12 
president’s address. 
other cereals and grasses. Eriksson’s experiments tend to show that 
it exists in more than one form or variety, if, indeed, they he not 
distinct species. These varieties do not differ from one another 
morphologically, so that by simple microscopic characters they are 
indistinguishable. Biologically, however, they are distinct. For 
instance, one form (Pucci nia graminis ) occurs on Bye, Barley, and 
Twitch, as the rust-spores taken from any one of these plants, and 
applied to another of them, will reproduce itself. A second form 
affects the Oat (as well as Milium effusum, Alopecums pratensis 
and Dactylis glomerata), hut this form when applied to Rye or 
Barley, does not reproduce itself on them, nor, conversely, does the 
form from Bye affect the Oat. A third form occurs on Aiva ccespitosa, 
a fourth on Agvostis, and a fifth on Poa. All these five forms 
have their cluster-cups on Barberry. In like manner, Eriksson finds 
with the rust of P. glumarum, that when it occurs on Wheat 
it gives rise to the same rust when applied to other Wheat plants, 
hut does not affect Barley, and, conversely, the form on Barley does 
not produce the disease on Wheat. If subsequent observation 
confirms these results of Eriksson’s, and there is no prima facia ? 
reason to think otherwise, a very important modification of our 
views on the nature of these parasites and of the diseases caused 
by them must take place. 
It is debatable whether these different forms, as Eriksson calls 
them, are sufficiently distinct from each other to be considered 
species. We are dealing with plants, it will be remembered, 
which are essentially parasites, and which belong to a class where 
differences in external appearance are not great. In the year 1874 
the late Dr. Georg Winter drew attention to the fact that the 
pretty little cluster-cup on the wild Garlic ( Allium ursinuni) was 
connected with a mildew ( Puccinia sessilis) on Phalaris 
arundinacea. In 1885 I found that the rare cluster-cup on Arum 
maculatum also had its Puccinia on the same grass ; and further, 
that although these two mildews on Phalaris were practically indis- 
tinguishable in the mildew state (although thei'c were more marked 
differences in the rust-stage), yet the ono which came originally from 
the Garlic would not affect the Arum, and the one which came 
