510 BACTERIAL DISEASES OF PLANTS 



380, sub. 1, and 382 to 384) to marked hyperplasia (Figs. 368 to 

 372 and 377). They have one element, however, in common and 

 in this they differ from all overgrowths due to active parasites, 

 that is, corresponding to the fleeting nature of the stimulus, their 

 growth is usually of short duration, whereas tumors due to para- 

 sites, because supplied with a continuous stimulating exudate 

 from the foreign organism, may continue their development 

 indefinitely. The physico-chemical stimuli are, however, I 

 believe, much the same in all cases where genuine hyperplasias 

 occur. To these let us now turn our attention. 



' IV. SPECULATIONS ON THE CHEMICAL AND PHYSICAL STIMULI 

 UNDERLYING TUMOR-FORMATION 



Classed according to the number and size of their component 

 elements, tumors are of three kinds: (1) simple hypertrophies 

 (cell enlargements); (2) hyperplasias (cell multiplications); and 

 (3) mixtures of the two, that is hyperplasias containing giant- 

 cells. The size and shape of the cells forming the hypertrophy 

 or the hyperplasia differ from tumor to tumor even in the same 

 tissue, thus in animals we have round-celled, oat-celled and spin- 

 dle-celled, large-celled and small-celled connective tissue tumors 

 (sarcomas). The nature of tumors varies also, of course, greatly 

 according to the nature of the tissue in which they originate, 

 since the cells of each organ have a histolog}^ and an inheritance 

 of their own. For this reason, connective tissue yields one type 

 of tumor, glandular tissue another type, vascular tissue a third 

 type and so on. 



An enormous amount of data has been accumulated on tumor 

 differences, that is on the gross and minute anatomy of tumors, 

 especially of human and animal tumors, because this has been 

 the easiest method of approach, but it is not the most interesting 

 side of the problem. That lies in quite another field, viz., in 

 the field of hypothesis and experiment dealing with their 

 etiology. 



All overgrowths, without reference to whether they are due 

 to parasites or have developed independently of them, appear to 

 me to be singularly alike in their chemical and physical origin 

 and physiological requirements, their diverse appearances being 



