188 ON THE PERONOSPOR^. 



previously described. When one of these conidia falls off and 

 reaches moisture, such as a drop of rain or dew, it immediately 

 breaks up into a swarm of from six to fifteen zoospores {e.g.^ P. 

 i?ifestajis, the potato fungus, and F. 7iivea^ the parsnip Peronospora). 

 These zoospores are little masses of protoplasm furnished with 

 two lash-like tails, by means of which they move about so rapidly 

 that it is difficult to follow their movements in the field of the 

 microscope. In about half-an-hour this swarming process comes 

 to an end, the zoospores become firmly attached to the cuticle of 

 the host, invest themselves with a thin cell-wall, and begin to 

 germinate. 



In all these cases, so potent is the mycelium that it is capable 

 of at once corroding, boring, and entering the epidermis of the 

 leaf, thus giving rise to the well-known brown or black spots so 

 characteristic of the fungus. Whilst on this subject, I cannot 

 omit quoting the careful observations of a recent observer, H. 

 Marshall Ward, published in the Quart. Journ. Micro. Science of 

 last year. He found that^ while the mycelium of the fungus 

 absorbed the protoplasm from the cells of the affected plant, it left 

 the starch-grains in the tubers of the dahlias and potatoes untouch- 

 ed ; and he desired to know how it was, that at a late stage in the 

 development of the fungus, the starch-grains, cell-nucleus, and even 

 cell-walls disappear ? He came to the conclusion that the remaining 

 changes in the cell-contents are effected h^ Bacteria, carried into the 

 invaded tissues by the hyphae of the fungus ; that these Bacteria 

 reduce the rest of the protoplasm and nucleus, first to a soluble 

 mass, and then cause the dissolution of the starch-grains. At 

 first, the action of the Bacteria is taken advantage of by the 

 fungus, but eventually the mycelium of the latter suffers from the 

 dominance of the former, and becomes in part a prey to its 

 companion, not, however, before it has formed its well-protected 

 oospores, which lie unhurt among the rotting debris. 



The mention of the oospore brings us back again to the repro- 

 ductive process in the Peronospora. The propagation of the fungus 

 by means of conidia and zoospores is only an asexual process. 

 There is also a sexual propagation, as in the Saprolegniece. The 

 sexual organs of the Peronospora are developed in the interior of 

 the tissue of their host. Spherically dilated ends of branches of 



