ON THE ANATOMY OF CHENOPODIUM ALBUM L. 
spirals we now find elements with scalariform and sometimes with reticulate 
walls. Various types of transitional forms are found between the spiral 
element on the one hand and the pitted vessel on the other. The meta- 
xylem, and of course the wood produced by the cambium, contain only 
pitted vessels. 
The xylem, as we have learned, forms a compact woody cylinder made 
up of a series of concentric, undulate zones of growth (PI. XVI, B), each 
zone in turn being a circle of collateral vascular bundles united with one 
another by intermediary or conjunctive tissue. Separating each zone of 
growth is a tangential band of parenchymatous tissue of varying width. 
Narrow, or sometimes broad, bands of parenchyma traverse the xylem in 
radial direction. These bands usually connect radially the individual zones 
of growth. Often, however, they pass through several zones, and in those 
instances closely resemble medullary rays. 
In cross section the cells of this conjunctive tissue appear like ordinary 
parenchyma, but in radial and tangential cuts a great variation in form 
manifests itself. There are groups of cells made up of the common sub- 
stitute fiber — a tracheidal element with simple pits. Other elements of 
this tissue are comparatively short or even isodiametric. Morphologically, 
then, this tissue is not homologous with rays, though it may function as 
such. 
The phloem is made up of three types of elements: the sieve tube, the 
companion cell, and the phloem parenchyma. Contrary to the conclusion 
of earlier investigators (i, 4), the sieve tube is 
the principal element of this tissue. The tubes 
form longitudinal series (PI. XVI, D) with occa- 
sional anastomosing of the elements of closely 
connected groups. On the whole, however, the 
course of the phloem groups is radial-perpen- 
dicular, with connections of the elements of the 
groups taking place only through the leaf gaps 
formed by the branching of the leaf-trace bundles 
and their subsequent fusion. 
The sieve tube is of medium size with an 
average diameter of 14 The end walls are 
usually slightly oblique, which makes it difficult 
to observe the sieve plates in strictly transverse 
sections. There are no sieve plates in the radial J'°- Chenopodium album: 
„ . Diagrammatic drawing 01 Sieve 
walls. The latter, however, are extensively .^^e and companion cell, 
pitted with the companion cells, which, as is 
usually the case, are connected by simple pits with the neighboring paren- 
chyma cells of the conjunctive tissue. Late in the season the plates of 
many sieve tubes become covered with callus ; this callus formation is most 
often observed in the phloem of the primary bundles. 
