i626 A TEXTBOOK OF THEORETICAL BOTANY 



erect, yellow hairs, which serve as nectar guides, on the front surfaces of 

 the two lower petals. The stamens in the immature condition are directed 



downwards but become erect as their 

 anthers dehisce, thus ensuring that the 

 head of the bee will come into contact 

 with them. After they have discharged 

 their pollen they bend down again and 

 make way for the styles, which now turn 

 upwards, presenting newly matured stig- 

 mas to the head of a visiting insect. 



In Aconitiim napeUus (Fig. 1483) the 

 flowers are protandrous. The large sepals 

 are brightly coloured and in conjunction 

 with the smaller petals attract humble 

 bees. The two upper petals are converted 

 into long-stalked, hood-shaped nectaries 

 which are covered by the upper sepal. It 

 has been shown that each species of Acon- 

 itiim has its own particular species of 

 humble bee and moreover that the shape 

 of the flower exactly fits a medium-sized 

 female bee. So close is this relationship 

 that species of Aconitiim are entirely depen- 

 dent for their survival on certain species 

 of humble bee and it is interesting to note that the distribution of the 

 plant genus closely follows that of its insect pollinator (Fig. 1484). 



Fig. 14S3. — Aconitiim luipellus. 

 Flower in section showing one 

 of the two petal nectaries under 

 the hood formed by the pos- 

 terior sepal. 



I 



Fig. 1484. — Map of the distribution of the genera Acotiitiim and Botnhiis, plant and polli- 

 nator. Interrupted line, Acotiitiim; dotted line, Bombiis. {After Kionjeld, from Kmith.) 



