As alike as
peas in a pod
The title
of this piece is an English saying used to describe the similarity of, for
example, identical twins. However, it does not stand up to scrutiny. Peas in a
pod often differ from each other quite markedly, for example in size. Further,
there are also variations in the likelihood of individual seeds in a pod
surviving to maturity. This is clearly shown by Mena-Alí and Rocha (San José, Costa Rica, pp.
449–455) for a tropical member of the Fabaceae, the tree Bauhinia
ungulata. They have investigated, over three seasons, the development
process from ovule to viable seed. The first step, fertilization by the
incoming pollen, was achieved successfully for between 70 and 95 % (the latter
in two years out of three) of the ovules. However, a large proportion of the embryos
were aborted in all three years so that only between 20 and 45 % of the
original ovules developed into mature seeds. Further losses occurred because of
damage by moth larvae and by beetles, giving final viable seed production of
between approx. 10 % and approx. 38 %. The authors also demonstrated very clear
position effects. Failure in fertilization was most likely for ovules that were
in more basal positions in the pod, i.e. further from the style, with a greater
distance for the pollen tube to travel. Similarly, early abortions (but not
late abortions) were much more frequent at more basal positions. The authors
suggest that this may be related to the vigour or quality of the pollen.
Rapidly growing pollen will quickly encounter ovules at the stylar end of the
pod, leaving the slower growing pollen to seek unfertilized ovules at a greater
distance. It would be very interesting to know which features of an embryo lead
to its abortion, but the overall effect is clear: ovules at the basal end of
the pod very often do not develop into viable seeds.
Professor J. A. Bryant
University of Exeter, UK
j.a.bryant{at}exeter.ac.uk
