miscellaneous: stimuli underlying tumor-formation 551 



development and relation to the larval cavity, so much so as to 

 have caused great wonder among cecidologists. In these galls, 

 the enlarged cells of the inner layer (Fig. 419 Hy) often contain 

 more than one nucleus (Fig. 424), i.e., the mechanism of cell- 

 division is more or less upset. 



It is worth while here to mention another type of plant tumor 

 in which cell-hypertrophy is a marked characteristic but in 

 which the nucleus takes even less part than in the preceding. I 

 refer to the root-nodules of legumes and, for the sake of a second 

 example, again to the club-root of cruciferous plants, the one due 

 to a bacterium, the other to a myxomycete. Here the parasite 

 is wholly inside the cells, not outside sending into it feeding or- 

 gans, as in case of the eel- worm. In both instances the parasit- 

 ized cells become enormously distended and filled with the bodies 

 of the parasite, the dividing cells being the remoter, non-para- 

 sitized cells. The nucleus is killed early in the invasion and all 

 the protoplasm of the cell is finally consumed and hence it 

 would seem as if the final enlargement of the parasitized cells 

 must be due to growth of the cell-wall uncontrolled by the proto- 

 plasm or to mechanical stretching, and this seems to be confirmed 

 by the fact that remote from these large parasitized cells there is 

 little evidence of hyperplasia, such as we might expect if soluble 

 cell-stimulating poisons were excreted. There is more or less 

 cell-multiplication in each of these tumors, often considerable, 

 but in their end terms, at least, these two types of overgrowth 

 are as far removed as possible from active hyperplasias, and need 

 not long detain us, although at one time much study was put 

 upon the club-root of turnips and cabbages (due to Plasmodi- 

 ophora brassicae) by various persons who thought they saw in it 

 certain resemblances to animal cancer. 



More interesting in every way are hyperplasias produced by 

 purely chemical means, such as I have illustrated in Figs. 364 to 

 377. We may take for discussion two examples, the acetic acid 

 tumors and the ammonia tumors. 



When cauliflower plants are exposed for a short time to vapor 



another part of the gall. Cross-section of the young worm lat X) surrounded by 

 multinucleate hypertrophied cells. Farther away is hyperplasial 'tissue. Medium 

 magnification. 



