CHARACTER OF THE TUMOR. 163 



In the crown-gall there is not only enormous proliferation of the 

 parencln'matic tissues (often in nestlike masses), but there occur in 

 the tumor also all the other tissues normal to the organs attacked, 

 although usually the woody tissues — the conductive ones — are greatly 

 distorted and reduced very much in volume. They are there, how- 

 ever, permeating the tumor in various directions. Some portions of 

 them are seen on sections as small ligneous islands and others as more 

 or less lignified short tubes; but these fragmentary appearances are 

 due simply to the direction of the knife cut, and careful dissection of 

 the part shows that the ligneous conductive tissues arise from the 

 base of the tumor and twist and branch in various directions through 

 it, becoming reduced to widely separated single vessels or pairs or 

 small groups of vessels in the remoter portions. This fact should, 

 perhaps, count for more, in our judgment, as to the analogies of these 

 tumors than the appearance of the rapidly proliferating parenchyma 

 cells, v.diich, however, strongly suggest malignant tumor tissue of 

 animals, as may be seen by referring to Plates XXVI to XXX. (See 

 also Plates VIII and XXI.) Undoubtedly many of the supporting 

 elements in crown-gall, perhaps all, grow out of the substratum along 

 with the growth of the tumor (PI. XXVI, lower figure, Y). In some 

 instances in large tumors it would seem as though some of the vessels 

 were produced in place from the tumor itself, but of this we are not 

 certain. 



Another suggestive likeness is the fact that in this disease, and 

 likewise in the olive-tubercle, there are well marked metastases, that 

 is, secondary tumors arising from within, at some distance from the 

 primary tumor, as the result of migrations, but as yet in the crown- 

 gall we do not know the mechanism of this migration, i. e., whether 

 the bacteria move independently through the tissues, setting up 

 irritations in more or less remote places, or whether the migration 

 takes place only within special host cells. In many plants cells push 

 through small pits in vessel walls, forming in the interior of the 

 vessels numerous rounded growths known as thyloses. These often 

 contain bacteria. If they should become dislodged, they might then 

 fall or be carried upward in the direction of the water current to 

 become, if still able to divide, the center of a new growth elsewhere, 

 ((uite after the manner of malignant animal tumors, assuming 

 metastases of the latter to originate always in this way. Observa- 

 tions of Hunger on the brown rot of tomato and of the senior writer 

 on the same and on mulberry blight show that thyloses are developed 

 in vessels as the result of bacterial infection. Usually, in thyloses 

 the irritation is temporary and tumors are not developed, although 

 in case of the roots of old cucurbits it is common to find woody 

 vessels compactly filled with a pseudo tissue composed of thyloses. 



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