CHAPTER VII. -—PHENOMENA OF VEGETATION,— PARASITES. 363 
larly, and consists in extreme cases in this, that germination does not take place 
independently of the host, but only when the spore capable of germination has 
reached the surface of the proper host. When it has done this it at once puts out a 
germ-tube at the point of contact which penetrates directly through the membrane, 
otherwise it perishes without germinating. This is the history of many Chytridieae, 
Synchytrium especially, which are entirely or partially intracellular in their vegeta- 
tion, of Completoria’ also, and, as it appears, of Protomyces macrosporus. This 
mode of penetration is also the normal one in some Chytridieae and Pythieae, though 
they are able also to put out small short-lived germ-tubes without contact with the 
surface of the host. A quite peculiar mode of proceeding, but approaching the 
above, has been observed in the swarm-spores of Cystopus and Peronospora nivea 
(Umbelliferarum) ; these spores put out germ-tubes in water which soon die away ; 
in drops of water on the surface of their host they come to rest usually on or close 
to the stomata of the latter and send their germ-tubes into them and then proceed 
with their further development. 
The other deviation from the general rule is observed in epiphytic parasites on 
plants, which continue their chief growth outside the host during the whole of their 
life, but send haustoria into its cells. Here the spores form germ-tubes independently 
of the host, but where the tubes are in contact with a cell of the host, they send out 
peculiarly shaped branches, which pierce through the wall of the cell and develope 
into haustoria. In the Mucorini, which. are more or less facultatively epiphytic 
(Piptocephalis, Syncephalis, &c.), a copious formation of mycelium and gonidia may 
take place independently of the host if sufficient food is supplied to the plant. The 
germ-tubes of the Erysipheae’, after a short increase in length, send a haustorium at 
once into an epidermal cell of the host and develope on the food thus supplied to 
them from it into mycelial hyphae, which successively form new haustoria similar to 
the first, If the young germ-tube does not encounter the epidermis of a suitable host 
it dies after a slight elongation. 
When the germ-tube penetrates through a membrane, which usually happens 
after it has grown for a short time in some other direction, its extremity bends round 
towards the wall which is to be pierced, presses upon it and then grows transversely 
or obliquely through it. In doing this it may maintain nearly the same breadth 
in the perforated membrane as it had outside it, or it may be considerably 
narrowed and contracted. But in certain cases, as for example in the sporidia of the 
Uredineae, the portion of the tube which passes through the outer wall of the 
epidermal cell is a very slender process, usually appearing only like a simple line even. 
when highly magnified; as soon as this process has entered the cavity of the cell its 
tip swells at first into a roundish and then into an elongated tube-like vesicle, and the 
entire protoplasmic content of the spore streams into it; the spore itself and the 
portion of the germ-tube which is outside the epidermis of the host are seen to be 
filled only with a watery fluid and soon disappear. The filiform process also which 
passes through the cell-wall then becomes indistinguishable, and the opening which 
it produced in the wall appears to become closed up again; in a short time after the 

1 Leitgeb as cited on page 160. 
? De Bary, Beitr., and Wolff, Beitr., as cited on pages 261, 262. See also Fig. 6. 
