i3i8 A TEXTBOOK OF THEORETICAL BOTANY 



filament bearing an anther with two tubular extensions, the outer ends of 

 which open by pores. Attached medianly to the anther are a pair of stiff 

 spines which spread across the space between the anther and the wall of the 

 corolla. The nectar is secreted on a white swelling formed on the ovary. 

 When an insect inserts its head into the flower and thrusts up its proboscis, 

 the latter is almost certain to touch one or more of the outer spines. The 

 movement causes pollen to fall out of the tubes on to the head of the visitor. 

 In this way pollen is collected in a position where, on reaching another 

 flower, it will be brushed off on to the stigma. In later stages of anthesis 

 pollen may fall directly from the anthers on to the stigma, thus causing auto- 

 matic self-pollination, but this will only happen when the pollen has 

 become completely dry and powdery, as it finally does. 



4. Social Flowers with Concealed Nectar 



The separation of this group from the previous one is due to Hermann 

 Miiller. He used it to distinguish the type of floral arrangement found in the 

 Compositae and in a few other genera such as Scabiosa and Armeria. In these 

 cases the whole inflorescence is concerned in forming a conspicuous floral 

 structure, the flowers being individually inconspicuous. So far as colour 

 is concerned we have two distinct types. First, there are yellow or white 

 flowers, or a combination of the two, in which case the insect visitors are 

 similar to those of flowers with partly concealed nectar; and secondly there 

 are red, blue or violet flowers in which the insects are those which vi^it 

 flowers with completely concealed nectar. 



We may consider as an example of this group the case of Scabiosa [Knaii- 

 tia) arvensis (Fig. 1236). All the members of its family, the Dipsacaceae, 

 are regarded as having flowers belonging to this pollination group. 

 The purplish flowers form capitula of about fifty flowers and are usually 

 protandrous, wdth nectar secreted from the surface of the ovary and con- 

 cealed by the corolla tube. There is a marked difference in the size of the 

 component flowers, those around the margin being much larger and their 

 corollas markedly zygomorphic, a feature even more obvious in the allied 

 garden species S. caucasica (See Fig. 1874, p. 1945). The calyx is extremely 

 short and bears 8-16 deciduous bristles, which extend only about half-way 

 up the corolla tube. This tube is made up of four petals, is about 10 mm. 

 long, and may be somewhat two-lipped. Nectar is secreted at the base of the 

 corolla tube and is protected by hairs, which line the inside of the tube. In 

 the shorter flowers, it is readily accessible to all but very short-tongued 

 insects. The flowers are markedly protandrous and in the first stage the 

 stamens project well beyond the corolla. The anthers mature in succession 

 and they turn on their filaments so that the longitudinal slit is directed up- 

 wards. After the pollen has been shed the anthers fall off and the filaments 

 wither. When the four stamens have completed their development and 

 disappeared, the style, which had previously remained short and immature 

 in the centre of the corolla tube, begins to elongate and grows until it is 

 almost twice as long as the corolla. It is a club-shaped structure with a 



