CHAPTER VII,—PHENOMENA OF VEGETATION.-—-PARASITES, 391 
localisation. Other species partly belonging to the same genera show the opposite 
behaviour. The mycelium of Puccinia Tragopogonis which bears aecidia for instance, 
spreads all through the host and produces its aecidia over the entire surface of the leaves, 
while that which grows from the aecidiospores and forms teleutospores is confined 
to small spots on the leaf. The same is the case also with Uromyces Pisi, the con- 
verse with Melampsora Géppertiana. 
The varying extent to which Tuburcinia Trientalis spreads (see page 180) 
according to the kind of spore from which it grows is a nearly allied phe- 
nomenon. 
But mycelia grown from similar spores appear in some cases to be narrowly 
localised or to spread widely according to the species of the host. Cystopus candidus 
for example often appears to be limited to narrow spots in the leaves of species of 
Brassica, into which it must have penetrated through the stomata of the full-grown 
leaves, while it behaves in exactly the opposite way in Capsella and Lepidium. Its 
mode of entrance in Brassica would be in accordance with experience, for I once 
observed that the germ-tubes of this Fungus were able to develope a mycelium in the 
full-grown leaves of Heliophila crithmifolia. It isnot improbable that some Uredineae 
are affected in the same way by a difference in their hosts, but the point requires 
further examination. 
In many Fungi the mycelia which spread through the host have everywhere 
the same characters, or at least they have no distinct and special characters at distinct 
places in the host; they may form their spores at any spots where the external 
conditions, as supply of air or peculiar mechanical relations, permit, and actually do 
so form them. 
On the other hand there are many species which assume different characters 
in different organs of the host, showing structural differences in the hyphae themselves, 
and especially capacity or incapacity for forming sporocarps and spores. It is obvious 
that conditions of nutrition must in general be the cause of this phenomenon, but we 
have at present no precise physiological account of the matter. We can only therefore 
speak provisionally in such cases of favourite places for the formation of one and 
another kind of spore. Some remarkable examples have been already briefly given 
in Chapter V, but we may add a few more in this place. 
Cystopus Bliti forms its gonidia only on the leaves of Amarantus Bliti, and its 
oospores only in the stems; Cystopus candidus forms gonidia in great abundance on 
all the aerial organs of its hosts, but I never found its oospores on the leaves; some 
species of Peronospora behave in the same way; P. Arenariae, Brk., for example, 
produces gonidia on all parts of the leaves of Möhringia trinervia, oospores almost 
exclusively in the parts of the flowers. Very many Ustilagineae form their spores only 
in or on the parts of the flowers of the host, some in the anthers, others in the ovary 
or ovule, others again (Sorosporium Saponariae, Ustilago Tragopogonis) on the whole 
surface of the corolla and of the parts inclosed by it. It has already been remarked 
that Peronospora Radii and P. violacea are also confined to the flowers of the host for 
the formation both of oospores and gonidia. In other Ustilagineae, as Urocystis 
occulta, certain parts of the leaves are ihe places where the spores are formed, in 
Ustilago hypodites chiefly the surface of the stem where it is covered by the leaf- 
sheath. Epichloe typhina always forms its stromata on the outer surface ofthe sheath 
