xi THE SPECIAL COLOURS OF PLANTS 313 



(3) In others there are levers or processes by which the 

 anthers are mechanically brought clown on to the head or 

 back of an insect entering the flower, in such a position as to 

 be carried to the stigma of the next flower it visits. This 

 may be well seen in many species of Salvia and Erica. 



(4) In some there is a sticky secretion which, getting on 

 to the proboscis of an insect, carries away the pollen, and 

 applies it to the stigma of another flower. This occurs in our 

 common milkwort (Polygala vulgaris). 



(5) In papilionaceous plants there are many complex ad- 

 justments, such as the squeezing out of pollen from a 

 receptacle on to an insect, as in Lotus corniculatus, or the 

 sudden springing out and exploding of the anthers so as 

 thoroughly to dust the insect, as in Medicago falcata, this 

 occurring after the stigma has touched the insect and taken 

 off some pollen from the last flower. 



(6) Some flowers or spathes form closed boxes in which 

 insects find themselves entrapped, and when they have fertilised 

 the flower, the fringe of hairs opens and allows them to escape. 

 This occurs in many species of Arum and Aristolochia. 



(7) Still more remarkable are the traps in the flower of 

 Asclepias which catch flies, butterflies, and wasps by the legs, 

 and the wonderfully complex arrangements of the orchids. 

 One of these, our common Orchis pyramidalis, may be briefly 

 described to show how varied and beautiful are the arrange- 

 ments to secure cross-fertilisation. The broad trifid lip of 

 the fiower offers a support to the moth which is attracted 

 by its sweet odour, and two ridges at the base guide the 

 proboscis with certainty to the narrow entrance of the 

 nectary. When the proboscis has reached the end of the 

 spin, its basal portion depresses the little hinged rostellum 

 that covers the saddle -shaped sticky glands to which the 

 pollen masses (pollinia) are attached. On the proboscis 

 being withdrawn, the two pollinia stand erect and parallel, 

 firmly attached to the proboscis. In this position, however, 

 they would be useless, as they would miss the stigmatic 

 surface of the next flower visited by the moth. But as 

 soon as the proboscis is withdrawn, the two pollen masses 

 begin to diverge till they are exactly as far apart as are the 

 stigmas of the flower ; and then commences a second move- 



