Anatomical observation was made to elucidate the process of the occurrence of pithy tissue of the radish root.
It is well known that the 2, 3, 5-triphenyl tetrazolium chloride (TTC) reaction, according to the difference in color intensity, reflects the physiological activity of fresh tissue in plants. In general, the tissue between the central part and the cambium ring in a cross section shows lower physiological activity than the others. Making this more in detail, the parenchyma which is apart from the conductive tissue is shown to become weak in the TTC reaction as the plant grows. Furthermore, if the esterified pectin is present in the cell wall of the parenchyma, a deep carmine color will appear by the hydroxamic acid reaction. Large parenchymatous cells which are far apart from the conductive tissue, however, show only a weak color reaction. On the other hand, staining with ruthenium red, though not specific for pectins, shows a reddish pink color in the presence of calcium pectate. Aged parenchymatous cells are stained with this dye. From these facts, it seems that the de-esterification and/or the splitting of pectin chain molecule is promoted and, consequently, the solubility of pectins in the middle lamella and the primary cell wall of parenchymatous cells changes as the radish root ripens. In this stage of root growth, it was found that the content of soluble matter in the tissues surrounding the vessels increased. These tissue are visible as transparent ones. These aged parenchymatous cells become large in size and thin in thickness of the cell wall, and are separated from the neighboring cells. According to the observations using the phase contrast microscope, the fusion of the cell plate melts away, but the union of the fibrillar bundle in the cell wall remains. As a result, the schizogenetic intercellular spaces are formed. In the observation, it was demonstrated that the occurrence of pithy tissue is accompanied with the dissolution of the middle lamella pectin and of the cementing material (probably protein) between the bundles of the fibrillar structure in the cell wall.
Young cells surrounding the vessels and the sclerenchymatous cells developed from the meristem show a positive TTC reaction. It is also noted that the esterified pectin recognized in the middle lamella and in the primary cell wall shows deep carmine color by the hydroxamic reaction. Recently, WATANABE drew attention to the fact that some young parenchymatous cells and sclerenchymatous cells in fleshy roots possess an ability of cell division in the xylem parenchyma. It seems that these cells have an appropriate amount of desoxiribonucleic acid, kinins and auxins as the materials and/or the agent that induce cell division in the xylem parenchyma. The schizogenetic intercellular spaces do not appear in such young tissues. It may be concluded that from these observations that pithyness in the radish root is a phenomenon accompanied with the senescene of parenchymatous cells.