MISSOURI BOTANICAL GARDEN. 



furthermore that fungus threads occur all through a given 

 section of a tree. 



The manner in which fungi influence their hosts varies 

 considerably. One may consider the distribution of the 

 mycelium within the host. There are but few references 

 to this point in discussions on fungi. Tubeuf * says : 

 " A large number of fungi have a mycelium which never 

 extends beyond a very short distance round the point of first 

 infection, and cause only local disease, frequently with no 

 perceptible disturbing effect on the host. Such is the case 

 with leaf spot diseases." Thus Frank f describes the 

 mycelium of Gloeosporium Lindemuthianum as caus- 

 ing a browning of the tissues as far as the mycelium 

 extends. The same is true of Cercospora. The mycelium 

 of Aecidium Rhamni on Rhamnus frangula has a 

 local distribution, J so also that of many Erysipheae, 

 for instance Microsphaera densissima also Uncinula necator 

 of which an interesting case was recently described by 

 Stevens. This localization of the mycelium may be due 

 to mechanical obstructions, such as the veins of a leaf, as in 

 Puccinia Podophylli, or to chemical reaction on the part 

 of the host. The large majority of fungi have a mycelium 

 which extends through large areas of their hosts. Wakker 

 (1. c.) classifies parasitic fungi according to their effects on 

 their hosts as producing either mechanical or chemical 

 effects. By mechanical effects he understands such as are 

 due to direct pressure. The vast majority affect their 

 hosts chemically. Here again two classes may be distin- 

 guished, such as produce chemical effects ' which will im- 

 mediately, or otherwise exert a direct destructive influence 



* Tubeuf, C. Freiherr von. Diseases of plants 16. (Eng. edit.) 



t Frank, B. Uber einige neue u. weniger bekannte Pflanzenkrank- 

 heiten. (Ber. d. deut. bot. Ges. 1 : 31. 1883.) 



J Wakker, J. H. Untersuchungen tiber den Einfluss parasitischer 

 Pilze auf ihre Nahrpflanzen. (Prings., Jahrb. f . w. Bot. 24 : 505. 1892.) 



Stevens, F. L. A peculiar case of spore distribution. (Bot, Gaz. 

 27 : 138. 1899.) 



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