204 THE BOOK OF NATURE STUDY 



and the pollinia are removed when the insect withdraws its 

 head. 



The method of removal of the pollinia can be studied by 

 inserting a sharply pointed pencil into the spur (Fig. 92, 6). This 

 reveals a further important feature in the behaviour of the pollinia. 

 As the viscid disc dries it contracts unequally, and as a result 

 of this the pollinia are bent forwards, and at the same time diverge 

 slightly from one another. The importance of this movement 

 will be evident if the position of the stigmatic surface is taken 

 into account. If it did not occur, the insect on visiting another 

 flower would tend simply to replace the pollinia in the cavities 

 of the anther lobes. Owing to the movement, however, the 

 tips of the pollinia now come in contact with the stigmatic sur- 

 faces situated below and to either side of the rostellum. This 

 surface is very sticky, and it will be found that when a pollinium 

 is brought in contact with it and then pulled away a greenish 

 stain remains on the stigma. The latter has removed a number 

 of pollen grains, but has not taken the whole pollinium. One 

 pollinium thus serves to pollinate a number of flowers. The 

 whole mechanism, on the one hand, makes self-fertilisation im- 

 possible, and on the other, is beautifully adapted to ensure cross- 

 pollination as the insects pass from flower to flower. 



So far we have considered the flower in the position it occu- 

 pies on the spike when fully open. If, however, the buds near to 

 the tip of the inflorescence be looked at, the labellum will be found 

 to stand at the back of the flower. This is its proper position, 

 and comparison with flowers lower down will show that it is only 

 brought into the anterior position occupied in the open flower 

 by a twist of the ovary. This explains the spiral course of the 

 ridges on the latter. The inferior ovary is composed of three 

 carpels and encloses a single cavity. The minute ovules stand in 

 three rows, corresponding to the lines of junction of the carpels. 



Later in the season inflorescences will be found bearing fruits. 

 The perianth has withered and fallen off. The ovary has en- 

 larged into the fruit, and the ovules have developed into the very 

 numerous and small seeds. The dry fruit opens when ripe by longi- 

 tudinal splits, and the seeds escape and are readily carried by the 

 wind. The young plant developed from such a minute seed 



