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JOURNAL OF THE ROYAL PIORTTCULTURAL SOCIETY. 



deadly than the race as it is found under natmal conditions. The 

 reactions of Sac char oniyces can also be at once altered by changing the 

 composition of the sugars in which it is cultivated. These points are 

 so familiar to students of bacteria and yeasts that perhaps the full 

 significance of them is hardly realized. 



Fungi are also exceedingly adaptable, as is shown by many well- 

 known facts. 



Thus the manner in which the various races of one species of 

 Puccinia and of other Uredineae have become so specialized that they 

 can infect only one out of many possible host-plants, seems to me to 

 show that they have managed to produce just the particular excretion 

 which in any one case will overcome the resistant excretion developed 

 in the particular flowering plant which is invaded. 



Nor have they lost the power of developing new excretions when an 

 occasion presents itself. 



Soon after the South African Nemesia was introduced into European 

 gardens, the Scotch Fir Eust managed to attack and establish itself 

 in a perfectly new host-plant, and in all probability a new biologic race 

 of this fungus will be formed.* 



This adaptability is not confined to the Uredineae, for quite similar 

 biologic races have been discovered in Ustilago, Claviceps, and 

 Ery'siphe. Special forms of all these fungi appear to exist, each adapted 

 to attack one particular species. 



MasseeI has recorded a most interesting experiment in which he, by 

 skilful and sympathetic assistance, made it possible for a fungus, which 

 had previously never had anything to do with Begonia, to become a 

 regular parasite of that plant and able to establish itself on perfectly 

 healthy leaves. Chittenden | has also suggested that the common 

 saprophytic fungus Cladosporium has become regularly parasitic on 

 the apple tree. Salmon' s§ experiments are also very interesting in 

 this connexion, as they give a clear explanation of the probable manner 

 in which infection occurs. 



Perfectly healthy leaves will resist the poisonous excretions of 

 invading fungi, whilst diseased leaves are unable to do so. Quite a 

 slight injury seems to be decisive in battles of this kind. 



If a small piece is sliced off a leaf, it is unable to resist the enemy : 

 when a hot razor was pressed for a few seconds against the upper 

 surface, the vitality of the leaf was just by so much impaired that the 

 fungus enemy could make good its hold. 



SoRAUER and Gieslar|| have also in several instances pointed out 

 that both fungi and insects can settle themselves on sickly or injured 

 plants, whilst those which are in full vigour are either immune or 

 recover after a slight struggle. 



The point which seems clearly brought out by all these experiments 



* Fischer, Atti Soc. Elv. d. Locarno, 1903. 

 t Massee, Journ. E.H.S. Vol. 28; Proc. R.S. Ser. B, 1904. 

 t Chittenden, Journ. R.H.S. Vol. 33. Part 2, p. 509. 

 ■ § Salmon, Trans. Royal Soc. 1904, p. 107. 

 II Sorauer, Zeit. f. Pjianzenhranhh. Band 10, Heft 5. 



