No abstract

Ten bud stages are defined and their profiles illustrated. A fate map of the developing bud of Hydra attenuata was made using vital intracellular marking. Marks made a t increasing distances from the young bud tip end up in increasingly more proximal regions of the bud. There is no major difference between the recruitment patterns of cells from above, below and lateral to the bud tip. The angular positions of cells on the parent is directly correlated with their final angular positions on the bud axis. Therefore, tissue is recruited in concentric rings around the young bud tip and is distorted directly outward into the bud column. At the youngest bud stages, the fate map of the bud extends about 180" around the parent.Tissue recruitment from the parent ends a t the time that tentacle rudiments appear on the bud. Thus, tissue recruitment and hydranth morphogenesis are separate processes.

No abstract

We have studied the role of ethylene in accelerating the lytic formation of gas spaces (aerenchyma) in the cortex of adventitious roots of maize (Zea mays L.) growing in poorly aerated conditions. Such roots had previously been shown to contain increased concentrations of ethylene. Ten day-old maize plants bearing seminal roots and one whorl of emerging adventitious roots were grown in nutrient solution bubbled with air, ethylene in air (0.1 to 5.0 μl l(-1)), or allowed to become oxygen-deficient in nonaerated (but not completely anaerobic) solution. Additions of 0.1 μl l(-1) ethylene or more promoted the formation of aerenchyma, with lysis of up to 47% of the cortical cells. The effects of non-aeration were similar to those of exogenous ethylene. When silver ions, an ethylene antagonist, were present at low, non-toxic concentrations (circa 0.6 μM), aerenchyma formation was prevented in ethylene treated roots and in those exposed to oxygen deficiency. Silver ions also blocked the inhibiting effect of exogenous ethylene on root extension. By contrast, the suppression of aerenchyma formation by silver ions under oxygendeficient conditions was associated with a retardation of root extension, indicating the importance of aerenchyma for root growth in poorly aerated media. Rates of production of ethylene by excised roots were stimulated by a previous non-aeration treatment. The effectiveness of Ag(+) in inhibiting equally the action on cortical cells of exogenous ethylene and of non-aeration, supports the view that gas space (aerenchyma) formation in adventitious roots 'adpted' to oxygendeficient environments is mediated by increased concentrations of endogenous ethylene. The possibility that extra ethylene could arise from increased biosynthesis of a precursor in root tissues with a restricted oxygen supply is discussed.

In the column of hydra, tissues continually move away from a region located just underthe whorl of tentacles. Above this subtentacular region, tissues proceed into the hypostome and tentacles; below it tissues pass into the buds or continue down the stalk. These movements represent a steady state pattern of tissue renewal in which column growth is balanced by tissue loss at the body extremities. But the existence of a subtentacular zone in which tissue appears stationary does not necessarily indicate that growth is restrictedto this region, as is commonly stated. The body column of hydra can be viewed as an expanding cylinder whose elongation is balanced by tissue loss at the two ends. In such a body there must be one region from whichtissue appears to emanate, regardless of how growth is distributed along the cylinder. Only the rates at which tissues move will be characteristic of the underlying growthpattern. In Hydra littoralis, the measured rates of tissue movement down the gastric column are consistent with the distributions of mitotic figures, which indicate that growth is spread out along the column.