Differences among flagellated and nonflagellated sperm in land plants
are striking, but close examination reveals similarities in pattern of
cytoskeleton and in nuclear structure. The microtubular cytoskeleton
of flowering plant sperm consists of microtubule bundles arranged obli
quely around the nucleus, terminating in cellular extensions. Microtub
ules are linked into bundles that branch and rejoin along the axis of
the sperm cell, forming a cytoskeleton that determines cell shape but
does not actively participate in cell movement. Generative cells and s
perm share a pattern of microtubules not found in somatic cells. This
pattern is initiated in the generative cell, one division before sperm
formation, a situation parallel to spermatogenous cell development in
vascular plants with flagellated sperm. Chromatin in flagellated and
nonflagellated sperm is condensed by specialized histones.