Pethybridge — On the Rotting of Potato Tubers. 545 



itself is usually empty, and closed off by a wall or walls from the antheridium 

 itself. Although a very careful watch was kept, at uo time could any passage 

 of protoplasm from the antheridium to the spherical portion of the oogonium 

 be made out, and no " fertilization track " has ever been observed. 



When the oogonium has attained its full size, its contents occupy the 

 upper spherical portion only, which is then completely filled with densely 

 granular protoplasm. Sooner or later tlie protoplasm gradually begins to 

 withdraw itself from contact with the oogonium wall, and the early stages of 

 this process resemble what one would expect to see if the oogonium were 

 being plasmolysed by being placed in a salt solution. Gradually a definite 

 sphere is produced within the oogonium which occupies only a portion 

 of its interior, and which at first appears not to be surrounded with 

 a definite wall, hence to be an oosphere. The wliole of the contents 

 of the oogonium, however, are not utilized in forming the oosphere : 

 small granules are to be seen left behind occupying the space between 

 the oosphere and the oogonium wall ; and, further, other portions of 

 the contents may frequently be seen forming what might be described as 

 trabecula-like structures joining the oosphere to the oogonium wall. When 

 one of these lies towards the base of the oogonium, it gives a false impression 

 as of a "fertilization track" proceeding from the antheridium (see fig. 17, 

 Plate XLIV). 



A definite wall now begins to be formed around the oosphere, which 

 ultimately becomes about 2f.i thick, and is very slightly tinged with a yellowish 

 brown colour, and tlius an oospore is produced. The contents at the same 

 time undergo a change : from being more or less uniformly distributed through 

 the developing spore, they become aggregated and more dense towards the 

 periphery, leaving a much less dense central portion, while a single, rather 

 large, oval refractive body is to be seen in the peripheral portion of the 

 contents of eacli oospore. Thus the fully ripe oospores of this fungus 

 resemble those of other members of the genus. The oospores vary slightly 

 in size, but as a rule their diameter is between 29;u and SO/i. One of them is 

 shown in fig 18, Plate XLIV. The wall of the oogonium is quite colourless, 

 and is not brittle as is the case with P. infestans, and pressure on the cover- 

 glass does not cause the oospore to be expelled from the oogonium. The 

 germination of these spores has been attempted, but so far without 

 success. 



From what has been said it will be seen that the oogonium when fully 

 developed is a pear-shaped or balloon-like structure, with its larger distal end 

 (in which the oospore develops, and which is about 36/^ in diameter) situated 

 on the top of the antheridium, and with its conical or funnel-shaped proximal 



SCIENT. PECO, R.D.S., VOL, XHI, NO. XXXV, 4 M 



