456 



PART III. — THE CLASSIFICATION OF PLANTS. 



however, prevent their return. When the pollen has reached the 

 stigma, its lobes (Fig. 293 A and B n) spring upwards, and thus 

 the anthers, which now begin to open, are made accessible to the 

 insects ; these, in their efforts to escape (Fig. 293 i), creep round 

 the anthers and some of the pollen adheres to them ; bj this time 

 the hairs in the tube have withered, and the insect escapes, dusted 

 over with pollen which, in spite of experience, it proceeds to convey 



in like manner to another flower. 

 Those flowers which are ready 

 for pollination have an erect 

 position, and the tube of the 

 perianth is open above so that 

 the insect can readily enter ; 

 after pollination the peduncle 

 bends downwards and the tube 

 is closed by the broad lobe of 

 the perianth, so that it is im- 

 possible for insects to enter 

 flowers which have been fer- 

 tilised. 



In the flower of Epipactis 

 (one of the Orchidaceae), the 

 anther is situated above the 

 stigma and does not shed its 

 pollen in isolated grains ; but 

 when a certain portion of the 

 stigma (the abortive anterior 

 lobe), known as the rostellum 

 (Fig. 294 h), is touched, the 

 two pollinia, together with a 

 mass of sticky substance {re- 

 tinaculum) derived from the 

 rostellum, are removed from the 

 pollen-sacs, adhering to the for- 



FiG. 293.— Flower of Aristolochia. ^Before, eign body (Fig. 294 F, /i). The 



and B after fertilisation; r the tube of the insect crecpS into the flower 

 perianth ; fc the cavity below ; n stigma ; a • i i i • i. • 



anthers ;i an insect ;fe/ovary. (After Sachs.) to obtain the honey whlch IS 



secreted in the cavity of one of 

 the leaves of the perianth, the labellum (Fig. 294 /) ; as it with- 

 draws from the flower, it carries away the pollinia on its head, 

 and on entering the next flower, deposits them upon the stigma. 



