366 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1955 



found to be useful as a means of immunization against a more virulent 

 strain. This virus appears to have a limiting membrane and a type of 

 morphological differentiation which can hardly be regarded as char- 

 acteristic of molecules. This structure probably represents the result 

 of the interplay of many molecules. 



The virus shown as No. 2 on plate 2 is influenza virus, which appears 

 as white fluffy balls of about 120 m/i. This virus was responsible for 

 one of the greatest outbreaks of disease within the knowledge of man. 

 It has been reliably estimated that approximately 500 million people 

 had influenza during the winter of 1918 and that of these about 15 

 million died. Here in the United States alone over 400,000 people died 

 within 4 months during that epidemic. In order to place this in 

 proper perspective it has only to be remembered that our total battle- 

 field casualties durmg World War I and World War II add up to 

 approximately the same figure. Here, therefore, is a single virus 

 disease which, in 4 months, caused as great a loss of life in this country 

 as occurred among our soldiers on the battlefield in two great world 

 wars. It is also noteworthy that in 1918 the causative agent of influ- 

 enza was unknown, for it was not mitil 1932 that human influenza was 

 shown to be due to a virus. 



No. 3 of plate 2 is the familiar tobacco mosaic virus which exists in 

 the form of rods 15 by 300 m/x. Of considerable interest is the fact 

 that Williams of the Virus Laboratory has shown that the rods of this 

 virus have a hexagonal cross section. Each rod or molecule appears 

 to have a crystalline structure. 



The virus shown at the right (No. 4) of plate 2 is known 

 as the latent mosaic of potato virus or sometimes as the healthy 

 potato virus. It consists of crystallizable nucleoprotein rods which 

 are thinner and longer than those of tobacco mosaic virus. This 

 virus is thought to be present in all, or almost all, of the potato plants 

 grown in the United States, yet it does not cause an obvious disease and 

 our plants appear healthy. However, it is not present in potato plants 

 grown in certain European countries. If the only experimental mate- 

 rial available were the potato plants of the United States, it would 

 be most difficult, if not impossible, to prove the existence of this virus. 

 Extracts from one potato plant would have no effect on a second plant 

 because it would already contain the virus. It is only by virtue of the 

 fact that potato plants without the virus are known to exist and the 

 fact that this virus causes obvious disease manifestations in certain 

 other plants that it was possible to establish the existence of the latent 

 mosaic of potato virus. In the absence of this information, this virus 

 would be regarded as a normal constituent of our potato plants. Now 

 it may be useful in any consideration of problems such as the nature 

 of genes or the cancer problem to wonder how many similar entities 



