3o8 GENERAL PLANT PATHOLOGY 



of the host, as with the common mildews, and send short haustoria 

 into the epidermal cells of the host on which they grow. Some fungi 

 have mycelial hyphae that grow in both ways, intracellularly and inter- 

 cellularly. Others, as a number of wood-destroying fungi, grow down 

 through the tissue of the host and ultimately kill it. Apical growth 

 is shown by some. The haustoria, as they enter a cell, may flatten out 

 against the cell wall, as in Piptocephalis. Such flattenings are known 

 as appressoria. The haustorium, which enters a cell, may become 

 branched, or dendritic, it may enlarge into a haustorial knob, or re- 

 main as an haustorial tube. Internal sclerotia are formed sometimes 

 in certain parasitic fungi. These are consolidated or hardened masses 

 of hyphae, which are associated with a resting period. 



Ordinarily when a spore falls on the surface of the plant, it produces 

 a germ tube, which by the action of a secreted ferment bores its way 

 through the epidermal cell walls and thus enters the host. Sometimes 

 it penetrates the cuticle, grows between it and the cell wall and grows 

 down between the membranes of the cells, as in Botrytis parasitica. 

 Occasionally, but not commonly, it enters through the stomata, or 

 sometimes through nectaries and stigmatic surfaces. However, there 

 are certain bacteria, such as those which cause the black rot of the 

 cabbage, which fall upon the drops of water excreted by water stomata 

 and by following the water back into the plant infect the cabbage 

 leaves. A cork layer is protection against infection. Fungi, however, 

 gain access to the interior of the plant in a variety of ways. Some 

 years ago^ the writer considered the way in which fungi enter living 

 trees and a restatement of the facts presented in that paper is 

 apropos. 



Occasionally the planted seed contains a dormant fungus (but not 

 as a mycoplasm in Eriksson's sense), which begins its growth, as soon 

 as the seedling plant emerges. The oat- or wheat-smut spores are 

 produced in the grain and consequently infect the cereal plant when 

 it is small, and at or near the surface of the ground. In other cases the 

 fungus penetrates the underground parts or the twigs of trees. Fungi 

 gain entrance to plants, through injuries caused by mechanic, meteoro- 

 logic, chemic, or other agents. Mechanic injuries are due to man, 

 animals, or other causes, such as the weight of snow, the rubbing of 



1 Harshberger, John W.: How Fungi Gain Entrance to Living Trees. Forest 

 Leaves, viii: 88-90, December, 1901. 



