54 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 leafspot diseases.’”’ Thus Frank ft 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,{ 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 
Puceinia 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.) 
+ Frank, B. Uber einige neue u. weniger bekannte Pflanzenkrank- 
heiten. (Ber. d. deut. bot. Ges. 1:31. 1883.) 
} Wakker, J. H. Untersuchungen iiber den Einfluss parasitischer 
Pilze auf ihre Nahrpflanzen. (Prings., Jahrb. f. w. Bot. 24 3 505. 1892.) 
§ Stevens, F. L. A peculiar case of spore distribution. (Bot. Gaz. 
27: 138. 1899.) 
32 
