POTATO-FITNGTJS. 117 



tissues discoloured by Phytophthora, and that, too, in the interior of 

 the cells. Its oogonia, in "various stages of maturity, were supported 

 in a characteristic manner (which I shall more fully describe here- 

 after) on cylindrical threads without septa, completely resembling in 

 structure those of Phytophthora, though, it is true, distinguished by 

 a different distribution of protoplasm, and by being much thinner 

 than the usual Phytophthora mycelium. It became, however, abun- 

 dantly clear that these thin threads were branches of other mycelium 

 which corresponded in thickness with the mycelium of Phytophthora, 

 and, like it, buried themselves among the cells of the Potato in the 

 intercellular passages. It was scarcely possible, especially in the 

 discoloured tissues, to draw a positive distinction between the two 

 kinds of intercellular filaments. 



The oogonia (1, a, h) are round cells. Their position on the my- 

 celium is either sessile on its outside, or they are inserted with a broad 

 base into the mycelium-tube. In contact with the oogonium lies an 

 antheridium — rarely two — which, as a rule, is club-shaped, and rises 

 close to the oogonium from the same tube (1, ri). Unimportant ex- 

 ceptions to this rule may here be passed over. A single oospore is 

 formed from the whole of the protoplasm of the oogonium. It 

 occujiies the cavity, is nearly globular, and at the period of maturity 

 lias the usual oospore structure described above, with very thin, 

 smooth episporium, which when mature is of a light yellowish-brown 

 colour, like the persistent wall of the oogonium. In the proper place, 

 then, and at the very time when they were sought for, there were 

 found oospores which might have belonged to Phytophthora infestann. 

 not only from their structure, but also because they seemed to spring 

 from thin branches of the intercellular mycelium of that Fungus. 

 They differed from all similar organisms which I knew, in their small 

 size, and in the peculiar insertion of the oogonium and antheridium. 

 To test the value of the conjecture that these were the oospores of 

 Phytophthora, an attempt was made to. obtain their speedy germi- 

 nation. To observe this, thin slices from one of the Potatoes in which 

 they were particularly plentiful, but in which other Fungi and In- 

 fusoria were relatively scarce, were placed in drops of water on object- 

 glasses, and the oospores were further isolated. Some of the oospores 

 sown in this way germinated, sending out, within twenty-four hours, 

 a tube, which became several times longer than the diameter of the 

 oogonium (Fig. 6, *) ; in the course of one or two days some of these 

 tubes also sent out several short branches, and then they ceased to 

 grow. Previous to this the extremity of the tube or of a branch 

 often swelled into a round bladder, into which all the protoplasm was 

 collected, and which was then cut off by a septum. The young plant 

 did not grow further. When the Potatoes which were experi- 

 mented on had been kept a few days longer in a moist atmosphere, so 

 as to prevent their drying up, and secure the complete maturity of the 

 oospores, new sowings were made. In this case the result was 

 different. Some of the oospores did not germinate ; but a large 

 nximber of others speedily seut out a thick, short, straight tube, which 

 grew to about the length of the diameter of the oospore, and then its 

 further growth lengthways ceased ; but soon its extremity suddenly 

 swelle<i out into a globular bladder. Into this, while it was swelling 



