262 M, C. COOKE, NOTES ON PODISOMA. 



The Podisoma, which is found exclusively upon the leaves of 

 junipers in Denmark, has also received notice. Oersted is con- 

 vinced that it is only a folicolous form of Podisoma juniperi, and 

 that the figure and description given by Corda* of Podisoma 

 juniperi minor belong to it, since the figures accord so exactly with 

 the forms found at Hoiljerg ; and he adds to this, " the place I 

 assign to it is perfectly exact, since it is identical in its spores with 

 that which thrives upon the branches." It must, therefore, be very 

 different from the Podisoma foliicolum of Berkeley, which, neither 

 in appearance nor fructification, resembles Podisoma juniperi. 



The question naturally suggests itself as to what are the proba- 

 bilities in favour of the accuracy of Oersted's observations. Are 

 the Rasstelias only conditions of the Podisomas ? We confess to a 

 little difficulty in accepting the conclusion as the matter stands. 

 We would not deny, neither can we affirm it. If true, how are we 

 to explain cases in which pear trees are infested with the Rastelia 

 without a Savin growing within a radius of very many miles ? Or 

 how can we account for the Rosstelia on the hawthorn in localities 

 where junipers are unknown ? Is an annual impregnation neces- 

 sary, or can one be produced without the other, under any circum- 

 stances ? Anyhow, this alternation of generations in plants, fixed 

 to determinate' spots, is a mystery far greater than such phenomena 

 in animals that are locomotive. Mysterious as it may be, we are 

 bound to accept the facts if satisfactorily confirmed, although we 

 cannot account for all the phenomena. 



If true, which is the early stage, the Razstelia or the Podisoma ? 

 We should elect in favour of Rastelia, commencing with the sper- 

 mogonia, because, if the spermatia have any fertilizing function this 

 would precede the germination of the Rcestelia spores. No sper- 

 matia having as yet been discovered in connection with the Podisomas 

 themselves, but always preceding the Rastelia. 



The Tremelloid Uredines have been grouped under two genera, 

 namely, Podisoma and Gymno sporangium. As far as we can judge 

 there seems to be no good grounds for this separation. The written 

 characters of the two genera present as a distinction the external 

 form alone, which is more or less expanded in Gynmosporangiwn, 

 and clavate or clavariasform in Podisoma. Internally and micros- 

 copically, as far as we can ascertain upon close examination, the 



* Corda, Icones Fung., vol. i, p. 8., t. ii. f. 122. 



