Notes. 
575 
generations as if we had here an essentially new process, as against 
those other sorts of Fungi which are dimorphic or polymorphic. 
There are nearly allied Uredineae in which all the spore-forms appear 
one after another upon the same mycelium. In my view the condition 
for the different kinds of propagation will also be unlike, and the 
regular alternation of the fruit-forms would be explained by the 
fact that by the development of the host-plant itself, as by its depen- 
dence upon the seasons, changes in it necessarily go forward, which 
will serve as direct occasion for the growth of the different spore- 
formations of the parasite. 
In Uromyces Polygoni , for instance, the aecidia appear only on the 
young plant, the uredospores and teleutospores on the older. In 
the heteroecismal Uredineae the special conditions for the individual 
fruit-formations are much more strongly distinguished still, so that 
other host-plants are necessary to bring the Fungus to the formation 
of aecidiospores or teleutospores. The time will probably come 
when these conditions may be more accurately recognized, and the 
Uredineae be cultivated on artificial substrata. Then it will appear 
whether these parasites do not behave just like the other Fungi, and 
cannot also be compelled to produce the different fruit-formations 
upon the same mycelium. A great obstacle to the cultivation of 
the Uredineae lies in our ignorance of the chemical composition 
of the host-plants. We are quite ignorant of the substances charac- 
teristic for the species, which, besides the usual food-stuffs, sugar, 
proteid, &c., are at any rate of decided importance for the develop- 
ment of the parasite. 
In all the cases now mentioned we have to do with the alternation 
of several generations, each of which is characterized by special 
propagation. In the unicellular Thallophytes the non-sexual pro- 
pagation coincides with the vegetative division. The propagation 
of the Desmidiaceae and Diatomeae by division corresponds to the 
propagation of Chlamydomonas by means of motile cells. In all of 
them the sexual process ensues after a series of divisions. Naegeli 
includes these processes under his conception of alternation of genera- 
tions, and even extends it to the Bacteria in which, after a series 
of generations by division, the cycle is closed by the formation of 
endospores. But if the term alternation of generations be limited 
to organisms with dimorphic propagation, an alternation of shoots 
might be spoken of in this case. 
