CHAPTER VII,—PHENOMENA OF VEGETATION,—PARASITES, 385 
&. Obligate Parasites. 
Section CIX. Most of the groups described in Chap. V contain a large 
number of Fungi which are obligate parasites, as will be seen by reference to the 
accounts there given; but within the class itself we meet with every gradation of 
adaptation, from the strictly parasitic mode of life to arrangements in which we may 
almost as well speak of facultative parasites, as of obligate parasites which are at 
the same time facultative saprophytes. Phytophthora omnivora' in the group 
of the Peronosporeae well exemplifies the transition from facultative parasitism. 
This Fungus is a destructive endophyte in many kinds of living Phanerogams, as 
Fagus and especially in young seedlings, in Sempervivum, the Oenothereae, and 
others. Other species of Phanerogams it leaves uninjured, as Solanum tuberosum 
particularly and Lycopersicum. Its development is rapid in proportion to the amount 
of water contained in the host, even when that amount exceeds the limits of a 
normal and sound state of the plant and becomes pathological, as when the land- 
plants just named are immersed in water. The development of the Fungus culminates 
in the formation of oospores, usually in large numbers, after the host has been killed 
by its assailant. The Fungus can also grow on dead organic, even animal, 
bodies in water and form an abundance of gonidia, but without arriving, as far as is 
known, at the formation of oospores. On this latter account it is better to place it 
among facultative saprophytes. The same may be said of its nearest relative Phy- 
tophthora infestans the Fungus of the potato-disease, with the limitation that the 
adaptation to a parasitic mode of life is more marked. 
For the same reasons the members of the Mucorini, Piptocephalis, Synce- 
phalis, and Chaetocladium, to which the expression facultative parasites was first 
applied, may be termed facultative saprophytes. They may be grown as saprophytes 
from spores in a nutrient solution and produce an abundance of gonidia, but accofding 
to present observations they only attain to their full development in the formation of 
zygospores when they live as parasites on other Mucorini in the manner described in 
a former page. 
The species also of the Ustilagineae (see page 179), in which the young plants 
can vegetate by sprouting or in the manner of the Hyphomycetes in solutions made 
from organic bodies, must be termed facultative saprophytes. But the saprophytic 
faculty is much less important to them than the parasitic mode of life, for it is as 
parasites only that they are able to produce the resting spores and sporophores 
which are peculiarly characteristic of their development. This is so, even if the round 
cells of intercalary origin obtained by Brefeld’ in large quantities from the mycelium 
of Tilletia Caries, when it is grown as a saprophyte in nutrient solutions, really 
have the characters of resting-spores; this can only be proved by observing their 
germination, and in default of this observation it remains unproved; moreover the 
characteristic sculpture on the surface of the spore-membrane of T. caries ‘could not 
be clearly seen’ on the products of cultivation. In the majority of the species 
which have been examined the saprophytic development does not go beyond the 

1 Bot. Ztg. 1881, p. 585. 
2 Hefepilze, p. 159. 
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