Xotes OK R. Acerinum and R. Pseudoplatani. 1S3 



NOTES ON RHYTISMA ACERINUM AND 

 RHYTISMA PSEUDOPLATANI. 



OVith Plate VII.) 

 By Rose Bracher. M.Sc. 



Introduction. 



During an investigation of the c\"tolog3" of the genus Rhytisma, 

 several obsen'ations relating to the general life-histor^' have 

 been made which have not hitherto been recorded. The object 

 of this paper is to give a preLLminar\" account of these observa- 

 tions which may sen'e as an introduction to the c\*tologicaI 

 study. 



The material used for investigation has been collected in 

 England and the United States. The Enghsh form gro\VLng upon 

 Acer Pseudoplatanus L. is separated by Miiller 1,1913) as a new 

 species, R. Pseudopiatani, while the American form, found on 

 Acer saccharinutn Wangenh., the common maple of the United 

 States, is called by Miiller R. acerinum. The life-histories of 

 the two forms studied are closely similar, and the few min or 

 diSerences which occur will be noted below. 



External Appearance. 



It may be of interest to record briefly the successive changes 

 in external appearance which occur on infected leaves. Leaves 

 were inoculated and closely watched for s\"mptoms. It was found 

 that the incubation period corresponded \\'ith that given by 

 Miiller; the first indication of the disease being minute water- 

 soaked dots which rapidly develop into small yellow spots about 

 1-2 mm. in diameter. Four or five days later, on the upper 

 surt'ace of the leaf, minute black dots appear %\-ithin each yellow 

 spot, and these spread and finally coalesce until the spot becomes 

 uniformly black. At first the black spot is smooth but in a few 

 days it becomes covered with a number of small pimple-Uke 

 projections about i mm. in diameter. These open at the top 

 by a circular pore through which small drops of a milk\' fluid 

 are exuded. This is the conidial stage of the fungus, and under 

 natural conditions lasts from June until August. \Mien this 

 stage is over the spot begins to thicken and extend in area so 

 that when the leaf falls the average diameter of the spot is 

 about 1-5 cm. The surface of this spot is smooth at first but 

 during the winter, when the leaves he on the ground, it becomes 

 raised into sinuous ridges which indicate the positions of the 



