4 CEOWN-GALL. AND SAECOMA. 



the leaf tumors is approximately that of the stem. "We have, then, 

 in place of the leaf traces and loose cellular structure of the ordinary 

 dorsiventral petiole or midrib, a firm structure consisting of a green- 

 ish pseudopith (of tumor tissue), surrounded by a cylinder of wood, 

 beyond which is a cambium and then a bark covered with cork. This 

 stem structure is sometimes very perfect, although less w^oody than a 

 normal stem — i. e., of a looser structure — at other times, incomplete 

 on one side, only a partial cylinder of wood being formed. This 

 stem structure may develop either in the central leaf trace or in one 

 or more of the side leaf traces, according as one or the other is pene- 

 trated by the advancing strand of tumor tissue. All stages of this 

 conversion have been seen, from petioles which still present their 

 external form and show only a slight disturbance in one of the 

 leaf traces to those in which all resemblance to an ordinary leaf has 

 disappeared, the petiole being displaced by a thick, firm, stemlike 

 growth more or less circular in cross section and totally unlike the 

 dorsiventral structure of a leaf. Even in such cases traces of the 

 unchanged wings of the petiole may be seen sometimes as marginal 

 appendages of the secondary tumor. 



Full details will be given in another bulletin as soon as the illus- 

 trations can be prepared. 



Approved : 



James Wilson, 



Secretary of Agriculture. 



Washington, D. C, June 17^ 1911. 



[Cir. 85.] 



o 



