XI THE SPECIAL COLOURS OF PLANTS 313 



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

 anthers are mechanically brought down on to the head or 

 liack 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 ])ollen, and 

 applies it to the stigma of another flower. This occui's in oui- 

 common milkwort (Polygala vulgaris). 



(5) In jDapilionaceous 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 Avhich catch flies, butterflies, and wasps by the legs, 

 and the Avonderfully 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 flower off"ers a support to the moth Avhich 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 

 s])ur, its basal portion depresses the little hinged rostellum 

 that covers the saddle -shaped sticky glands to Avhich 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 being withdrawn, the two pollinia stand erect and parallel, 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 hj the moth. But as 

 soon as the proboscis is Avithdrawn, the tAvo 2)ollen masses 

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

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



