68 



Inside the Living Cell 



infecting healthy plants. When introduced into the plant, an extremely 

 small quantity infects the plant with the disease and it also multiplies 

 extensively so that, after a time, a much larger quantity of infective 

 material than that introduced can be obtained from the plant cells. 

 It therefore appeared that this substance had one of the characteristic 

 features of life, viz., the ability to multiply in an appropriate en- 

 vironment. 



1^= Tooo^* 

 a millimeter 



FIG. 15. Relative sizes of viruses and bacteria 



A considerable number of viruses has now been isolated in what 

 appears to be a pure condition, some of them in a crystalline state. 

 Crystallinity simply means that the particles all have the same well- 

 defined regular shape so that they will pack together in a lattice 

 arrangement, forming crystals which have regular geometrical shapes. 

 More recently many viruses have been photographed under the elec- 

 tron microscope and their shapes and sizes made visible (see Plates 

 8 and 10). It is found that they vary greatly in size and shape. Some 

 are spheres; others, like the tobacco mosaic virus, are cylindrical 

 rods; others resemble a tadpole with a tail. The spherical ones easily 

 pack into a crystalline form, and the long cylinders line up in one 

 direction. 



