parr] | THE MECHANISMS OF FLOWERS. 183 
long; so that the less specialised bees, such as Andrena and 
 Halictus, are not shut out from the honey. 
q (2) The ale cohere with the carina at one point on each side 
(¢, 2, 8, Fig. 59), so that both rotate upwards and downwards 
together; this rotation is facilitated by the very thin and weak 
claws of the alz and carina, which adhere for the greater part of 
their length to the staminal tube, but are free anteriorly. Since 
_ the alz project considerably beyond the carina, they play the part 
| of a long lever-arm in this action. The long anterior end of the 
| yexillum has a similar action. 
(8) The return of the parts to their original position is brought 
- about chiefly by the vexillum and the alz, in addition to the part 
_ played by the calyx-tube. The strong, broad claw of the vexillum 
overlaps the other petals and the reproductive organs, being 
_ closely applied to them above and at the sides; by its elasticity it 
? returns forcibly to its own place after it has been moved, and forces 
the organs within into their place also; the anterior part of 
the taminal column is also kept in check by the ale, whose 
superior basal lobes are modified into two vesicles (v) which lie close 
; _ together upon the upper part of the column. 
_ Visitors: A. Hymenoptera—Apide : (1) Apis mellifica, L. §, very ab., s. 
_ and cp. ; (2) B. pratorum, L. 9%, s.; (8) Megachile Willughbiella, K. ¢; (4) 
 Halictus tarsatus, Schenck, ?, s. ; (5) H. maculatus, Sm. 9, ep. ; (6) Andrena 
| fulvicrus, K. 9,8. B. Diptera—(a) Syrphide: (7) Volucella bombylans, L., 
| 8.3 (b) Conopide : (8) Myopa buccata, L., s.; (9) M. testacea, L.,s. C. Lepid- 
re ptera—Fhopalocera : (10) Pieris brassice, L., s. Of these visitors it is only 
the bees that act in the manner described and effect cross-fertilisation regularly ; 
| i s other visitors only do so occasionally. See also No. 590, I1., and No. 609. 
The Dutch Clover was one of the first Papilionaceous 
flowers in which the necessity of insect-visits for full productive- 
ness was demonstrated. Darwin found that when insects were 
xcluded by means of a fine net the plant was only one-tenth 
as productive as when insects were freely admitted (152). 
99. TRIFOLIUM FRAGIFERUM, L.—This species agrees generally 
‘in the characters of its flower with 7. repens ; but the flowers are 
‘much smaller, the calyx-tube is only 2 mm. long, and the ale are bent 
‘outwards. I have seen the hive-bee visiting its flowers abundantly. 
‘Though the hive-bee as a rule keeps very strictly to one species 
| of flower for a time, yet, where 7. fragiferum and T. repens grow 
_ together, it visits the flowers of both indiscriminately. 
