CHAPTER VII. PHENOMENA OF VEGETATION. PARASITES. 385 



b. 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 1 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 according 

 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 2 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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