Rust Fungi. 105 
rest. Maire points out that in Puccinia malvacearum, formerly 
described as a Lepto-form, the germination of the spores is imme¬ 
diate or retarded according to external conditions, and hence 
concludes that there is no essential distinction between the Lepto 
and the Micro-forms. 
Maire discusses many of the interesting features presented by 
the general biology of these fungi. His verdict upon Eriksson’s 
mycoplasm hypothesis is an open one, but the arguments advanced 
against it by Marshall Ward have lost none of their force with the 
lapse of time. 
Maire enters upon a consideration of the possible ways in 
which so many Rust Fungi have developed a hetercecious habit. 
According to Fischer’s views, a rust, at present heteroecious, may 
have formerly been pleophagous, and in the process of evolution 
may have come to develop its aecidial stage only upon certain of 
its former hosts and its teleutospore stage upon others. Dietel, 
however, holds the view that the ancestral forms of the hetercecious 
rusts possessed only teleutospore stages and that other spore 
forms were subsequently elaborated ; the teleuotspore stage then 
lost the power of completing its development on the original host 
and by some means or other passed to a different host. In a 
recent book Olive 1 supports the same view. He considers it 
likely that the present gametophytic host is relatively the primitive 
one, and that heteroecism has been evolved by the passing of the 
sporophyte generation to a new host. Olive mentions the fact that 
the sporophytic generation of many rusts has a tendency to a 
pleophagous habit. Thus Melampsorella caryophyllacearum attacks 
only Abies in its gametophytic phase, while its sporophyte is found 
on no less than five different genera. Nevertheless the gametophyte 
is sometimes plurivorous, and Maire mentions the extraordinary 
case of Puccinia Isiacecc, whose teleutospore stage is formed only 
upon Pliragmitcs communis, but whose aecidia are produced upon 
no less than twenty-two hosts, belonging to nine different families. 
It is interesting to note that in the only known hetercecious Asco- 
rnycete, viz., Sclerotinia Ledi, there would appear to be no relation 
between the alternation of hosts and the gametophytic and sporo¬ 
phytic stages of the parasite. As yet we know very little of the 
physiology of heteroecism, and until more knowledge is forth¬ 
coming it is difficult to see how we can obtain any clear indication 
of the manner in which a hetercecious mode of life has been 
evolved. F T B 
’ Olive, E. W. Phytopathology, 1911, Vol. 1, p. 139. 
THE CLASSIFICATION OF LICHENS. 
Since the publication in 1894 of the first part of this Mono¬ 
graph 1 many advances have been made in the study of lichenology, 
and consequently the classification used by Crombie has been 
1 “A Monograph of British Lichens,” Part I, by the Rev. J. M. 
Crombie, M.A., F.L.S., 1894. Part II, by Annie Lorrain 
Smith, F.L.S., 1911. Published by the British Museum, 
