CHAP, xiii.] Fungoid Diseases of Trees 277 



Fungi may be either saprophytic or parasitic. The former, 

 fortunately including most fungi, are only to be found on dead 

 and decomposing substances, and are therefore the accom- 

 paniment or consequence of sickly growth and disease, and 

 not its cause. Those belonging, however, to the latter class, 

 attack healthy living plants, and are a direct cause of disease, 

 often terminating in death; but many of them continue in 

 a sort of saprophytic growth when once the disease they have 

 occasioned has terminated the life of the plant attacked. All 

 the fungi called rusts ( Uredineae] are parasitic. 



The reproductive organs of fungi are developed in or on 

 receptacles, sporangia, sporophores, or spore-producers, and 

 consist of germinative cells of different kinds roughly classi- 

 fiable as Spores, Sporidia } and Gonidia or Conidia. 



Spores (including ordinary spores, and also Uredospores, 

 Aecidiospores, Teleutospores &c., according to the different 

 circumstances relative to their formation and development), 

 are the generic name for these germinative cells without dis- 

 tinction ; whilst Sporidia are secondary spores formed on 

 a Promycelium developed by the germination of hibernating 

 or resting spores ; and Gonidia or Conidia are those formed 

 at the point of usually erect-growing mycelial threads. 



When any spores that settle on plants find the conditions 

 (damp and warmth) favourable to their development, germina- 

 tion follows, i. e. they develop delicate, thin-walled, mostly 

 colourless, tube-like processes (mycelial filaments or hyphae\ 

 sometimes filled with a golden-yellow oily fluid. These tubes, 

 though often undivided, are as a rule divided into simple 

 cells (septated), their contents being protoplasm and cell-sap. 

 With the exception of the mildew-fungi, which merely vege- 

 tate on the exterior, the infection of leaves, fruits, bark and 

 wood always commences from the outside by the hyphae 

 pushing their way either through the stomata or the epidermal 

 cells of young leaves or bark, or effecting an entrance into 

 the interior of the plant through the unprotected portions of 



