596 



" Corky Scab " of Potatoes. 



[NOV., 



a very few irregularly globose protoplasmic bodies, aggregated 

 round the nucleus of the cell (Fig. 4). These bodies range 

 from 3-5 to 4-5 fi* in diameter, and for a certain period of 

 time possess the power of spontaneous movement, slowly rotat- 

 ing on their axis or around the nucleus. When fixed and stained, 

 these amoeboid bodies are seen to possess a single nucleus. 

 Various stages of fission of the amoeboid bodies may be seen, 

 preceded by the division of the nucleus into two daughter nuclei. 

 How these amoeboid bodies gain an entrance into the cell of the 

 host has not yet been observed, but such invaded cells are 

 always the most internal of the cells affected by the parasite, and 

 are always immediately adjoining other cells containing the 

 organism in a more advanced stage of development. It seems 

 probable that the organism, both in the plasmodial and also 

 in the amoeboid condition, may be able to invade new cells 

 by passing through the pits present in the cell wall. The un- 

 attacked cells of the potato are replete with starch grains, and 

 contain shreds of protoplasm and a nucleus, which sometimes 

 retains its normal form, but is usually more or less crushed and 

 deformed by the pressure of the starch grains. It appears 

 highly probable that the cells are invaded by the parasite for 

 some time before the amoeboid bodies surrounding the nucleus 

 can be seen, for when they are present the starch has as a rule 

 disappeared from the cell. Stained sections support this view, 

 as minute, irregularly-shaped masses of a protoplasmic nature, 

 which stain deeply with a watery solution of Congo red, are 

 present amongst the starch grains in cells bordering on the 

 fringe of the obviously infected portion of the tuber. It may 

 be mentioned here that a watery solution of Congo red has been 

 found to differentiate very clearly the protoplasm of the parasite 

 from that Of the potato cell ; the former, becomes much more 

 intensely coloured than the latter, and of a different shade. 

 This may be due to a dinerence in functional activity and to 

 the difference in age of the two kinds of protoplasm. 



A second phase of development is reached when the amoeboid 

 bodies, after repeated division, blend together to form a 

 Plasmodium, which at this stage nearly fills the cell in which it 

 was formed (Fig. 5). This plasmodium presents an irregularly 

 nodulose appearance, and for some hours slowly continues to 

 change its form, the surface nodules gradually disappearing 



* ix = micromillimetre or T oVo millimetre. 



