- 

 braced by its threads, which are of a fine lake colour : the stalks, 

 when thus attacked, become yellow, and die speedily : and as 

 the champignon is propagated by radiation, vve soon see in the 

 liicern fields many circular spots tlius deprived of vegetation. 

 The author reconmiends planters to dig around the infected 

 places deep ditches, to prevent the creeping filaments from 

 spreading, taking care to throw tlie earth within the circle, in 

 order not to extend the evil instead of checking it. 



One of the most difficult points in botany consists in fixing 

 accurately the limits of the species, not regarding as such the 

 varieties of soil and climate ; and the chief way to avoid this 

 error, is not to admit among the characters of the species the 

 peculiarities of organisation, the mutability of which has been 

 ascertained beyond a doubt. M. Desvaux having applied this 

 method to the rose trees, and perceiving that several of their 

 pretended species differed only in characters which were various 

 in the same individual, succeeded in reducing considerably the 

 nominal species of this genus. He has shown, for example, that 

 the common wild rose (rosa caninoj presents twenty-one varie- 

 ties, the differences of which might be expressed by descriptions, 

 but which pass insensibly into each other ; and that thirteen of 

 these varieties have been improperly raised to the rank of species 

 by certain authors : six otiicr pretended species have also been 

 displaced from this raiikj and assigned to the ruse of the alps : 



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