The Correlation of Flower-and Fruit-Structure in Carica Papaya
Author(s): P. J. Wester
Source: Bulletin of the Torrey Botanical Club, Vol. 35, No. 3 (Mar., 1908), pp. 141-146
Published by: Torrey Botanical Society
Stable URL: https://www.jstor.org/stable/2479258
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The correlation of flower- and fruit-structure in Carica Papaya
P. J. WESTER
The papaya (Cctricct Papaya L.) is a small tree, -dioecious, or
rarely bearing perfect flowers. The normal staminate flower is
funnel-shaped with a long slender tube, the lobes being shorter than
the tube and with ten anthers inserted in the throat of the corolla,
the pistil being abortive. The normal pistillate flower is larger
than the staminate and has distinct petals and a sessile ovary, which
is large, round or angular, and contains numerous ovules. The
stigma is sessile and five-rayed, with rays ultimately branched to
six or more flattened lobes, and the stigmatic area extending a
short distance around upon the dorsal surface (FIG. i, a; for the
sake of clearness only one of the five rays is shown).
a b c d
FIG. I. Pistils of Caruca Papaya showing different forms of stigma. (Enlarged
about i/5. )
During the summer of 1904 the attention of the writer was
called to a rather unusual difference in the shape and size of the
individual fruits on a papaya growing in the garden of the Sub-
tropical Laboratory. The plant, being old, ceased to bear in the
autumn and died later, but a cutting had fortunately been rooted
successfully during the summer, and was planted out in the
winter. Recalling the interesting fruits noted on the parent tree,
141
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142 WESTER: CARICA PAPAYA
the development of this plant was watched with great interest. In
July, I905, the first flowers appeared. For several weeks all
blossoms were staminate. In August several hermaphrodite flowers
opened, which are rather rare in this species. By far the greater
number of the flowers were staminate, but as the plant grew larger,
quite a number of perfect flowers appeared in the axil of every
leaf. On examination it was found that the structure of these
flowers varied to a remarkable degree. Three distinct types of
flowers were easily recognized: (i) those resembling a pistillate
flower more than a staminate, large, tube one-half as long as lobes,
anthers inserted in the throat of corolla, ovary large, angular,
stigmatic end superior to anthers, ovules numerous, stigma nor-
mal; (2) those with the characters of both the staminate and pis-
tillate flowers equally present, tube longer than in the type
described above, ovary not so large, one or more of the stigmatic
rays abortive, in some instances only a rudimentary ray being
present (FIG. I, c and d): (3) flowers approaching more closely a
staminate flower in structure than either of the two classes men-
tioned, having the tube and lobes of equal length, ovary small, slen-
der, ovules few, stigma reduced to a stigmatic area at the apex of
the ovary (FIG. I, b), inferior to or level with the anthers. It was
noted also that the pollen masses were released just before or
about when the flowers opened where it was inferior to or on a
level with the anthers and in some instances, at least, where this
organ was superior to the anthers.
In order to determine whether the development and formation
of the fruit might be correlated with the structure of the flower, a
series of notes was taken upon a number of flowers all on the same
tree. In most instances the bisexual flower-buds were bagged
with small paper sacks before they opened, the sack being retained
until the flower had faded and the ovary was beginning to develop.
Numerous small black ants frequent the flowers of the papaya to
get the honey secreted, and to prevent a possible cross-pollination
by these insects a broad band of cotton was tied around the trunk
of the plant, which prevented their ascent. It may be mentioned
here that bees or other large flying insects do not seem to act as
fertilizing agents in this species, the pollen evidently being carried
by the wind, or, more probably, by small insects, from the stami-
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WESTER: CARICA PAPAYA 143
nate flowers to the pistillate. All the male flower-buds on the
plant were picked before they opened to prevent a possible con-
veyance of pollen from them by the wind to the flowers not bagged.
Twenty-three flowers were numbered as they opened anid the dif-
ferences in their structure noted. Seven of these were not bagged.
Of the flowers that did not set fruit, two had small ovaries with
stigma reduced to a stigmatic area at the apex of the ovary, one
of these two not being bagged. Five fruits were injured by insects
so that they dropped or ripened prematurely and one was cut to
give the other fruits more room. The first fruit was picked April
23, I906, and the last June 26. As they were tested each fruit
was weighed then cut through the center longitudinally with a
sharp knife and an outline of the fruit and cavity traced on a paper.
The dried seed from each fruit was also weighed. That the her-
maphrodite flowers do not need external aid in pollination and
that they are fertile with their own pollen was fully demonstrated,
as in no instance were the flowers hand-pollinated. The chances
that the unbagged flowers were fertilized by the wind were also
exceedingly slight, as no male plants were growing in the vicinity.
The supposition that the characters of the flower might be corre-
lated with the form and size of the fruit was fully borne out and is
best illustrated in the accompanying photograph (FIG. 2) of the
tracings of six fruits. By referring to the following notes corre-
sponding to the numbers above the outlined fruits it will be seen
that where the ovary was small and slender, with rays in the stigma
nearly aborted, the fruits grew comparatively small, cylindrical and
oblong, almost solid, with exceedingly small seed-cavity containing
few seeds, while where the pistil was normal, or nearly so, the
fruits grew large, more or less angular, with the apical end dis-
tended, and the cavity containing a large number of seeds.
