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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 

 m 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; CtU-, DIVIDES 



"The host cell now contains, of abnor- 

 mal bacterial products, (a) these escaped 

 endotoxines, (&) 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." 



