THE VIRUS DISEASES OF PLANTS 427 



Properties of the Virus 



Leaving the discussion of the means of transmission from 

 plant to plant and the carriage of the virus within the plant, 

 one may next consider some of the properties of the virus, which 

 have been chiefly investigated by Allard and Chapman in the 

 tobacco mosaic. In this case the virus can be filtered through 

 the Chamberland, Berkefeld, and Kitasato filters without being 

 retained, though it passes less readily through those with finer 

 pores. Infectivity was 91 per cent, with the Chamberland, 

 63 per cent, with the Berkefeld normal, 47 per cent, with the 

 Berkefeld fine, and 40 per cent, with the Kitasato, in one series 

 of tests. ^ It is entirely checked by filtration through the 

 Livingstone atmometer porous cup and through powdered talc 

 I inch thick. (The cucurbit mosaic virus filters through 

 the Berkefeld, but not through the Chamberland filters tried.) 

 The whole of the virus is carried down with the precipitate 

 obtained by treating with 45 to 50 per cent, alcohol and 

 with an aluminium hydroxide precipitate, without its in- 

 fectivity being injured. It is quickly destroyed by alcohol 

 of ys to 80 per cent., formaldehyde of more than i to 

 800 destroys it in thirty-one days and 4 per cent, very 

 quickly, copper sulphate readily destroys the infectivity of 

 the virus under certain conditions, but mercuric chloride 

 and carbolic acid have little effect. Heating to over 80° C. 

 for five minutes sometimes destroys it, but it can sometimes 

 withstand 90° C. for the same time. It is quickly killed 

 near the boiling point. The dried leaves remain infective 

 for very long periods, giving as much as 100 per cent, infection 

 after three years. The bottled juice without preservative has 

 remained highly infective for fifteen months, though it had 

 undergone putrefaction, and the juice treated with toluol has 

 been found infective after three years. The virus obtained 

 from evaporated juice loses its infectivity more quickly. Dilu- 

 tion of the virus to i to 1,000 does not reduce its infectivity, 

 and at i to 10,000 some infection can still be obtained, but 

 higher dilutions generally fail. The virus of cucurbit mosaic 

 behaves in exactly the same way in regard to dilution, but is 

 much less resistant in other respects than that of tobacco 

 mosaic. It is destroyed by formaldehyde, phenol, and copper 

 sulphate in 0-5 per cent, solutions, and by mercuric chloride at 

 a strength of i to 2,000. Heating above 70° C. renders it non- 

 infectious and drying the infected tissue has also the same 

 effect. The expressed juice is seldom infectious for more than 



1 Chapman, loc. cit., p. 107. 



