138 HISTOLOGY OF MEDICINAL PLANTS 
walls. This thickened part of the porous end walls of two sieve 
cells is called the sieve plate, and it may be placed in an oblique 
or a horizontal position. 
In a longitudinal section the sieve tubes are seen to be 
slightly bulging at the sieve plate, and through the pores extend 
protoplasmic strands. The strands are united on the upper 
and lower side of the sieve plate to form the protoplasmic strands 
of the living sieve tubes and the callus, layers of dried plants. 
This callus is frequently yellowish in color, and in all cases is 
separated from the cell wall. In certain plants the sieve plate 
occurs on the side walls of the sieve tubes in contact with other 
sieve tubes. 
SIEVE PLATE 
Sieve plates on cross-section (Plate 46, Fig. 2) are polygonal 
in outline, and the pores are either round or angled. Large 
sieve tubes and sieve plates occur in pumpkin stem; but, almost 
without exception, in drug plants the sieve tubes are small 
and the sieve plate is inconspicuous. When the drug is pow- 
dered, the sieve tubes break up into undiagnostic fragments. 
When studying sections of the plants, the extent, size, and 
arrangement of the sieve tubes must always be noted. 
MEDULLARY BUNDLES, RAYS, AND CELLS 
Function 
The medullary ray cells are the lateral conducting cells of 
the plant. They conduct outwardly the water and inorganic 
salts brought up from the roots by the vessels and tracheids; 
and they conduct inwardly toward the centre of the stem the 
food material manufactured in the leaves and brought down by 
the sieve cells. The medullary rays thus distribute the in- 
organic and organic food to the living cells of the plant, and © 
they conduct the reserve food material to the storage cells, and, 
lastly, they function in certain plants as storage cells. 
Occurrence 
The form, size, wall structure, and the distribution of the 
medullary ray bundles, rays, and cells are best ascertained by 
