176 



BRITISH PLANTS 



the nearest opening arum, and enter it. The birthwort, 

 or Dutchman's - pipe (Aristolochia), is another fly-trap, 

 formed on similar hues to the arum, but in this case it is 

 a single flower which acts as a trap, not an inflorescence 

 (Fig. 74). 



Pollination by Animals other than Insects. — In England 

 this is negligible. If it happens, it is only accidental. 

 In the Tropics, however, sun-birds and humming-birds, 

 with brilliant plumage and long, pointed 

 beaks, regularly visit flowers for the sake 

 of the honey, and indirectly act as 

 pollinators. Certain flowers — e.g., Aspi- 

 distra — are said to be pollinated by 

 slugs. 



Devices which tend to Prevent Self- 

 Pollination. 



1. The separation of the male and 

 female organs (stamens and pistils) in 

 space — that is, the stamens are iu one 

 flower, and the pistil in another. Be- 

 tween these any pollination at all must 

 be cross, and a pollinating agent is 

 necessary to carry the pollen to the 

 stigma. Both flowers may occur on the 

 same plant, but in diiferent flowers 

 (monoecious, Gr. monos, single ; oikos, 

 home). They may occur in the same 

 inflorescence, mixed with bisexual 

 flowers, as in many UmbeUiferse and 

 Compositae, or they may occur on dif- 

 ferent shoots, as in the hazel. In dioeci- 

 ous plants (Gr. di, two) the pistillate 

 flowers occur on one plant and the 

 staminate flowers on another — e.g., willow, poplar, box, 

 perennial dog's-mercury, etc. 



2. Dichogamy (Gr. dichos, apart), the separation of the 

 sexes in time — that is, the stamens and pistil of the same 

 flower do not ripen together. When the stamens ripen 

 first, the pollen is shed before the stigmas are receptive, 

 and when the pistil is mature there is no pollen available 

 from the same flower. When the stamens ripen first, the 



Fig. 74. — Longi- 

 tudinal S E c- 

 TiON 01' Unfer- 

 tilized Flower 

 OF Aristolochia. 

 (Reduced.) 



o, downward- 

 pointing hairs ; 

 6, perianth; 

 c, ovary ; d, stig- 

 mas; e, stamens. 



