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out. The cambium consists of three or four layers of cells in depth, 
with nothing to distinguish it from cambium of ordinary collateral 
bundles. 
The elements of the xylem are arranged according to the 
normal type. Following these elements radially from the cam- 
bium, first are found one or two reticulated ducts; after these come 
from one to several with annular markings; after these and next to 
the pith are one or two marked spirally. All of these ducts are 
very small in diameter; no porous ones werefound. The libriform 
tissue is not well developed, the walls being hardly thicker than 
those of the surrounding parenchyma. Surrounding the reticulated 
ducts are small wood-parenchymatic cells. 
It has already been said that the bundles are collateral and 
open. This statement requires some modification, as in the older 
portion of the organ the cambium of the bundles is no longer 
evident, its place being taken by differentiated phloem elements. 
There is also an indication of the formation of a cambium 
ring. Such a ring never really occurs even in the oldest portion of 
the organ; but the bundles continue to develop both phloem and 
xylem elements until the ordinary method by which herbaceous 
stems accommodate themselves to this growth is no longer suffi- 
cient. This method, namely, the dilatation of the cells lying near 
the bundles, is beautifully illustrated here by the extreme size of 
the cells between the bundles. In the early stage of their devel- 
opment they are no larger than the cells of pith or rind, but as 
they become older they increase rapidly in both tangential and 
radial diameter. 
This process, however, appears insufficient to keep pace with 
the growing cambium, and they now become meristematic, form- 
ing new walls which are at first tangential; later, radial walls are 
formed. In this manner arise clusters or bands of relatively small 
cells, extending from bundle to bundle. While these small cells 
appear like the ordinary meristematic tissue of stems whose cam- 
bium ring is formed after the bundles appear, they do not continue 
meristematic; at least, in the organs studied there was little evidence 
that these small cells produced lasting tissue of any kind, and none 
whatever of the formation of phloem and xylem elements. One 
or two other variations from the common type of dicotyledonous 
