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JAMA 100 Years Ago
January 5, 2005

THE ORIGIN OF PARASITISM.

Author Affiliations
 

JAMA 100 Years Ago Section Editor: Jennifer Reiling, Assistant Editor.

JAMA. 2005;293(1):100. doi:10.1001/jama.293.1.100-b

The remarkable predilection shown by certain species of microbes for certain hosts, and the relative indifference of those parasites to other organisms, has long been the subject of zealous speculation and inquiry. Great as has been the advance in our knowledge of the underlying causes of natural immunity during the last few years, it can not be said that any final explanation is even yet in sight. It is, indeed, apparent that the ardent prosecution of studies into the germicidal and antitoxic qualities of immune sera has caused a temporary suspension of activity in other lines of investigation. The broader biologic aspects of natural immunity sometimes have been neglected. It may well happen that studies in plant pathology will be found to throw light on some of the questions of animal parasitism that have hitherto been shrouded in obscurity. For some time botanists have known much concerning the various modes by which parasitic fungi gain access to the interior of the host plant. Some enter through the stomata, some by piercing the walls of the epidermal cells or the guard cells of the stomata. Marshall Ward, in his study of the Bermuda lily disease, discovered the interesting fact that the fungus concerned (a species of Botrytis) effects its entrance into the host plant by secreting at the tip of the germ tube an enzyme which softens the substance of the cell wall. Quite recently Massee has brought out important facts concerning the influences that determine the attack of a specific host by a specific fungus. It is pointed out that while the spores of a parasitic fungus will germinate on the damp surface of the leaf of any plant, yet the germ tubes will enter the tissues and infect only the particular kind of plant on which the fungus is known to be parasitic. It is a legitimate inference from this fact that the tissues of the particular host possess a special and peculiar attraction for the parasite in question. This hypothesis has been tested experimentally, and it has been found that under certain circumstances positive chemotropism plays a prime part in inducing infection. Through the use of certain chemical substances shown to attract certain fungi, an otherwise immune host plant has been successfully infected. In other words, the entrance of a parasitic fungus into the tissues of a healthy plant depends, at least in some cases, on the presence of some positively chemotropic substance in the cells of the host. It is stated that a saprophytic fungus can be gradually educated to become an active parasite for a certain plant by introducing a positively chemotropic substance into the tissues of that plant. It is possible also by similar means to induce a specific parasitic fungus to invade a new host hitherto not attacked by this particular parasite. In this connection it has been pointed out that a remarkable coincidence exists between the behavior of fungus spores toward their host, and that of the pollen grains when placed on the stigmas of flowers of their own and of other species. The marked positive and negative chemotropic properties shown in the attraction of pollen tubes are apparently closely related to the similar manifestations of the germ tubes of parasitic fungi. The strict association of specific parasite and specific host may, perhaps, be explained on a similar chemotropic basis.

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