PLANTS AS DISEASE PRODUCERS 307 



account of the fungi which cause specific diseases. It remains for this 

 discussion to consider fungi as the causes of diseases in general. Fungi, 

 using the word in the broadest sense to include the bacteria and slime 

 moulds, are responsible for an extraordinary number of diseases. The 

 entrance of the organism into another is known as infection. Nothing 

 like the infection of animals where the microbe, or its poison, circulates 

 in the blood, and finds lodgment in most of the organs is found with 

 plants. Infection follows, when a fungous spore germinates and pro- 

 duces an infecting hyphae, which grows into the cells^ or between the 

 cells of the host, it may be reaching to the ends of the plant. As disease 

 is induced by parasitic fungi, the parasite which enters the host and 

 spreads through it must absorb and utiHze the plastic and other sub- 

 stances of the plant, which is parasitized. Thus, we can divide the 

 endophytic hyphae into the intercellular hyphse such as we find in the 

 oomycetous fungi and Puccinia simplex. With such hyphae ^he 

 protoplasmic and other contents of cells are utilized by the formation 

 of haustoria of different forms and kinds, which penetrate the interior 

 of the cells. The second kind are the intracellular hyphae, which as in 

 the disease of the plane tree, Gnomonia veneta, grow lengthwise and 

 crosswise from cell to cell. 



The growth of the hyphae between and through the host cells is 

 accompanied by the formation of soluble ferments. These dissolve the 

 substance of the cell walls of cellulose, or woody walls with lignin and 

 pigment deposits. The hyphae live on the products of solution.^ 

 Hence timber may be damaged in two ways: by the formation of minute 

 pores and apertures through it ; or by a solution of the cell- wall materials. 

 The wood loses in strength and in weight and becomes "rotten." 

 This rotten condition, however, is reached in a multiplicity of ways, for 

 every parasitic fungus that lives in the wood of growing trees destroys 

 the wood in a manner peculiar to itself. Starch grains are decomposed 

 also in the cells, likewise crystals and tannin, for by the disappearance 

 of the latter, the smell of sound wood is lost. Hartig has described 

 the several methods in his ''Text-book on the Diseases of Trees." 



Then too, we have the epiphytic fungi which live on the surface 



^ Sometimes the h3^hae grow toward and surround the nucleus as the nucleus 

 exerts a chemotactic influence. Such hyphae may be termed nucleotropic as in 

 Puccinia adoxce. 



^ Consult Smith, Erwin F.: Bacteria in Relation to Plant Diseases, ii: 76-89. 



