644 Wallace.—On the Stem-Structure of 
Accessory Bundles at the Base of the Stem. 
A number of accessory bundles are present at the base of 
the older stem. These begin a short distance above the 
hypocotyl and increase in thickness toward the base of that 
organ. They are visible externally as prominent ridges, 
especially on the hypocotyl, giving to transverse sections of 
this region a very sinuous outline (Fig. 5). The bundles 
run in the cortex, and there is usually one opposite each 
primary bundle. They follow the course of the cotyledonary 
trace-bundles in the hypocotyl, where there is one opposite 
each of the six bundles in the upper part, and one opposite 
each of the four bundles in the lower part. The meristem 
in which these new bundles appear is a product of the layer 
of cells which lies immediately outside the sclerenchyma. 
If, according to the statement in De Bary’s text-book 1 , ‘the 
ring of sclerenchyma (in Cucurbitaceae, &c.) belongs to the 
plerome and marks its outer boundary,’ then it is evident 
that we have to deal here with a meristematic endodermis of 
a cambial nature. In its earlier condition the same layer is 
differentiated as a starch-sheath, a fact supporting the above 
view of its morphological value. This starch-sheath is fairly 
well marked in transverse sections of the hypocotyl of the 
seedling (Fig. 6). 
Opposite each primary bundle the cells of the endodermis 
elongate in a radial direction and divide tangentially, giving 
rise to rows of cells, not unlike periderm, which spread out in 
a radiate manner from each sclerenchyma-segment (Fig. 12). 
Between the sclerenchyma-segments, i.e. opposite the primary 
medullary rays, this formation is interrupted. From a certain 
number of radial rows a bundle is formed by some of the 
cells becoming converted into xylem-elements and others 
into phloem. Between xylem and external phloem the cells 
remain meristematic and function as a normal cambium, which 
adds to the thickness of the bundle in the usual manner. In 
time the wood comes to be surrounded more or less com- 
1 De Bary, op. cit., p. 419. 
