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 Papaya esate 
germ-tube or branch of the myce- [2 Pp which have g i d on the epi is of Faba vulgaris, the 
germ-tube penetratinginto astoma. c germ-tube which has passed through the 
er H =} 7. 7 stoma sinto the parenchyma of a leaf of Fada and there ramified: cis a portion 
lium either grows into the Interior of a transverse section through a leaf of Fada, the cell-wall of the spore and 
of the host through a natural the piece of the germ-tube which is outside the leaf not being shown. Magn. 
195 times 

FIG. 163. Uromyces app 7 a 


opening in it, or it pierces /hrough 
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 emier the host by the stomala 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 tlfe 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 
