  We investigate some aspects of the spectrum of D-branes and their
interactions with closed strings. As argued earlier, a collection of many
D-strings behaves, at large dilaton values, as a single multiply wound string.
We use this result and T-duality transformations to show that a similar
phenomenon occurs for effective strings produced by wrapping p-branes on a
small (p-1)-dimensional torus, for suitable coupling. To understand the decay
of an excited D-string at large dilaton values, we study the decay of an
elementary string at small dilaton values. A long string, multiply wound on a
circle, with a small excitation energy is found to predominantly decay into
another string with the same winding number and an unwound closed string
(rather than two wound strings). This decay amplitude agrees, under duality,
with the decay amplitude computed using the Born-Infeld action for the
D-string. We compute the absorption cross section for the D-brane model studied
by Callan and Maldacena. The absorption cross section for the dilaton equals
that for the scalars obtained by reduction of the graviton, and both agree with
the cross section expected from a classical hole with the same charges.
