why do citrus fruits have sections?

Una Smith una at doliolum.biology.yale.edu
Wed Jul 27 09:04:28 EST 1994


Debra Rosenberg <drosen at world.std.com> wrote:

>Does anyone happen to know why citrus fruits like oranges and grapefruit 
>come in sections, while other fruits like apples and peaches do not?

There are several major ways that plants have evolved for constructing
fleshy fruits.  Oranges etc. belong to the family Rutaceae, which is
unique in filling its "locules" (the spaces in which seeds are produced)
with "juice sacs" that are attached to the outer walls.  Each of the
locules probably represents a structure, known as a "carpel" that was
at one time distinct, but now has fused together with its neighbors.
The outer skin of most citrus fruits shows no sign that the interior
is sectioned, but this is a result of the fusion occurring early in
the development of each individual fruit (rather than early in the
evolution of the Rutaceae).

Apples belong to the family Rosaceae, along with the common rose;  the
apple that you buy at the store is in many ways essentially the same as
a giant rose hip.  The seeds are produced inside small locules that are
air-filled at maturity, and the flesh of the apple is produced partly
from the carpels (usually 4) but mostly from a "receptacle" that has
grown over the carpels.  Again, this happens early in development, so
the division between the carpelary tissue and the receptacular tissue
is generally invisible.  If you bake apple pies often, you may have
noticed that the core sometimes comes free from the flesh of quartered
apples, without actually being cut through:  this is where the two
tissues meet. 

Fruits are thought to have evolved to take advantage of mammals and
other animals that could be lured by tasty, high-quality food and 
might be used to disperse a plant's seeds.  Most existing families
of flowering plants are thought to have evolved *before* there were
any small mammals or birds, and thus had already become committed
to a number of other strategies.  So fleshy fruits are an example of
convergent evolution, as some members of many disparate families
evolved to exploit the new resource that mammals offered.  The
Rutaceae used tissue inside the locule of their carpels, while the
Rosaceae used tissue outside the locule, or even exterior to the
carpels.


>Does it have anything to do with keeping the fruit from drying out?

The short answer is no.

A better question would be why are these fruits (citrus) so wet in
the first place.  In part, it's because humans have exerted artificial
selection on the plants, by breeding for more "desirable" traits, but
we have only taken advantage of an existing trait.  Why the Rutaceae
have "juice sacs" is still a mystery, as the evolutionary history of
the angiosperms (flowering plants) is still not well understood.  Most
botany textbooks touch on this, and they and more technical references
are listed in the classic color atlas, "Flowering Plants of the World",
which is in press again, by Oxford University Press.  One of the best
textbooks available is by Katherine Esau;  look in used book stores or
your library.

Hope this helps,

-- 
	Una Smith			smith-una at yale.edu

Department of Biology, Yale University, New Haven, CT  06520-8104  USA



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