It has been proved that the secretion contains a digestive ferment capable of rendering proteid matter soluble.
R, Optical section of leptoid (sieve-tube segment) of Phanerogam, with two proteid (companion) cells.
The whole cavity of the cell is sometimes stuffed with proteid contents.
The sieve-tubes differ, however, from the tracheids in being immediately associated, apparently constantly, not with starchy parenchyma, but with parenchymatous cells, containing particularly abundant proteid contents, which seem to have a function intimately connected with the conducting function of the sieve-tubes, and which we may call proteid-cells.
Of the suspended substances, grains of caoutchouc, drops of resin and oil, proteid crystals and starch grains may be mentioned.