National Geographic : 1913 Jan
THE NATIONAL GEOGRAPHIC MAGAZINE by a single needle prick on a healthy leaf. Here no normal cells remain; the whole body is a tumor composed of fleshy tissue and woody fiber, the minute dots all over the picture being the nuclei of the tumor cells, from which further proliferation will take place. It is not a simple matter to obtain photo-micrographs showing the bacteria actually in the cells, but on Plate XII we have photographs of eight different levels in a cell, and the irregular rod-like bac teria are easily distinguishable. These bacteria are described by Dr. Smith as follows: "The galls on Paris daisy are due to a white schizomycete named Bac terium tumefaciens. This organism is a short rod multiplying by fission and mo tile by means of a polar flagellum. It can be grown in many sorts of culture media, but does not live very long upon agar. It forms small, round, white colo nies in agar or gelatin poured plate." HOW THE PARASITE WORKS The fight between the infected cell and the bacillus is most interesting, and shows how the cell responds to the stimulation and reproduces itself. This is how Dr. Smith conceives it to take place: "The relation between host and para site in this disease may be regarded as a symbiosis (or condition in which two dis similar organisms live together), in which the bacterium has the advantage. The bacterium derives its food from the cells of the host and drives them at a break neck speed. It gives to them in return its waste carbon dioxide for the use of their chloroplasts." (Chloroplasts are the bodies in the cell which contain chlo rophyll or green coloring matter, and are the most important bodies concerned in the making of starch from the water in the cell and the carbon dioxide of the air.) "The bacterium does not destroy the cells of the host, but only stimulates them into an abnormal and often exceed ingly rapid division. "This stimulus, it would seem, takes place through the following delicate ad justment of opposing forces: Within the host cell the sensitive parasite produces as one of its by-products an acid. As this acid accumulates it stops the growth of the bacteria and destroys a portion of them without, however, destroying the host cell. The membranes of these dead bacteria, which have now become perme able, allow the diffusion into the host cell of bacterial endotoxines." (Endo toxines are poisons produced by the bac teria, but held within them while alive, and only escaping when the membranes of the dead bacteria disintegrate.) THE CELL DIVIDES "The host cell now contains, of abnor mal bacterial products, (a) these escaped endotoxines, (b) a certain amount of weak acid (acetic ?), (c) some ammo nia, and (d) an excess of carbon diox ide. Under the stimulus of one or more of these poisons the nucleus (or point from which proliferation commences) divides by mitosis (the usual but more complex of the two methods by which cells multiply). In process of division the nuclear membrane (the envelope in closing the nucleus) disappears and the contents of the nucleus flows out into the cell. The dormant bacteria under the stimulus of this nuclear substance renew their activities in the daughter cells until again inhibited, whereupon the daughter cells divide. By this rocking balance, in which first the parasite and then the host cell has the advantage, the tumor develops rapidly and independently of the needs of the plant." This rapid growth of the tumor, inde pendently of the needs of the plant, and the tumor-strand, which produces the secondary tumors with structure of the primary tumor, show very clearly the cancerous nature of the disease, and its development closely parallels what takes place in cancer in men and animals. Dr. Smith is very careful to point out that he considers that his discoveries have no absolutely direct bearing upon human cancer, and the following closing words are characteristic: "Nothing in this bulletin should be construed as indicating that we think the organism causing crown galls is able to cause human cancer, but only that we be lieve the latter due to a cell parasite of some sort."