Physiological studies on the bulbing and dormancy of onion plant. II

Abstract

Histological studies of onion scale leaf and leaf sheath were carried out in relation to the response to photoperiod.1. The stomata and the palisade tissue developed well in the foliage leaf blade, but they did not differentiate in leaf sheath. The stomata were clearly found in the leaves of 7mm in length, but not in the leaves of 3mm, and the palisade tissue differentiated in the leaves longer than 3mm.2. The remarkable differences were found in number and size of parenchyma cells and intercellular space between the tissue of scale leaves and of foliage leaf sheath. Foliage leaf sheath consisted in less number of parenchyma cells and much more intercellular space than scale leaves of corresponding leaf location. The size of parenchyma cells in leaf sheath were fairly constant and small in spite of their age, while a lot of isometric cells in scale leaves grew much larger with the development of bulb.3. It was found that the number of cell layers in transversal section of leaf sheath tended to increase gradually from outer leaf to inner one. This tendency was more remarkable in the scale leaf than in leaf sheath.4. In the scale leaf tissue vascular bundle did not develop sufficiently, that is, the vessels and sieve tubes were not found so clear as those in foliage leaf sheath.5. A great number of undifferentiated parenchyma cells were found in the terminal part of scale leaves. When the plant was shifted from long day condition to short day condition, they elongated and developed to palisade tissue. This showed that the development of leaf blade tissue was inhibited in the scale leaves by long day condition.6. It was shown that long day treatment induced a rapid increase in plant height followed by scale leaf formation, but the initial scale leaf differentiated immediately after the growth increment per unit time in plant height arrived at a maximum.7. From the results mentioned above, the physiological mechanism of bulbing was discussed.