For a group of closely related Rhododendron species in section Vireya, it is shown that success of
interspecific pollinations is significantly dependent on male/female style length ratio (SLR), and apparently
independent of whether both species belong to the same taxonomic subsection. Crosses with
SLR < 0.2 or SLR > 6 were unsuccessful, with the probability of success increasing as SLR approached
1.0. When SLR was < 0.2 pollen tubes of the shorter-styled species were unable to reach the ovary.
When SLR was > 5-6, pollen tubes of the longer-styled species penetrated deeply into the ovary but were
rarely observed to enter ovules. Disparate style lengths are therefore a major incongruity factor among
Vireya rhododendrons, and may contribute to species isolation.
Frequent overgrowths of pollen tubes within the embryo sac are characteristic of a number of
interspecific crosses in the genus Rhododendron (Ericaceae). The combined techniques of sectioning,
squashing and whole-ovule clearing have confirmed that in ovules showing this phenomenon the pollen
tube fails to terminate growth and release sperms on entry into a synergid; instead it continues to grow
beyond the synergid and egg cell, often filling the main body of the embryo sac with a coiled and
distorted mass. Such ovules fail to develop further. The occurrence and possible causes of this error
syndrome are discussed.
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