CHAPTER VII. — PHENOMENA OF VEGETATION. — PARASITES. 387 



(Oidium Tuckeri, Brk.) ; its sporocarps are perhaps the objects described as Uncinula 

 spiralis, which grow in North America on a native vine. This dissimilar promotion 

 by hosts of different species of the otherwise similar course of development of the 

 Fungus makes no difference in the phenomena in question. Parasites which go through 

 their whole course of development on a single host of a particular species are termed 

 aidoecious or autoxenons. All the parasites described in preceding chapters are examples 

 of this class, with the exception of one group to be named presently. It is not perhaps 

 altogether superfluous to remark, that the larger part, or at any rate very many species, 

 of the Uredineae, in which the alternation of generations is most copiously differ- 

 entiated, are autoecious. 



Thus the entire development of Uromyces Phaseolorum is completed on species 

 of Phaseolus, that of U. appendiculatus on the Vicieae, of Puccinia Tragopogonis on 

 Tragopogon, of P. Pimpinellae on Myrrhis or Chaerophyllum, of P. Falcariae on 

 Falcaria Rivini, of P. Violarum on species of Viola, and so on. 



The contrary is the case with a number of Uredineae which form aecidia. These 

 are obliged to change their host with the separate sections of their alternating genera- 

 tions in order to complete the course of their development, like the Cestodes and 

 other parasitic worms. They are accordingly termed hetej-oectous, or still better 

 metoecious or metoxenons as changing their place of habitation or host 1 . 



I was myself the first to establish this metoecism in the case of Puccinia graminis, 

 in which the phenomenon or at least its consequences were recognised more than a 

 hundred years ago by agriculturists, who rightly maintained against the botanists that 

 grain grown in the neighbourhood of shrubs of Berberis were liable to be attacked by 

 rust, that is by Puccinia graminis. This parasite exhibits the pleomorphism and 

 alternation of generations of the Uredineae which form aecidia in its greatest variety of 

 form (see page 279). Its teleutospores pass the winter on old stems of wild and 

 cultivated grasses, especially Triticum repens, while the germ-tubes from the sporidia 

 which are developed in the ensuing spring penetrate into the epidermal cells of 

 Berberis vulgaris, more rarely into those of a Mahonia, and never into a grass. They 

 develope rapidly in Berberis into a mycelium which produces aecidia but never forms 

 uredospores or teleutospores, and if the germ-tubes of the aecidiospores find their way 

 into the stomata of suitable Gramineae they develope there and there only into a 

 mycelium which produces uredospores and teleutospores. The germ-tubes of the 

 uredospores in their turn develope only on the Gramineae, and in the manner which 

 is common to all uredospores. 



Later investigations have shown that an analogous change of hosts takes place in 

 many other species. 



The aecidia of Puccinia Rubigo vera and P. coronata which also -live on Gramineae 

 or Cyperaceae in the other sections of their development are confined, the former to the 

 Boragineae, the latter to species of Rhamnus ; those of P. Moliniae to Orchis ; those 

 of P. Caricis to Urtica and of P. (Caricis) limosae to Lysimachia thyrsiflora ; Uromyces 

 Dactylidis forms its aecidia on the leaves of common species of Ranunculus 2 , and its 



1 See on the terminology Bot. Ztg. 1867, p. 264; on other points see the literature of the Uredineae 

 enumerated above on page 286. 



a See also Cornu, Comptes rend. 1882, 94, p. 1731. 



C C 2 



