EFFECT OF PARASITIC FUNGI ON THEIR HOST. 15 



cell attacked which is at once killed before it can enlarge or 

 otherwise react to the influence of the intruder. Good examples 

 of such parasites are presented by some of the Chytridiaceae 

 — the Archimycctcs of Fischer — which, as a rule, inhabit only 

 isolated cells of their respective host-plants. This mode of 

 nutrition is equivalent to that of the Myxomycetes and Mycetozoa, 

 which absorb the cell-contents after completely enveloping the 

 living cell, or after slipping inside or sending a haustorial process 

 into it. 



A second series of parasites consists of those which live 

 on the contents of the host-cell, and give it time to react to 

 the stimulus exerted by the intruder. The reaction generally 

 results in a cell-enlargement or fungus-gall, which in the simpler 

 cases includes one , cell only. The gall harbours one or more 

 parasites, which gradually use up the cell-contents. As examples 

 we have Olpidium tumaefaciens and 0. uredinis} Pseudolpidium 

 saprolegniae, Olpidiopsis saprolegniae, RMzomyxa liypogaea} etc. 

 A specially striking case is that of Plcotrachelns fulgcns, which 

 causes the rudiment of the sporangiophore of Piloholus Klcinii 

 to become hypertrophied and gall-like.^ 



We have as a third series those parasites which penetrate 

 into living cells and absorb their contents, at the same time 

 stimulating the host-cell to abnormal and increased growth, as 

 well as some surrounding cells not directly in contact with the 

 fungus. In this case the parasite exerts a far-reaching effect, 

 and produces a gall composed of more than one cell. Species of 

 Synchytrium are examples. The fungus itself penetrates into one 

 cell only, which enlarges ; but simultaneously the surrounding 

 cells grow and multiply to form a wall or rampart enclosing the 

 cell originally attacked. Other parasites do not absorb the 

 host-contents as a whole, but only withdraw osmotic substances 

 by means of delicate processes of the fungus-hyphae. These 

 haustoria penetrate the wall of the host-cell, but the fungal 

 protoplasm inside them remains separated from the host-proto- 

 plasm by a delicate membrane. In the case of the vine-mildew 

 and some other Erysipheae, the cells thus preyed on turn brown 

 and die. With other related forms {e.g. Sphacrotlieca castagnci), 



^ See Fischer's Phycomycefes. 



- This causes a slight swelling of the root-hairs of various plants and absorbs 

 their content. 



^Zopf, Be.itrd(j€ zur Physiol, u. Morphol. nied. Organismen, ii. 1892. 



