THE FORMS OF FLORAL ORGANS. 103 



and the only genera with irregular flowers of the same order, 

 are adapted to, and mainly visited by the larger species of 

 bees. Similarly of conspicuous and regular flowers of 

 Bosacece, Pnmus comviimis has twenty-seven visitors ; Spircea 

 JJlmaria, twenty-two ; Euhus fruticosus, sixty-seven ; Fragaria 

 vesca, twenty-five ; Cratcegus oxyacantha, fifty-seven. On the 

 other hand, of irregular flowers, Digitalis jptirjpurea has only 

 three useful visitors ; Linaria, nine or more species of bees, 

 and Orchis mascula only eight. 



As an instance of a long-tubed regular flower, Lonicera 

 ccerulea may be mentioned. It is adapted to humble-bees, by 

 which it is chiefly visited. Similarly, the flower of the 

 Honeysuckle, the Jobes^of ^ which a re scai-cely if_ at all 

 UT^^ual, admits only a few_lepidopterous insects which can 

 reach the honey. So, too, Asperula taurina, which has a tube 

 Q'to 11 mm. long, is visited by nocturnal Lepidoptera. 



^The Origin of Ikregularity. — With reference to the 

 theoretical origin of irregular whorls, I assume that they 

 have all descended from regular ones through external 

 influences.* "With regard to terminal, regular flowers the flow 

 of sap is directed equally, radially, and in all directions on 

 reaching the floral receptacle, and there is no inherent 

 cause to m.ake a terminal flower zygomorphic, or to induce 

 one or more parts of any whorl to grow differently from the 

 rest. Hence the primary cause of irregularity must come 

 from without, and I regard this cause as issuing from the 

 insect itself ; namely, the mechanical influence of its weight 

 and pressures. To this external irritation the protoplasm of 

 the cells responds, and gives rise to tissues which are thrown 

 out to withstand the strains due to the extraneous pressures 



* The fibro-vascular cords of tlie pedicel are arranged at regular 

 intervals, and are perfectly symmetrical around the medulla in irregular 

 flowers, just as they are in the case of regular ones. 



