It has been mentioned that although plants do not form antibodies they have the capacity to develop a certain resistance, in a sense a degree of “acquired immunity”, to some virus diseases. This has been carefully studied with the aid of the microscope using high power microscopy. This is how it worked. After a plant has been infected with a particular virus for some time it may “grow away” from the disease, which means that it no longer shows symptoms. Nevertheless the virus is present in the tissues and the plant is to all intents and purposes now a “carrier “, a phenomenon which has already been described. Such a plant cannot be reinfected with the same virus, though it can, of course, be infected with other viruses.

We can follow this sort of acquired immunity in plants a step further. It sometimes happens that there exist two viruses which are very closely related when examined under a microscope, so closely related that one is called a “strain” of the other. It also may happen that of these two closely related viruses, one is a mild or a virulent strain while the other produces a severe disease. Now if the plant be first infected with the mild strain of virus, which may be so mild as to produce practically no symptoms, then the plant is immune to infection with the severe virus. This has been carefully observed under the microscope. You may wonder at this point that there is no difference from vaccinating for smallpox, but there are in reality great differences. First of all there is as yet no evidence of the formation of antibodies in plants comparable to those formed in animals. Secondly, this type of immunity in plants depends upon the universal presence of the mild virus in the tissues of the plant. In other words, it seems to be a case of “first come first served “, and the second virus cannot enter so long as the first one is in complete possession of the field. If, however, as sometimes happens, some part of the tissue is not completely invaded by the first virus, then the second virus may gain an entry into the plant at that point. Even so, it will not be able to spread far if the rest of the tissues are permeated by the first virus. This has been observed under the microscope using high power microscopy.

A quotation from Aristotle says: “Nature makes so gradual a transition from the inanimate to the animate kingdom, that the boundary lines which separate them are indistinct and doubtful.”
Many things have been studied and discussed about the virus but perhaps many are still inclined to ask the question: “But what is a virus?” We have learned something of what viruses can and cannot do. Still some may ask: But what are they and where do they come from?” In order to try to answer this legitimate question let us discuss the various theories concerning the nature and origin of viruses. A consideration of the nature and origin of viruses almost inevitably leads to a discussion of the nature and origin of life, so that speculation must obviously play a large part in such a discussion. The question which the layman invariably puts to the virus worker is: “Are viruses living things?” One reply to this question, of course, is: “Define what a living thing is and I will tell you,” Because there is no exact criterion of life.” Scientific researches endeavored to answer the various questions raised with the aid of the microscope using high power microscopy.

We have seen already that viruses show certain activities which are characteristic of living things; as what has been observed by the scientists under the microscope using high power microscopy. For instance viruses have the power to multiply or reproduce themselves and they also ” mutate ” or change into closely similar strains. We have also discussed that it is possible to purify and concentrate certain viruses until they become crystalline and behave in other ways like a chemical rather than a microorganism. This has been observed closely by scientist who performed laboratory experiments with the use of the microscope. So you will be able to understand something of the dismay of the biologist who must now readjust his ideas First to conceive of an agent, which although behaving like a chemical, yet has that fundamental attribute of life, the power to reproduce itself ; and secondly, to revise his conception of what he means by a microorganism. On the other hand, to be quite fair one must also consider the dismay of the chemist who may be forced to contemplate a mutating molecule.

We know that viruses are extremely small and that the majority of them are far beyond the resolving power of the best lenses which use visible light. Indeed the microscope played a very significant part in the study of the viruses. Some, indeed, are so small as to be within the range of molecular size and are invisible to the naked eye. Now a molecule is defined as the smallest particle into which matter can be divided without changing its chemical properties. Are some of the viruses, then, protein molecules which have the power of reproduction under certain conditions? If they are molecules, then the particles of a given virus must all be identical in size and shape and must all have the same properties. These different shapes and appearances of the viruses have been closely scrutinized under the microscope by the scientific investigators.