LIFE AND THE CELL 43 



any destruction of its infective properties. An infinitive amount of the 

 virus will increase many times over when inoculated into a normal plant. 

 The symptoms of a diseased plant are the mottling of the leaves due to 

 alternating patches or spots of light green or yellow, and dark green, but 

 under certain conditions the mottling may be masked. 



In 192 1 a new concept of the nature of the tobacco mosaic was sug- 

 gested. The substance of this concept was that it was a product of the host 

 cell, a gene, perhaps, that has revolted from the shackles of coordination, 

 and having the property of reproduction, continued to produce disease 

 only in the living plant cells. 



As tobacco mosaic virus is the most outstanding in having properties 

 which are easily worked with, as stated above, and as it is typical and 

 representative of all viruses it has been experimented with extensively. 

 Countless numbers of tobacco plants have been grown and infected 

 artificially. The diseased plants after a certain time were ground up, pressed 

 and the tobacco juice containing the virus extracted. Protoplasm, in gen- 

 eral, contains proteins, fats and carbohydrates. Certain enzymes are pro- 

 tein splitters or digesters. Proteolytic pepsin, as noted before, is an organic 

 protein digester. This enzyme was added to some of the plant juices in a 

 test-tube and kept under suitable conditions to see if it would act on the 

 virus. After a certain length of time a small amount of the solution was 

 rubbed on the leaves of some healthy plants. No infection resulted after 

 repeated tests, as the protein causing the disease had been digested. Pepsin 

 is specific in its action; it will not act on fats, carbohydrates, hydrocarbons 

 or salts. Therefore a sound conclusion that the virus is protein in nature 

 can be made. 



Certain chemicals such as ammonium sulfate or dilute alcohol will precipi- 

 tate proteins. They were added to some of the diseased tobacco juice to 

 which pepsin had not been added. A solid precipitate was thrown down. 

 A bit of the supernatant fluid was rubbed on healthy leaves. No infection 

 resulted. A different picture was represented when a neutral liquid, as 

 water, was added to the precipitate and it was dissolved and then rubbed 

 on normal leaves. Diseased plants resulted. These two experiments proved 

 without doubt that the infective agent resided in the protein molecules. 



To further prove the nature of the virus, the precipitate was again dis- 

 solved in a neutral liquid and ammonium sulfate compound was added. 

 Crystals were formed from the solution. These crystals were refined by 

 ten successive fractionations and recrystallizations. By this technique all 

 impurities as well as all living matter was separated out. Why do we say 

 that living matter was ehminated? Because no protoplasm is known to pos- 

 sess the property of crystallization. Did you ever see a crystalline gonococ- 

 cus, amoeba or a "crystalline chicken" either in a coop or walking down 

 Fifth Avenue on Easter Sunday or any other day for that matter? 



Now if these crystals infect healthy plants far-reaching results can be 



