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 obHque 

 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 forni the protoplasmic strands 

 of the hving 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 b}' 

 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 



