292 The Phenomena of Morphogenesis 



Gautheret, Levine, Riker, de Ropp, and P. R. White have been especially 

 active. 



The inception of a crown gall seems always to come by means of a 

 wound. In the early stages of this infection the reactions of the host cells 

 produced either directly or indirectly by the parasite are much like wound 

 reactions, but in the young gall the new cell walls soon lose the regular 

 arrangement found in wound tissues. Reparative wound calluses and those 

 formed at grafts often resemble the early stages of crown gall, but as the 

 gall develops, rapid cell division occurs in the outer layers and a large 

 mass of callus is formed. The great difference between crown gall and 

 ordinary callus, however, is that the latter is self-limiting and soon be- 

 comes quiescent whereas gall tissue is capable of indefinite and amor- 

 phous growth. This is a fact of particular morphogenetic significance. 

 There is now good evidence that the cells of crown gall have undergone 

 a permanent change in character. This seems to involve an acquirement 

 of the capacity for autonomous growth, which may result, Rraun ( 1958 ) 

 believes, from the permanent activation of a series of systems by which 

 growth substances are synthesized. In normal cells these systems are 

 precisely regulated and growth ultimately stops. Normal cells in cul- 

 ture require auxin from an outside source but crown-gall cells do not. 

 Crown gall thus differs from most other galls, which are self-limiting and 

 do not grow indefinitely. In some cases the crown gall matures, stops 

 enlarging, and undergoes some histological differentiation. Nests of vascu- 

 lar cells, chiefly xylem, appear in it (Fig. 11-11), and the primordia of 

 roots and shoots may develop. The cytological and histological changes 

 in the development of a typical crown gall have been described by Ther- 

 man (1956) and Kupila (1958). 



The process of conversion of normal cells into tumor cells is a gradual 

 one. Its inception depends both on a wound stimulus and on the pres- 

 ence of an auxin ( Rraun and Stonier, 1958 ) . Tumors differ in appearance, 

 in the degree of their organization, and in their capacity for growth. These 

 differences may be the result of their location on the plant, the virulence 

 of the strain of infecting bacteria, or other factors. Crown-gall tissues 

 can be grafted into normal ones, and these may be carried through an 

 indefinite series of graft transfers. There is no good evidence that they 

 induce adjacent normal tissue to form tumor cells, though temporary 

 alterations may take place there. Gall tissues can readily be grown 

 in culture and can then be grafted back to normal ones and form galls. 

 In all these cases their cells remain unchanged. 



In a few plants secondary tumors may develop, often at some distance 

 from the original gall or primary tumor. In certain cases this results from 

 an infection near the apical meristem and a subsequent separation of 

 the secondary from the primary gall by growth. Sometimes, however, 



