The cellulose walls of parenchyma cells are stained blue with hæmatoxylin and by chlorzinciodide. Cellulose is completely soluble in a fresh copper ammonia solution.
SIEVE TUBES
Sieve tubes are downward-conducting cells. They conduct downward proteid food material. This fact is easily demonstrated by adding iodine to a section containing sieve tubes, in which case the sieve tubes are turned yellow.
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.
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.