miscellaneous: stimuli underlying tumor-formation 545 



rounded by and closely appressed to the big cells (Fig. 414) but I 

 have not actually seen their sucking organ inserted into them. 

 I have seen these large multinucleate cells in very early stages 

 of gall-development where the worms had been present only a 

 few days, but always already the head of the young worm is 

 in close contact with the cell-membrane where by means of its 

 mouth-parts it is able to feed. From this it might be thought 

 that the stimulus to growth must reside exclusively in the saliva 

 or other excretion from the mouth-parts of the feeding worm, 

 and so long as we had no counter observations this explanation, 

 which is not yet altogether excluded, appeared to be reasonable 

 and sufficient. But there is in Florida on orange-roots, as Cobb 

 has shown, a free-living parasitic nematode, closely related to 

 Heterodera radicicola, which also sends its mouth-parts into many 

 root-cells, but no tumors result, and the explanation I have to 

 offer for this marked difference in response is not that the orange 

 cannot respond by overgrowths like other plants, since it gener- 

 ally responds quickly to crown-gall inoculations but that, the 

 greater part of the body of the orange-root nematode being out- 

 side of the plant, the anal excretions are voided into the earth 

 and do not reach the tissues, whereas in the Heterodera radicicola 

 the anal excretions are voided into the tumor and are, I believe, 

 the chief cause of its development. The cells nearest to the 

 worm receive the greatest volume of stimulus and in these cells 

 not only is the protoplasmic membrane paralyzed so that it 

 allows a great influx of water and foodstuffs into the cell, but 

 the karyokinetic mechanism of the nucleus is also partially 

 paralyzed so that the cell cannot divide and the result is not only 

 a repeated mitotic division of the nucleus within the cell (I have 

 seen 30 nuclei in a cell and Nemec has seen more than 500 in 

 cells of Vitis gongylodes) but also at times amitotic division with 

 an enormous growth and stretching of the cell-wall until the 

 cell often becomes several hundred times its normal size, gorged 

 with water and foodstuffs which serve the worms for nutriment. 

 Later there may be nuclear fusions, all the nuclei merging into 

 one or several mulberry-like conglomerate masses (Nemec). 

 These cells are totally unlike any normal cells of the plant and so 

 large that, in thin slices of the gall examined superficially, it 



