Silica deposition in relation to ageing of leaf tissues in Sasa veitchii (Carriere) Rehder (Poaceae: Bambusoideae)

BACKGROUND AND AIMS

Silica deposition is one of the important characteristics of plants in the family Poaceae. There have been many investigations into the distribution, deposition and physiological functions of silica in this family. Two hypotheses on silica deposition have been proposed based on these studies. First, that silica deposition occurs passively as a result of water uptake by plants, and second, that silica deposition is controlled positively by plants. To test these two apparently contradictory hypotheses, silica deposition in relation to the ageing of leaf tissues in Sasa veitchii was investigated.

METHODS

Tissues were examined using a light microscope and a scanning electron microscope equipped with an energy dispersive X-ray microanalyser.

KEY RESULTS

The deposition process differed depending on cell type. In mesophyll tissue, fusoid cells deposited large amounts of silica depending on leaf age after maturation, while chlorenchyma cells deposited little. In epidermal tissue, comprised of eight cell types, only silica cells deposited large amounts of silica during the leaf's developmental process and none after maturation. Bulliform cells, micro-hairs and prickle hairs deposited silica densely and continuously after leaf maturation. Cork cells, guard cells, long cells and subsidiary cells deposited silica at low levels.

CONCLUSIONS

The significance of these observations is discussed in relation to the two hypotheses proposed for silica deposition in Poaceae. The results of the present study clearly indicate that both hypotheses are compatible with each other dependent on cell types.

Proposed distal margin for resection of rectal cancer

To determine the adequate distal margin, particularly from the point of extent of lymph node metastasis, 2,333 lymph nodes from 44 patients with rectal carcinoma were evaluated, using a clearing method. The tumors were divided into two growth patterns; infiltrative and localized. Lymph node metastasis was histo-pathologically examined with special attention focused on nodes on the distal side of the tumor. The intramural spread was also given attention. The proximal pararectal lymph nodes often contained malignant cells, whereas the distal ones were rarely involved, and if involved, they were present within 1 cm from the tumor. Pararectal lymph node metastasis and intramural spread were seen with a range of 1 cm and 0.5 cm, respectively, in the localized type and 1 cm and 2.1 cm, respectively in the infiltrative type. Based on these findings, the distal margin for surgical resection of rectal carcinoma is considered to be 2 cm for the localized type and 3 cm for the infiltrative type.