  We propose a new theoretical interpretation of the electron energy-loss
spectroscopy results of Pichler {\it et al.} on bulk carbon nanotube samples.
The experimentally found nondispersive modes have been attributed by Pichler
{\it et al.} to interband excitations between localized states polarized
perpendicular to the nanotube axis. This interpretation has been challenged by
a theorist who attributed the modes to optical plasmons carrying nonzero
angular momenta. We point out that both interpretations suffer from
difficulties. From our theoretical results of the loss functions for individual
carbon nanotubes based on a tight-binding model, we find that the nondispersive
modes could be due to collective electronic modes in chiral carbon nanotubes,
while the observed dispersive mode should be due to collective electronic modes
in armchair and zigzag carbon nanotubes. Momentum-dependent electron
energy-loss experiments on individual carbon nanotubes should be able to
confirm or disprove this interpretation decisively.
