1892.] NEW-YORK MICROSCOPICAL SOCIETY. 85 



meristematic, but becomes the first layer of the phelloderm. With 

 the next division of the mother cell the outer cell is added to the 

 cork, and, thus alternating, the successive zones are formed until 

 about eight layers of phelloderm appear, when the growth once 

 more becomes centripetal and so continues. In the younger 

 stems a layer of cork is added each year. This appears to con- 

 tinue through life, for in the periderms of older stems the layers 

 of cork so closely correspond to the annual zones that the loss of 

 bark would easily explain the deficit. Thus the plant seems to 

 be exceptionally regular in the formation of all its parts. The 

 cork cells are tabular, appearing at first with delicate walls filled 

 with sap (Fig. 20). If complete suberification of the walls 

 results the cell contents remain as a brown, resinous mass, filling 

 the cell. Sanio holds that air never appears in those cells which 

 contain this residue of cell life. But cases are not wanting in 

 which the contents seem to have drawn away from the walls, 

 leaving an air cavity. This separation may be due to the dis- 

 turbance of the cell either from chemical reaction or cutting. In 

 many cells the cutinization is slight, in which case the inner 

 lamina grows to the exclusion of nearly the entire lumen and 

 becomes much furrowed by canals (E). It is worthy of note 

 that these thickened and brown-colored cells usually alternate, 

 giving a marked stratified appearance to the cork. The walls 

 are transparent, even in the oldest cells, readily showing through 

 the green of the phelloderm. They adhere strongly by their 

 tangential walls, and when the pressure of growth finally severs 

 them they curl back from the break, causing the scale-like appear- 

 ance of the bark. Contemporaneous with the cork-meristem 

 appears the first trace of the cortical sheath. Certain cells of 

 the parenchyma adjoining or near the bast begin to thicken, and 

 eventually become short-celled sclerenchyma, the lumen being 

 nearly excluded and the pit canals much branched (Fig. 21). 

 A zone more or less interrupted is formed, sharply separating the 

 plerom and the periblem. Secondary bundles often arise outside 

 the primary groups. This element is a very characteristic feature 

 of the outer cortex, and affords exceptionally good material for 

 the study of the origin and the development of sclerenchyma. 

 The dilatation of the fundamental mass of the periblem keeps 

 pace with the growth of the stem. In so doing there is a loss in 



