386 DIVISION III. MODE OF LIFE OF THE FUNGI. 



production of gonidia ; and it may be assumed, though it has not been yet distinctly- 

 proved, that they may form germs capable of infecting, and that by means of these germs 

 they attack the host-plant and thus return to a parasitic life, somewhat in the way 

 described at page 373 in the case of Cordyceps. 



The facts stated in pages 265, 339 show that the behaviour of Exoascus and 

 Exobasidium is quite similar to that of the Ustilagineae. 



The example of Cordyceps leads to the mention of Claviceps, and Epichloe 

 typhina also approaches near to the latter genus in the points in question. The 

 gonidia (and possibly also the ascospores) of these Fungi may develope small mycelia 

 producing fresh gonidia in the manner described on page 227. That this facultative 

 saprophytism in its various degrees is a frequently recurring phenomenon in parasitic 

 Ascomycetes is to be expected, though more stringent proof of it is in most cases 

 still to be desired. 



Of all the Ascomycetes the Lichen-fungi, according to our present knowledge, 

 must be mentioned first as examples of strictly obligate parasites, after them the 

 Erysipheae and Polystigma (section LXIII). It has still to be ascertained what are 

 the exact conditions in this respect in the large number of parasites in the groups 

 of the Hysterineae and Phacidieae. The Peronosporeae also contain excellent 

 examples of the class which we are considering, for most species of Peronospora 

 and all of Cystopus are strictly parasitic, the first stages only of germination being 

 completed outside the host. This is the case also with Protomyces and many Chy- 

 tridieae, some of which even commence germination on the surface of the host 

 (see Chapter V). Lastly, only the strictest parasitism is known in the group of the 

 Uredineae so rich in forms ; they germinate if sufficiently supplied with water, and 

 their further development takes place only on the proper host. 



SECTION CX. The Fungi which are parasitic on plants naturally exhibit within 

 the limits of the chief phenomena of parasitic vegetation and its effects, which were 

 pointed out on page 359, a variety of special adaptations in respect of their choice 

 of a host and their spreading in, upon, or along with it. The reaction of the host itself 

 which varies in each case corresponds again to the spread of the Fungus. We call 

 attention to the following facts which are of general interest in relation to these 

 points, again referring the reader to the former sections of this work and to the 

 special literature of the subject. 



As regards the choice of the species to serve as a host, the rules stated on 

 page 359 are of the first importance. Most parasites living on plants require a single 

 proper host for the completion of their whole course of development, though they may 

 enjoy a larger or smaller room for choice between different species more or less nearly 

 allied to one another. Of all the hosts that are possible for a species of Fungus some 

 may be more favourable to their development than others. Cystopus cubicus for 

 example flourishes and forms abundance of gonidia on the leaves of species of Trago- 

 pogon, Podospermum, and Scorzonera, but forms oospores almost exclusively on 

 Scorzonera, especially on S. hispanica ; oospores are extremely rare on Tragopogon 

 in my experience. So it is with the Uredineae and species of Erysiphe. The best 

 known Erysiphe is the Fungus of our grape-vine ' which in Europe only forms gonidia 



1 See De Bary u. Woronin. Beilr. III. 



