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 l 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 2 , 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 25i, 262. See also Fig. 6. 



