FUNGI 317 



divided primarily into such as feed on the decaying bodies of plants 

 and animals and dead organic substances — saprophytes — and those which 

 attack living hoi^xts— parasites. Between the two extremes of strict 

 saprophytes and strict parasites there are intermediate forms. Some 

 saprophytes, which ordinarily live throughout their course of develop- 

 ment as such, have the power of living as parasites either wholly or 

 during a part of their course of development. Such are called faculta- 

 tive parasites. Similarly some parasites which ordinarily live as such 

 have the power of passing at all events a part of their lives as sapro- 

 phytes. Such are facultative saprophytes. The lichen-forming fungi 

 which live socially with algae may be placed in another category. Most 

 fungi are saprophytes, and it is obvious, from the fact that so many are 

 confined to specific substrata, that there is much variation in the nutri- 

 tive adaptations of such forms. These adaptations are, however, more' 

 clearly marked in the case of the smaller number which lead parasitic 

 lives. Some are confined to single species of host-plants ; many range 

 over allied species, some of them attacking plants outside the group 

 mostly aifected, or exempting from attack certain species within the 

 group. Others, again, may be said to be omnivorous parasites, attack- 

 ing plants or animals of diverse groups. AVith regard to the predisposi- 

 tion of the host to the attack of the parasite, it is impossible for the 

 most part to say exactly wherein it lies. Reference may be made to the 

 case of species of Pythium which as facultative parasites attack Phanero- 

 gams, &c. The amount of water present in the host determines there 

 the degree of predisposition to attack. \Vhile a sickly condition may 

 constitute a predisposing cause to the attack of a parasite in some- 

 cases, it is by no means so in the majority of instances. It has been 

 asserted that certain cultivated plants, such as cereals and the potato- 

 plant, have by cultivation acquired an ' inherent tendency ' to certain 

 parasitic diseases, whereas it is obvious that the growing together 

 of vast numbers of these plants furnishes opportunity for the spread of 

 diseases which, in the absence of other evidence, may be taken to 

 account for extensive outbreaks. Parasites commonly attack their hosts 

 by the penetration of the membranes of the uninjured host, though 

 cases are numerous where the entry is made by means of the stomates, 

 or of wounded surfaces only. Most are endophytes, but a small number 

 ■ — e.g. Erysiphe — are epiphytes, which send haustorial branches into the 

 body of the host. The result of attack is either the destruction of the 

 host, or the production of deformities by anomalous processes of growth 

 in the parts affected. 