Unfortunately, no fruits matured from flowers where the stigma
was rayless, as the fruits dropped, being injured by insects. The
following notes all refer to hermaphrodite flowers:
No. 2. October 5, I905. Flower bagged; tube long; ovary
slender; stigma reduced to stigmatic area at apex of ovary with
one short ray like FIGURE I, C; mature April 23 ; weight of fruit
6oo grams; seed-cavity small; weight of dried seeds 2 grams.
(FIGURE 2, no. 2.)
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144 WESTER: CARICA PAPAYA
No. 4. October 7, I905. Flower not bagged; tube long;
ovary slender; stigma reduced to stigmatic area at apex of ovary
with short ray like FIGURE I, C; mature April 23; weight of fruit
345 grams; seed-cavity very small, containing few seeds; weight
of dried seed one gram. (FIGURE 2, no. 4.)
No. 7. October IO, I905. Flower bagged; tube long;
ovary slender; stigma reduced to stigmatic area at apex of ovary
with three short rays, about like FIGURE I, d; fruit mature April
23; weight of fruit 520 grams; seed-cavity small, containing few
seeds; weight of dried seed 2 grams. (FIGURE 2, no. 7.)
No. I5. October I7, 1905. Flower bagged;. tube short;
ovary large and angular; stigma normal like FIGURE I, a; fruit
mature May 8; weight of fruit 2 I I 5 grams; seeds numerous and
more than lining cavity; weight of dry seed I 5 grams. (FIGURE
2, no. I 5.)
No. i6. October I8, I905. Flower bagged; tube short;
ovary very large and angular; stigma normal like FIGURE I, a;
fruit mature May 31; weight of fruit I620 grams; cavity well
filled with seed; weight of dried seed I 5 grams. (FIGURE 2, no.
I6.)
No. 17. October i8, I905. Flower bagged; tube short;
ovary very large and angular; stigma 4-rayed, otherwise normal;
fruit mature June 6; weight of fruit I920 grams; weight of dried
seed i6 grams. (FIGURE 2, no. 17.)
The fruits developed from flowers in which the stigmatic rays
were almost entirely abortive contained several undeveloped ovules,
and the fruits were smaller than those developed from flowers
where the stigmatic area was large. No. 14, flower bagged, was
an interesting exception. The flower to this fruit was similar to
that of no. I 5. On maturity the fruit weighed 580 grams. Of a
large number of ovules very few had developed to seed, which,
dried, weighed only 0.75 gram. The general outline of the fruit
was similar to that of no. I 5 and the seed-cavity was proportion-
ately as large. It is possible that the inferior size of this fruit
may have been due to imperfect pollination, so many ovules hav-
ing failed to develop, and it may partly be the cause of the small
size of nos. 2, 4, and 7 also, although it is believed that a differ-
ence in the structure of the pistil is the greatest factor in determin-
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WESTER: CARICA PAPAYA 145
ing the growth and development of the fruits, in view of the fact
that the number of ovules is small and the wall of the ovary thick
where the stigma is small, while the number of ovules is great and
the cavity large and the ovary wall thin where the stigmatic area
is large.
The seed from the different fruits was saved and planted in
I906. A large number of these seedlings perished, unfortunately,
in the hurricane in October and the severe freeze of December
the same year. Of those that survived a very much larger num-
ber proved to be hermaphrodite plants than is the case with seed-
lings growni from fruits commonly found in the market.
FIG. 2. Outlines of fruits of Carica Papaya. (About I/6 natural size.)
Since the above notes were made, the flowers on a large number
of papaya plants have been examined. Few hermaphrodite plants
are found, but, in all trees bearing bisexual flowers that have been
noted, the variation in the structure of the flowers, as stated above,
has been recorded.
The papaya, as commonly propagated at present, has too large
a percentage of worthless seedlings to be commercially profitable.
Good material for cuttings is, from the nature of the tree, difficult
to obtain and is so difficult to root without bottom heat -seldom
or never used in rooting plants in the tropics or subtropics -that
this mode of propagating a good variety does not commend itself.
It has been demonstrated at the Subtropical Laboratory that in-
arching can be done, but this is necessarily a slow and cumber-
some method of propagation.
An attempt to originate a variety of papaya by inbreeding
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146 WESTER: CARICA PAPAYA
would probably be mole successful if the flowers were hand-pol-
linated with their own pollen than if pollination were accomplished
with pollen from flowers differing in structure. It seems quite
probable that in a batch of seedlings grown from an individual
fruit with small seed-cavity and few seeds - the flowers of course
being bisexual - some seedlings would have a greater percentage
of flowers that would develop this type of fruit, which would be
preferable commercially to fruits with large cavities containing
numerous seeds. It is probable that by inbreeding and rigid
selection for several generations a variety of papaya bearing all its
fruits of this type might be originated, which, the flowers being
self-pollinated, would reproduce itself tl-ue to the variety.
SUBTROPICAL LABORATORY, MIAMI, FLORIDA.
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