162 CROWN-GALL OF PLANTS. 



accomplish under conditions favorable to it is the formation by cell 

 division of a protective cork layer about the diseased area somewhat 

 as the tissues of the animal body for the same purpose inclose tuber- 

 culous nodules or syphilitic gummata with a fibrous mass of connective 

 tissue. According to the current medical classification of tumors, 

 crown-galls belong with the infectious granulomata apart from the 

 true tumors, but it is not likely that such classifications represent 

 anything more than a temporary phase of progress in pathology, 

 because they rest largely upon absence of knowledge. One by one 

 as the causes of tuberculosis, lepra, syphilis, actinontycosis, etc., 

 have been discovered these diseases have been removed by medical 

 writers from the domain of tumors and classed as specific inflamma- 

 tions, but logically if parasites should be discovered all the remaining 

 malignant growths would have to be removed, leaving nothing but 

 the empty pigeon hole for tumors. 



Unlike teratomas, these tumors do not have a restricted growth 

 comparable to a defective normal growth. Teratomatous growths 

 are frequent in plants, but quite unlike the cell development here in 

 question. Neither are crown-galls to be regarded as degeneration 

 processes. We have in plants certain disease phenomena, namely, 

 cedemas, which seem to be more like degeneration processes in animals 

 than are the growths here described. In oedema, which is believed to 

 be nonparasitic, we have swelling from excess of water supply and 

 more or less enlargement of parenchyma cells, but it does not usually 

 pass beyond simple hypertrophy and does not involve such hetero- 

 geneous hyperplasiac tissue changes as are conspicuous in the crown- 

 galls. Some of our gum diseases of unknown origin show somewhat 

 similar degenerations, formation of internal cavities, with enlarged 

 cells in the walls. In crown-galls no abscess cavities have been 

 observed. 



Cancers occur in a variety of animals, and no good reason has been 

 advanced why they should not occur in plants. These tumors are 

 morbid new tissue developments tending to weaken and destroy the 

 plant, and their structure does not suggest galls due to insects. 

 Insect galls are usually of quite specific structure and definitely 

 restricted growth, whereas the crown-galls are of indefinite structure 

 and indeterminate growth. As a working hypothesis, we may regard 

 insect galls as duo to a localized and fieeting stimulus of a chemical 

 nature not unlike the more generalized and prolonged stimulus which 

 leads to cell division in crown-galls. The determination of the imme- 

 diate cause of cell division in the one would probably throw much 

 light on the other. 



Certain resemblances to malignant animal tumors may be pointed 

 out in more detail. 



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