412 



PART IV. CLASSIFICATION. 



visiting it is obliged to take up the same position at each visit ; 

 consequent^ after it has visited a flower of the one form, when 

 it visits a flower of the other form, it touches the stigma of the 

 latter with the same part of its body with which in the first 

 flower it brushed the anthers, and thus the pollen which it 

 carried away with it from the anthers of the one flower is trans- 

 ferred to the stigma of the 

 other. Observations made by 

 artificially transporting the 

 pollen have shown that fer- 

 tilisation is most complete 

 when the pollen of stamens 

 of a certain length is con- 

 veyed to the stigma of a 

 style of the same length. 

 The same is the case with 

 trimorphic plants, e.g. Oxalis, 

 Lythrum Salicaria: in these, 

 three forms of flowers occur 

 with three different lengths 

 of styles and stamens. 



The flower of Aristolocliia 

 Clematitis (Fig. 243) is pro- 

 togynous ; insects can pene- 

 trate without difficulty down 

 the tube of the perianth, 

 which is furnished on its 

 internal surface with hairs 

 which point downwards, and 

 they thus convey to the 

 stigma the pollen they have 

 brought with them from 

 other flowers ; the hairs, how- 

 ever, prevent their return. 

 When the pollen has reached 

 the stigma, its lobes (Fig. 

 243 A and B ri) spring up- 

 wards, and thus the anthers, which now begin to open, are made 

 accessible to the insects ; these, in their efforts to escape (Fig. 

 243 i), creep round the anthers and some of the pollen adheres to 

 them ; by this time the hairs in the tube have withered, and the 



FIG. 213. Flower of Aristolochia : A before, 

 and B after fertilisation; r the tube of the 

 perianth ; kf the cavity below ; n stigma ; a 

 anthers ; t an insect ; fc/ovarr. (After Sachs.) 



