36 R. MARKHAM 



frequent intervals, and it seems certain that many of our ideas about the 

 constitution even of the best understood viruses will have to be revised. 



The present concept of the smaller plant viruses is that they consist of 

 tubular or globular bags made up from small protem subunits of a limited 

 variety, and enclosing a central mass of ribonucleic acid which may consist 

 of one chain, or a limited number of chains, of nucleotides. More and more 

 the protein subunits, either singly or as polymerized protein resembling the 

 virus itself, are being encountered in infected plants. This phenomenon is 

 clearing up some of the anomalous results which have been reported in the 

 literature, and study of these proteins may lead to better understanding of 

 virus multiplication. 



The following chapter is an attempt to summarize some of the present 

 information about those plant viruses which have been studied sufficiently 

 to give a consistent picture. An attempt has also been made to point out 

 some of the pieces of information which are missing. In this, many of the 

 plant viruses which have been studied have been omitted from discussion 

 because the information that has been obtained about them is trivial or 

 incomplete. Many plant viruses, too, are only known from the disease which 

 they cause, and possibly also from electron micrographs. A number of viruses 

 which have been purified and even crystallized are also omitted because 

 none of the work on them has yet been published. 



II. The Purification of Plant Viruses 



The problems of isolating plant viruses from diseased plants are numerous 

 and far from completely solved. Plant cells in general consist mainly of a 

 large vacuole filled with cell sap and surrounded by a thin layer of proto- 

 plasm in which are embedded the cell nucleus and the various plastids. It is 

 not known with certainty how the virus is distributed in such cells, although 

 it is probable that it develops in the cytoplasmic layer, and it is certain that, 

 in the case of the tobacco mosaic virus, a large proportion of the virus 

 particles is eventually deposited inside the vacuolar sap in the form of crystal- 

 line masses (Fig. 1). The problem then is to remove the virus from as much 

 of the plant material as possible and with the minimum loss. This has only 

 proved practicable because many viruses have such unique physical and 

 chemical behavior that it has been possible to destroy or otherwise remove 

 extraneous matter from them by relatively simple procedures. However, 

 lest it be thought that the isolation procedures for obtaining virus particles 

 from plants have been perfected, it is as well to realize that less than 10 % 

 of the known plant viruses have been purified to date. 



The first requirement for the purification of a plant virus is that there 

 should be a source of material containing sufficient virus to make the isolation 



