THE ANGIOSPERMAE 1341 



but does not enclose, the flowers, and the inflorescence axis is covered with 

 flowers to the top and not prolonged as a sterile organ, the spadix. The 

 pollination mechanism of Arum macidatum is one of the most remarkable 

 in the plant kingdom and it will be described in detail under the Family 

 Araceae (see p. 2017). 



[d) Pinchtrap Mechanisms 



The peculiar and highly specialized mechanisms included in this section 

 are characteristic only of certain families or genera and in each case the 

 process is quite diflPerent, although the result achieved may be similar. 

 In general the insect is either forced by some obstruction to follow a parti- 

 cular route within a flower whereby it comes into contact with the anthers or 

 stigma, or alternatively the whole anther may become attached to the body 

 of the insect who, in visiting another flower, leaves the pollen on the stigma. 



(«) Clip mechanism. The most important family so far as this mechanism 

 is concerned is theAsclepiadaceae. Almost all the genera ofthe family exhibit 

 some modification ofthe same basic arrangement. This is discussed in rela- 

 tion to the morphology of stamens (p. n 89) and again under the description of 

 the family (p. 1883), and need only be briefly referred to here. There occur 

 in the flowers a number of curious clips, each of which consists of a thin, hard 

 plate, and bears two pollinia one from each of an adjacent pair of anthers. 

 Owing to the shape of the clip, like the letter "c" (see Fig. 1796, p. 1884), 

 it grips round the foot, proboscis or bristle of an insect visitor and is for- 

 cibly torn away when the insect leaves the flower. In this way the pollinia 

 are removed and carried away by the insect. If the insect visits another 

 flower of the same type a funnel-shaped cavity in the flower leading down 

 to the stigmatic surface guides the pollinia into a position where they stick 

 fast to the stigma and become severed from the clip. The clip remains 

 attached to the insect but meanwhile another clip may become attached, 

 and two more pollinia, from the second flower, are removed when the 

 insect leaves it. The number of clips attached to an insect is therefore an 

 index of the number of visits it has made to this kind of flower. From the 

 fact that insects have been observed with not only a number of clips, but also 

 a number of pollinia attached to them, it would appear that the mechanism 

 is not entirely satisfactory and there is no certainty that the pollinia will 

 inevitably reach the stigma when the insect visits another flower. Various 

 kinds of insects, bees, butterflies and flies all visit species of Asclepias, for 

 all can reach the nectar which the flowers offer. 



Another asclepiad, the South American Araiijia albens, sometimes culti- 

 vated for its white flowers, is pollinated at home by Humble Bees. In culti- 

 vation the flowers are visited by moths, which are trapped. The anthers 

 have rigid wings and the moths get their probosces caught in the tapering 

 slits between these wings and die as prisoners. 



{b) Orchid type. The foregoing method of attaching the pollen in masses 

 to the body of an insect visitor is similar in principle to that found in the 

 Orchids. Here the insect is encouraged by the offer of nectar to thrust its head 



