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PART I11.] THE MECHANISMS OF FLOWERS. 87 
the two posterior are transformed into long-stalked nectaries, only 
accessible conveniently to humble-bees. The well-marked pro- 
terandry insures cross-fertilisation if humble-bees visit the flower, 
and, as a rule, prevents self-fertilisation in absence of insects 
(609). 
17. Aconitum Lycoctonum, L.—The structure of the flower 
is for the most part similar to that of A. Napellus, but the 
helmet and the nectaries are so exceedingly long that only an insect 
_ whose proboscis is very exceptionally long can reach the honey. 
In point of fact, in the plains (in a wood at Thiile, near Paderborn) 
I have found it visited exclusively by Bombus hortorum, L. @ 
_ (proboscis, 21 mm.), whose proboscis is the longest among lowland 
Fia. 29.—Aconitum Napellus, L. 
| A.—Flower in first (male) stage. The anthers have become erect, have dehisced, and are coated 
_ with white pollen. nes 
e | B.—The same flower in side view. a, a hole bitten by Bombus mastrucatus. 
C.—Ditto, in longitudinal section. 
D.—Essential organs in the first (male) stage. a’, anther bent backwards and not yet dehisced ; 
_ a’, ditto, becoming erect ; a3, ditto, erect, dehisced, and covered with pollen; a‘, ditto, empty, 
and bending backwards ; a5, ditto, empty, and bent quite backwards, 
E.—Essential organs in the second (female) stage, 
A—C, nat. size; D, B, x 2. 
__ humble-bees, and on the Alps exclusively by B. opulentus, Gerst. 
(22 mm.), whose proboscis is the longest among Alpine species. 
B. opulentus has not as yet been observed upon any other flower, 
either by Prof. Gerstiicker, who discovered and described it, or by 
| _ myself (609). 
