264 Salmon,OnS pecialization of Parasiti sm in the Ery siphaceae. 



number of closely crowded simple branches, the conidiopliores, are 

 produced. Frora the apex of each of these conidia are rapidly ab- 

 stricted in basipetal succession, so that when mature each conidio- 

 phore bears a long moniliform chain of spores. Under favoui^able 

 conditions the spores are produced so rapidly that the Oidium 

 patches become covered with a powdery mass of heaped conidia, 

 which disperse in a fine cloud on the host-plant being touched, or 

 blown by the wind. The Oidium of E, Graminis differs from all 

 other Oidia, so far as I have observed, in possessing a bulb-like 

 swelling at the base of the conidiophore. This bulb, which is the 

 first part of the conidiophore to be formed (see flg. 2) is always 

 present, and I have observed it in the fungus on numerous species 

 of Bromus, Loiium, Acena, Triticum, Festuca etc. 



The conidia of the Oidium of E. Graminis are usually oval- 

 cylindrical in shape, and about 30 — 40X15 {i; but sometimes tliey 

 are broadly oval and smaller, about 25X16 {jl, they are very 

 minutely apiculate or bluntly warted at one or both ends (see 

 flgs. 3, 5, 7)1). 



Now, although no differences in morphological characters could 

 be found in the Oidium of E. Gramims growing on dififerent hosts, 

 yet, as will be seen later, many of these Oidia proved to be distinct 

 ,biologic forms.' On this account the fact observed that certain 

 Oidia differ from others in the colour of the ripe or old conidia, 

 seen in the mass on the leaf of the host-plant, becomes of much 

 interest. The powdery masses of old spores are buff or frequently 

 yellowish in the case of the Oidium on Bromus [B commutatus^ 

 B. hordeaceus etc.); on Oats they are rose-coloured; on Festuca 

 elatior var. pratensis the spores seen in the mass are tinged with 

 a pale buff" colour; whilst on another grass (at present unidentified) 

 the spores remain permanently snowy white 2). 



The fact of the occurrence of these apparently distinctive 

 characters becomes of special interest when we remember that the 

 Suggestion has been made by some authors that a ,biologic form' 

 may be the starting-point from which a morphological species is 

 evolved, — in other words, that ,biologic forms' are incipient morpho- 

 logical species. I have observed similar differences of colour in 

 forms of the conidial State of E. taurica Lev. 



The other Oidium used in the experiments was that of E. 

 Polygoni on Trifolium pratense and Pisum sativum. Plauts of T. 

 pratetise covered with this Oidium were gathered in September, 



^) The ^Torula papillata^ described by Bonorden in Bot. Zeit., 1861, 



E. 195, and still maintained as a species under the name Oidium papillatum 

 y Saccardo(Syll. Fung. IV", 46) is clearly nothing eise than the Oidium of 

 E. Graminis. The description given by Bonorden is as follows: sporis ovatis 

 magnis utrinque subpapillatis hyalinis albis; pedicellis simplicibus haud 



septatis et subinflatis. Hab. In foliis vivis Graminum in Guestphalia 



Die Sporen sind oval und an beiden Enden papillentörmig zugespitzt.' 



2) It was doubtless the rosy colour of the spores of the Oidium on Oats 

 which led Bonorden to describe it as a distinct species under the ,name 

 ,Torula ruhellu' with the characters' sporis rubicunais subrubiginosis. See 

 (2, p. 196\ 



