362 A TEXTBOOK OF THEORETICAL BOTANY 



from plant to plant by grafting, and in 1892 it was proved that this was due 

 to a substance which could pass through a fine porcelain filter that held back 

 even the smallest Bacteria. Furthermore an infinitesimal dose of this sub- 

 stance rapidly spread throughout an inoculated plant and apparently increased 

 enormouslv in amount in doing so. These facts led to the conclusion that 

 the substance in question was a living and growing material, whose particles 

 were too small to be microscopically visible. Thus arose the idea of a 

 " virus " as a type of living organism lying in a region much below the 

 smallest Bacteria. It has since been shown that many of the commonest 

 diseases both of plants and animals are attributable to such filterable viruses, 

 and the subject has advanced to the front rank of importance. 



It is a curious fact that we only know of viruses as parasites, that is, we 

 can only recognize their presence by the efl^ects on their hosts. No such thing 

 as a free-living or saprophytic virus is known, and it is difficult to see how 

 they could be discovered if they exist. This argues against the idea, once 

 popular, that viruses are a primitive form of life, lying between the living 

 and the non-living, with particles which are intermediate between large 

 molecules and the smallest visible organisms. 



Dilution and filtration experiments with virus-containing juices have 

 proved that there are definite virus particles, which vary between 10 and 

 200 m/a in diameter {mfi = j oVmi^O- They vary greatly in their power of resist- 

 ance. Some are easily killed by drying, heat or poisons. Others will remain 

 active for years in the dry state or survive in absolute alcohol for forty-eight 

 hours. This resistance to agents which normally kill living organisms very 

 rapidly, combined with the fact that some viruses can be apparently pre- 

 cipitated from suspensions and redissolved in water, has suggested doubts 

 whether all viruses are really of the same nature. The belief has been growing 

 that whereas some viruses are possibly ultra-bacteria others are simply toxic 

 substances. The latter view seems to have been definitely proved by the 

 isolation of several viruses in fully crystalline form, and of some others 

 as liquid crystals. Such substances certainly cannot be considered to be 

 living agents. The apparent growth of such viruses in the host plant can 

 only be accounted for by supposing that they catalyze the production of 

 similar molecules in the cells of the host. In this respect their action is 

 analogous to that of the genes in a normal cell. 



Among plant diseases caused by viruses are the important group of 

 mosaics (Figs. 350 and 351), so called because they cause mottling of the 

 leaf. The contagious variegations first referred to belong to this group. 

 Flowers may also be affected, and the coloured streaking or " blazing " seen 

 on Tulip petals is an example which must be familiar to many. Other viruses 

 cause stunting or malformation of growth, yellowing of leaves, fruit dropping, 

 or crinkling and rolling of leaves (Fig. 352). They are seldom fatal to the 

 host, but they usually cause a marked diminution of vigour which is accom- 

 panied by a serious reduction of crop, so that their agricultural effect is 

 immense. They are very seldom transmitted through the seed, but plants 

 like the Potato, which are habitually propagated by vegetative means, carry 



