CHAPTER VII.— PHENOMENA OF VEGETATION. — PARASITES. 



361 



spores ; they appear in nature rather as epiphytic growths on the walls of cavities in 

 the bodies of animals which are easily accessible from without, such as the passages of 

 the ear and the bronchi. 



In most cases the spore of the parasite begins the emission of a germ-tube 

 independently of the host, either after simple absorption of water or by appropriation 

 at the same time of food-material produced outside the host. If the tubes or the 

 hyphae which proceed from them then come into contact with the host, they fasten 

 upon it in the way peculiar to each species. The most common case of the kind is 

 when the spore finds its way by some mode of dissemination or other to the surface 

 of the body of the plant or animal, and puts out germ-tubes which penetrate into the 

 body. Parasites which, like Ancylistes Closterii and Polyphagus Euglaenae, attack 

 unicellular organisms living in societies, send out mycelial branches from the individual 

 first attacked, and these can fasten upon fresh individuals and by degrees on entire 

 aggregates of the host-cells. Some 

 facultative parasites of higher or- 

 ganisms, Sclerotinia for example, 

 Agaricus melleus and others of 

 R. Hartig's tree-destroying species, 

 behave in the same manner, since 

 any of the hyphae or mycelial 

 strands are able to make their way 

 into new individual hosts. 



The act of penetration is 

 accomplished in two ways ; the 

 germ-tube or branch of the myce- 

 lium either grows into the interior 

 of the host through a natural 

 opening in it, or it pierces through 

 the firm membranes of the surface of the body of the host. The one or the other 

 mode is adopted according to the species and the kind of spore ; it is seldom that both 

 occur promiscuously. 



Many examples of the first kind are supplied by those endophytic parasites on 

 plants, in which the germ-tubes enter the host by the stomata only. All the uredo- 

 spores and aecidiospores of the Uredineae for instance germinate on the moist 

 epidermis of phanerogamous plants. The germ-tube grows in a curve on the surface 

 of the epidermis, and when its tip reaches a stoma it descends into it, usually after it 

 has first become vesicularly swollen outside the stoma, and then passes on into the 

 air-space which lies beneath it. Here it increases rapidly in size and receives the 

 entire protoplasm of the germ-tube, while the rest of the germ-tube outside the spore- 

 membrane dies away. The extremity of the tube which has thus penetrated into the 

 host can now put forth branches which develope into mycelial hyphae (Fig. 163). 

 These germ-tubes enter the stomata of any phanerogam, but only develope further in 

 the species which is the proper host of the particular parasite ; they wither away in all 

 other species in the subepidermal air-space. The short germ-tubes from the sporidia 

 of Leptopuccinia Dianthi, DC. proceed in a similar manner. If a sporidium of this 

 plant germinates in the neighbourhood of a stoma of the host, its germ-tube grows 



FIG. 163. Uromyccs apperidiculatus. a uredospores germinating in water. 

 b uredospores which have germinated on the epidermis of Faba 'vulgaris, the 

 germ-tube penetrating into a stoma, c germ-tube which has passed through the 

 stoma x into the parenchyma of a leaf of Faba and there ramified : c is a portion 

 of a transverse section through a leaf of Faba, the cell-wall of the spore and 

 the piece of the germ-tube which is outside the leaf not being shown. Magn. 

 195 times 



