288 S a 1 m on , On Specialization of Farasitism in the Erysiphaceae. 



Bronies and theii' Puccinia, and answered in the negative. Mars- 

 hall Ward has mvestigated so fully the structui^al peculiarities 

 (thickness of cellwall and cuticle, ,bloom,' size, number, and dis- 

 tribution of hairs, distribution of chlorophyll-tissne, vascular bundles 

 and sclerenchyma) of the leaf of all the species which were used in 

 the main series of experiments described above that we are able, 

 by means of the data given by this autlior, to answer the question, 

 also in the negative, in the case of the Bromes and their Oidium. 



We have seen, for example, that the two species Bromus 

 tectorum and B. brizaeformis, amongst a number of allied species 

 which remain immune, are susceptible — the former species remark- 

 ably so, the latter to a less degree — to the attacks of the three 

 Oidia which giow on B. mterrupüis, B. hordeaceus, and B. commii- 

 tatus. Now an examination of the comparative tables of morpholo- 

 gical and anatomical peculiarities of the species of Bromus in question, 

 given by Marshall Ward, fui'nishes no clue to the reason why 

 these two species should be liable to infection; and we are forced 

 to the conclusion that with respect to the species of Bromus and 

 their Oidia liability to infection or the power of resistance is, to 

 quote the words of Marshall Ward (1. c), ,not to be referred to 

 any observable structural peculiarities but must depend on some 

 internal factor or factors of the host-plant.' 



Experiments were also made using an Oidium which occurred 

 on two cereals, Wheat and Oats. 



First, as regards the Oidium on Wheat, Triticum vulgare. 

 This was found constantly to be unable to infect ßarley, Oats, 

 or ßye (Exper. nos. 17, 20, 29). Li Exper. no. 53 the spores of 

 this Oidium were sown on Triticum vulgare, T. Spelta and Agro- 

 pyron repens. Füll infection resulted on both the species of Triti- 

 cum, but no infection occurred in the two pots of Agropyron repens. 

 In another experiment (Exper. no. 62) the Oidium again fully in- 

 fected T. Spelta and passed over A. repens. A summary of these 

 experiments is given at Table 10. 



The Oidium on Avena satimi was then used in a series of 

 experiments. This was found in the flrst experiment (Exper. no. 

 12) to be unable to infect Wheat and Barley, also (using potted 

 plants) Festuca elatior var. pratensis, Poa annua, and Dactylis 

 glomerata. The Oidium was then tried twice (Exper. nos. 21 and 

 30) on Rye, but failed to infect in both cases. Avena brems was 

 then inoculated with the Oidium on A. sativa, with the result that 

 füll infection was obtained (Exper. no. 42). In the next experiment 

 (Exper. no. 46) the Oidium on A. sativa was sown on A. 7iuda^ 

 A. brems, A. sterilis, and A. strigosa, and on all these species füll 

 infection resulted. The Oidium was then sown (Exper. no. 54) on 

 A. Orientalis, A. strigosa, ArrhenatJierum elatius, Triseium pratetise, 

 Phleum pratense, Festuca heterophylla, F. elatior var. arundinacea^ 

 Poa annua, Dactylis glomerata, Alopecurus pratensis^ and Lolium 

 italicum.. Infection resulted on A. orientalis and A. strigosa only, 

 the other species being completely passed over. A summary of the 

 experiments in which the Oidium on Acena sativa was used is given 

 at Table 11. 



