miscellaneous: stimuli underlying tumor-formation 543 



(ten times as plentiful as in the tubers). The shoots also 

 contained an excess of oxidizing enzymes and a great excess of 

 organic acids — 2 to 3 times as much acid as the flesh of the 

 mother tubers.^ The shoots had, therefore, every requisite 

 for normal growth except a sufficient water-supply. This lack 

 of water prevented elongation and the development of the 

 respiring and transpiring organs, and the hyperplasias were 

 brought on as the result of the increasing acidity and oxygen- 

 hunger of the tissues. The stomata over these hyperplasias 

 were always wide open (Fig. 413), and here this condition could 

 not have been due to excessive water-supply, but must have been 

 due rather to oxygen-hunger or to purely physical tensions. 



We may now consider another and quite different type of 

 overgrowth, viz., that found on the roots of a great variety of 

 plants. I refer to the tumor due to Heterodera radidcola (the 

 common gall-producing eel-worm), which tumor is a destructive 

 disease on many cultivated plants. Here the larval forms of 

 the parasite live, from the beginning, wholly buried in the root- 

 tissues. As soon as the worm is hatched it begins to feed on the 

 surrounding cells, and immediately there is a tumor response on 

 the part of the infested plant. This response, however, is 

 quite unlike that under the obturated lenticels. The tumors are 

 irregular, soft and large, and composed chiefly of a few cells 

 enormously hypertrophied, each cell containing from a half- 

 dozen to twenty or more nuclei (sometimes se\'eral hundred), 

 which either have divided amitotically or have not pushed ordi- 

 nary nuclear division as far as to the formation of cell-walls. 

 These giant-cells appear in the earliest stages of the tumor, soon 

 after the eel- worms have hatched; and always, in thin sections 

 of the very young tumors, one may find the young worms sur- 



^The tests were made as follows: weighed quantities (35 grams) of (1) the 

 mixed shoots and of (2) the mixed flesh of the tubers were wrapped in surgeon's 

 gauze and crushed in the same manner in a mortar, extracted 10 minutes in 100 cc. 

 of distilled water at 80°C., filtered, and immediately titrated. 



Bacillus phytophthorus, down to A''. Intumescences also at X and Y on the young 

 side shoots. Level of soil-surface at L. White McCormick potato inoculated 

 in the top of the shoot April 10 and foliage destroyed as early as April 13 or 14 

 (see X, Fig. 211). Photographed April 18. X 2. 



