44 Thompson—The Structure and Development of 
cells, which appear in longitudinal section as clear refractive 
fibres of considerable length, lies on the outer margin of the 
vascular bundle portion of the stem. The bundle cylinder 
consists first of a zone of external phloem about six cells deep. 
Most of the cells are still embryonic, with large nuclei and 
abundant protoplasm, some few have differentiated into sieve 
tubes. In longitudinal section the sieve plates can be recog- 
nized. The septa are large, transversely placed, and bear 
either four or three sieve plates with numerous perforations. 
The cambium layer is clearly defined by its regular brick- 
shaped cells with large nuclei. 
The wood is a broad zone, occupying more than a third of 
the area of the section, and is traversed radially by the 
oblong, deeply pitted cells of the medullary rays. A longitu- 
dinal section through the wood shows numerous spiral tracheze 
in the inner or protoxylem region ; external to this are both 
short and long tracheids, whose walls are thickened and 
deeply pitted. Large vessels are numerous in the outer por- 
tion of the zone. 
On the inner side of the wood lie four large rounded patches 
of internal phloem extending into the pith. These patches 
are two to three times broader than the external phloem zone, 
and consist also of sieve tubes and undifferentiated phloem 
elements. The inner margins of the phloem patches are 
bounded by a two-celled layer, which may be termed a phloem 
sheath (Plate IX, Fig. 1, Z. s.). This is sharply differentiated 
alike from the adjoining pith cells and from the phloem. A row 
of somewhat similar but smaller cells separates the outer mar- 
gin of the phloem patches from the wood, and immediately in- 
ternal to this roware the patches of medullary cambium. The 
cambium cells have the usual brick-like shape, thin walls and 
large nuclei. The cells of the sheath are rounded and in 
close contact with each other. They have thickened pitted 
walls and are conspicuous by their size and the large amount 
of chlorophyll and starch they contain. The pith cells are 
