INFLUENCE OF THE ORIGIN OF THE CELLS ON FERTILISATION. 9TT 
flowers. To this class belong Scrophularia nodosa, Mandragora 'vernalis, Scopolia atropoides, 
Plantago media, Lunula pilosa, Anthoxanthum odoratum, &c. Among protogynous flowers 
Aristolochia Clematitis is characterised by striking and peculiar contrivances. 
In Fig. 489 A is shown a young flower cut through lengthwise; the stigmatic sur- 
face fi is already in a receptive condition, but the anthers are still closed ; a small 
fly /, which has brought on its back a mass of pollen from an older flower, makes its 
way in through the narrow throat of the perianth, and runs about in the globular 
swelling k ; as many as from six to ten flies are not unfrequently found in one flower. 
They are shut up and cannot escape, because the throat of the perianth r is furnished 
with long hairs moving as on a hinge, which present no impediment to the entrance of 
the insect, but prevent its escape like a trap. While the insect is moving about in 
the cavity, its back laden with pollen comes into contact with the stigmatic surface 
and pollinates it, in consequence of which the lobes of the stigma curve upwards, as 
is shown in Fig. 489 B, n. As soon as this has taken place, the anthers, previously 
closed, open; they are laid bare by the change in the position of the stigmas, and 
are rendered accessible by the withering up of the hairs at the bottom of the cavity 
of the flower, which has now become wider. The flies which have now carried their 
pollen on to the stigmatic surface can therefore creep down to the open anthers, 
where the pollen again becomes attached to them. By this time the throat of the 
perianth r has again become passable, the net-work of hairs in it having died and 
withered away after the pollination of the stigma. The insect, laden with the pollen 
of this flower, can now escape, and again performs the same work in another flower. 
But while the changes which have been described are taking place inside the flower, 
its position has also altered. As long as the stigma is still receptive, the pedicel is 
erect and the perianth open outwards (Fig. 488 / /), so that the visiting flies find a door 
hospitably open. But as soon as the pollination of the stigma has been effected, the 
pedicel bends sharply downwards just beneath the ovary, and when the flies, again 
laden with poflen, have flown out of the flower, the standard-like lobe of the perianth 
above the mouth of the tube (Fig. 489 B) closes, preventing the entrance of the flies, 
whose visits would now be useless. 
(2) Flowers in which the anthers and stigmas are mature at the same time, but self- 
fertilisation is hindered or pre'vented by the position of the organs and by mechanical contri- 
'uances. The pollen is in these cases also usually carried to the stigma by insects, but 
generally in such a manner that the stigma can only be pollinated by the pollen from 
another flower, though sometimes, as in Asclepiadeae, pollination from the same flower 
is not impossible in addition to cross-fertilisation. The contrivances for this purpose 
are astonishingly numerous, and sometimes so complicated that their purpose can only 
be detected by very careful investigation. To this category belong, for example, the 
various species of Iris, Crocus, and Pedicularis, many Labiatae, Melastomaceae, Passiflor- 
aceae, and Papilionacese. Among the most interesting examples are the Asclepiadeae, 
in which however the contrivances could be explained only by lengthy descriptions and 
a large number of illustrations^. In Sal-via pratensis and some other species of this genus 
the mechanical contrivance for preventing self-fertilisation and for ensuring crossing^ is 
extremely beautiful and easy to understand. Fig. 490 represents a flower of S. pratensis 
seen from the side; at n is the two-lipped stigma in a receptive condition; and indicated 
by a dotted line inside the upper lip of the corolla is the position of one of the two sta- 
mens. If a pin is inserted into the tube of the corolla in the direction of the arrow, the 
two stamens spring out, as indicated at 0 ; if a humble-bee inserts its proboscis in order 
to obtain the honey, the open anthers strike t,he back of the insect, and some of the 
pollen adheres to a particular part ; when the bee places itself in the same position in 
^ For a fuller description, see R. Brown, Observations on the Organs and Mode of Fecundation 
in OrchidetE and Asclepiadeae ; Trans. Linn, Soc. 1833, and Hildebrand in Bot. Zeit. 1867, No. 33. 
^ For further details, see Hildebrand. Jahrb. für wiss. Bot. vol. IV, 1865, p. i. 
