Developing sieve tubes have all the parts common to a living cell; but when fully mature, however, the nucleus becomes disorganized, but a layer of protoplasm continues to line the cell wall. Sieve tubes (Plate 46, Fig. 1) are composed of a great number of superimposed cells with perforated end walls and with non-porous cellulose side walls. The end walls of two adjoining cells are greatly thickened and the pores pass through both 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. [Illustration: PLATE 46 1. Longitudinal section of sieve tube (_Cucurbita pepo_, L.). 2. Cross-section of sieve tube just above an end wall--sieve plate.] 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 powdered, 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 inorganic 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 studying: first, the cross-section of the plant; secondly, the radial section; and, thirdly, the tangential section.