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Report on Mrs Barber's paper.

No. 1745

Mrs Barber's paper seems to me worth publishing, because the dependence of the fertilization of a plant on one kind of insects alone, though not an unknown case, is very rare. Nor has any instance been recorded, as far as I can remember, of the access of other insects, being prevented by a mechanical obstacle requiring strength to be overcome. All the figures would of course be desirable and ornamental, but It would quite suffice to give on wood two of the adjoining flowers, one with a bee entering and one without. The outline fig 4 should also be given, as this shews better than fig 1 the contracted tube. It would be well to give the outline fig 3. if anyone can explain to the artist the structure of the fold embracing the base of the pistil, which is not clear to me.

Charles Darwin

To the Pres. & Council of Linnean Soc.

Down. May 10th 1869. )

[2]

[Notes by 'F.C']

No. 1745.

Read April 15 1869 F.C /

June 3 1869

Council — Print in Journal with woodcuts marked in Mr. Darwin's report— F.C

Duvernoia adhatodoides by Mrs Barber

Commund by Dr Hooker V. P. &c &c

[3]

Explanation of Plate

No 1. constricted tube

2. Stamen

3. Opened tube

4. blossom after fertilization has taken place.

5. Lower divisions of Corolla

6. Style and ovary

7. head of Bee shewing wedge-shaped beak which contains the proboscis.

8. ripe capsule natural size.

9. two halves of capsule shewing position of seeds.

10. seeds held in grooved process, and placenta.

11. Grooved process.

12. Duvernoia Adhatodoides with the Bees working amongst its blossoms. — natural size.

 

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On the fertilization and dissemination of Duvernoia adhatoides

D. Adhatoides (E. Mag. in Dreq. Comm.). Of this handsome species of Acanthaceæ, I received dried specimens and young plants from T. H. Bowker, Esqr of the Frontier Armed and Mounted Police. They were obtained from the forests near Fort Bowker, on the Bashee river, more than two hundred miles beyond the Colonial boundary, where this handsome plant was not uncommon, adding another charm to the many which adorn those lonely but most interesting woods, and forming together with its innumerable snowy blossoms, and broad dark green leaves a beautiful and conspicuous object amidst the surrounding scenery.

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The species is a fine ever-green shrub or small tree, attaining the height of from eight to ten feet, with numerous somewhat quadrangular jointed branches, and pink striped white flowers.

The season of its bloom, which commences rather late in Autumn, extends in duration over nearly three months, until near mid-winter.

The simple erect flowering branches or spikes of this plant contain several rows of buds; these are arranged in threes, and placed alternately opposite on the inflorescence, the buds of these tri-florate groups blossoming in succession — hence the protracted period of its flowering season.

The blossoms of D. adhatodoides are mainly if not entirely dependent upon insect agency for their fertilization and this work is, as far as I have been able to ascertain performed solely by the large black and yellow

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Carpenter Bee (a species of the genus Xylocopa) this bee, upon all bright and sunny days, is an assiduous labourer amongst the flowers of this plant, creeping into each in succession, and with its powerful wedge-shaped proboscis or beak (see fig. No. 4.) forcing open the constricted tubes, (fig. 1.) which is done by inserting this wedge-shaped proboscis into the fold which envelopes the style (fig. 3.) to accomplish this, the Bee seizes hold of the lobed projections of the lower divisions of the corolla, and drawing its body up, forces its head and thorax into the flowers, and this movement brings the hairy thorax of the Bee into the upper lip of the ringent corolla (fig. 4.) beneath which are placed the style and stamens (fig. 4.). The insect when retiring from the flower brushes out and carries away upon its thorax great quantities of the pollen

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and in entering another blossom, in the same manner as described above, the thorax of the Bee, laden with pollen masses, is forced into another ringent blossom; and then coming into contact with the style (which projects somewhat beyond the stamens) secures its fertilization, and also obtains from the adjacent stamens a fresh supply of pollen to be carried on in like manner to the next blossom.

No Lepidopterous insect or small Bee or fly could possibly effect the fertilization of D Adhatodoides neither could they obtain nectar from its constricted tubes; for the cunning manner in which they are closed would defy their utmost efforts.— And herein lies the mystery of this plant, its wonderful evidence of a divine guardianship a protecting Power which cares and provides for all— it is solely to this large Bee that

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this plant is indebted for its fertilization and it is to this Bee alone that she yields her honeyed stores, the Bee is rewarded with an abundant supply of nectar in fact he is paid for his work "and the labourer is worthy of his hire.''

It is interesting to watch these insects busily employing themselves amongst the blossoms of this plant, whilst all other insects pass it by as utterly unworthy of their attentions; surely the one was made for the other, the flowers for the Bee, and the Bee for the flowers. I tied a piece of muslin over a flowering branch of the which prevented the Bees from entering its blossoms, and this branch produced no seed. I may likewise remark that the spikes which blossom late in the season, after these large Bees have retired to winter quarters and have become dormant, also produce no seed.—

However, I think it not improbable that occasionally a flower or two may be

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fertilized by the wind blowing one branch against another.

D. Adhatodoides is not only remarkable for the manner in which its fertilization takes place, but likewise for the method of its dissemination, for the way in which the seeds of this plant are scattered abroad, is not more curious than clever, although in this respect it is not singular for many of the species of the order Acanthaceæ possess, to a certain degree, the same peculiarities.

The erect wedge-shaped capsules of the plant in question usually contain four seeds, these by abortion are often reduced to two, they are placed near the apex (fig. 9.) above the long elastic portions of the capsule, each seed is subtended by a rigid subulate, grooved process (fig. 11.) which proceeds from the placenta (apparently a continuation of it) and is prolonged half-way round the seed, which is

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held in its upright position by this curved groove — when the seeds are matured and perfectly ripe, and the capsule has become hard and dry, a contraction takes place along the opposite sides of the long spring shaped portions of the capsules, causing each to bend diametrically against the other, until at length it explodes or bursts with great force, producing a sound like that caused by the explosion of a small percussion pistol-cap, and at the same time throwing the two halves of the seed-vessel to some distance away from the plant, often if there are no intervening branches of tree to obstruct their passage, six or seven yards, and, with a favouring wind, often much further. The seeds in their flight through the air though dry and ripe, do not fall to the ground, they are held in their upright position by the grooved process until the

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apex of the capsule overbalanced by its weight, turns in falling, giving the seeds which are thus cleverly carried to their destination, a free passage to fall to the earth, at some distance away from their parent plant, where without incommoding it, there may be sufficient room for the future generations of D. Adhatodoides to spring up and fulfil their destiny.

M.E. Barber

Highlands—

Nov. 12, 1867.

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No. 1745.

SP.57

[pencil sketch of a bee's head]

head enlarged seen in front + Xylocopa trepida Fab.

To be substituted for no. 7 of the colrd drawing.

No. 1745

SP.57

[coloured plate]

 Barberis Duornoya.

Barber Mrs

No. 1745