8 AECD-3446(App. ) 3s pletely undifferentiated but elongated cells, which must serve as conducting tissue, to phloem with an apparent organization (in cross section) into sieve tubes and companion cells. However, no sieve plates were observed. The xylem elements are very much shortened with reticulate or spiral thickenings. —- The conducting tissues are rather regularly arranged in a cylinder around a centra) pith in each individual swelling of the multiple tumor. mt differentiates. An internal phloem The available material was not well fixed for cytological pur~ poses, but the nuclei appeared normal. The parenchyua contains cluster orystals especially near the lactiferous ducts and starch grains are particularly abun~ dant in the outermost layers of parenchyma. The lactiferous ducts are not well developed as in the normal stem and there appear to be no functioning seoretory cells. Aside from the proportionately small amount of conductive tissue, the tumors appear histologically surprisingly normal. There are no giant cells, the _ tissues maintain a regular arrangement, and there is no excessive proliferation of eny one tissue. There is simply a general “ground mass" of parenchyma with relatively little xylem and phloem. The striking histological feature of the tumors is the large number of : growing points or primordia most of which fail to continue development. primordia show a wide range from normality. These In normal primordia, apical growth is retarded early and ensuing development and growth is due to intercalary activity in addition to some mlocalized cell division (15). rordia, this intercalary growth seems to faile In the tumor pri- The procambial strands differ- entiate into some semblance of a conductive tissue, but no new tissue seems to develops The meristematic cells enlarge and become parenchymatous (Figures 8a and 8). <A phellogen must differentiate in some cases near the surface, but very often the tissue it produces is sloughed off. The surfaces of the pri- mordia often appear to be suberized and some peripheral tissue sloughs off rot UNIVERSITY ancyyy - OF WAsH LInlvEs BRARIES <# UNIVERS?ry , UNIV. OF Wisi oy :