What structural advantage is provided to stems that have
theirvascular tissue arranged in a ring near the periphery of
thestem?

As the plant grows, existing bundles grow larger and thicker providing structural support.

Explanation:

 In the dicot stem, rather than being scattered throughout the plant interior, the vascular bundles are arranged in a ring, with pith concentrated at the core of the stem.

In each vascular bundle, the xylem and phloem are separated by a substance called vascular cambium.

As the plant grows, existing bundles grow larger (rather than new vascular bundles being generated, as in monocots). The vascular cambium operates by producing new xylem and phloem cells, which in turn pushes the old cells outward and forces the bundle to grow.

Note: Development in woody dicots, includes a lateral system of nutrient transport in which vessels called rays extend horizontally from the phloem to the pith, carrying nutrients to the pith for storage or to the phloem for dispersal.

the answer that best describes the relative refractory period in the neuron is the resting membrane potential is restored, but concentrations of sodium and potassium are not restored to their original state.

the refractory period is the time during which another stimulus given to the neuron will not lead to a second potential. thus, because na+ channels are inactivated during this time, additional depolarizing stimuli do not lead to new action potentials.  

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