Jan. 8, 1874] 



NATURE 



189 



gradations both between species, and between different 

 ages and conditions of one and the same species. 



St. George Mivart 

 ( To be continued^ 



BEES VISITING FLOWERS 

 /^N the cliffs at Llwyngvvril, near V,Mvao\iX)\, Latityius 

 ^-^ syli'isfiis grows in large patches, and is freely 

 visited by humble-bees. Where a plant grows in con- 

 siderable masses, a great number of bees are naturally 

 attracted, and the competition among them becomes 

 severe. In this case the flov/ers are not sucked in the 

 usual manner, but the bees bite holes through the corolla, 

 and obtain in this way illegitimate access to the honey. 

 Hermann Miiller has shown that when flowers grow in 

 any quantity, they are so diligently worked at by the bees 

 that only comparatively a few contain any nectar ; it is 

 therefore important for the bees to find out as quickly as 

 possible whether a flower is worth anything or not. 

 These holes, bitten through the corolla, enable the bees 

 to visit the flowers more quickly, and are thus a great 

 saving of time. He also says that, although the bee 

 which first gnaws the hole loses time in doing so, yet the 

 advantage of being able to get the honey from the young 

 and as yet unvisited flowers, fully makes up for the loss 

 of time. 



In L. syli'cslris, as in many Leguminosx, the honey is 

 secreted wiihm a nectary formed by the filaments of nine 

 ' of the stamens soldered together. The trough-like cavity 

 thus formed is covered in above and converted into a 

 tube, by the tenth stamen. But at the base, where ihe 

 trough enlarges into a bulb, the stamen is not wide 

 enough to cover it, so that it leaves a pair of holes pierc- 

 ing the tube one on each side. It is through these " nectar- 

 holes," as they are called, that the bee, after pasi.ing its 

 proboscis down the tube of the corolla, or, as in the case 

 already mentioned, through the holes bitten at its base, 

 gains entrance to the siaminal tube, in its search for 

 I nectar. 



1 In L. sylvcstris the hole is gnawed through the tube of 

 J the vexillum, close to the edge of the calyx, and exactly 

 lover the left nectar-hole. (Throughout this paper I mean 

 'the right and left of an observer looking at the front of 

 the flower.) I think the reason of this constant choice of 

 the left side of the corolla is that the left nectar-hole is 

 usually somewhat larger than the right. I found this to 

 be ihe case in sixteen out of twenty-tour specimens of the 

 wild L. syhicstris, and in eleven out of sixteen in the garden 

 variety (the Everlasting Pea). It is difficult to say how 

 the bees have acquired this habit. Whether they have 

 discovered the inequality in the size of the ncctar-holes in 

 sucking the flowers in the proper way, and have then 

 ,^ utilised this knowledge in determining where to gnaw the 

 ' hole ; or whether they have found out the best situa- 

 tion by biting through the vexfllum at various points, and 

 have afterwards remembered its situation in visiting other 

 f'ovvers. But in either case they show a remarkable 

 power of making use of what they have learnt by expe- 

 f rience. 



The united filaments not only form the nectary, but 

 also a sort of casing in which the ovary is enclosed ; and 

 out of which the growing pod has to break its way as it 

 increases in size. In Vicia cracca it does so by lifting up 

 the tenth stamen, but in most Latliyri the filament is too 

 stiff to allow of such a movement, and the growing pod 

 was to squeeze its way between it and the edge of the 

 trough formed by the nine united filaments. In doing 

 this it enlarges and at last splits open one of the nectar- 

 holes. In L. sylvcstris the left nectar-hole, usually the 

 larger of the two as I have before said, is almost always 

 the one which is thus opened. \xi L.pratensis, on the 

 other hard, where the nectar-holes are equal, the pod 



emerges indifferently to the right or left of the tenth 

 stamen. 



I am inclined to believe that the want of symmetry in 

 the growth of the pod and the inequality in the size of 

 the nectar-holes are in some way correlated, and that both 

 are connected with a third asymmetrical character in the 

 flower of this species. In most Lathyri the brush of 

 hairs on the pistil is directed straight backwards towards 

 the stalk of the flower. This is the case with L. pra- 

 icnsis, and also with the flower-buds of L. sylvcstris, 

 while very young ; but, as they get older, the pistil rotates 

 on its own axis, so that, in the adult flower, the brush is 

 turned towards the left. I have often watched the bees 

 sucking the flowers of the Everlasting- Pea in the ordinary 

 way, and have observed that the pistil, in consequence of 

 being slightly bent as well as twisted on its own axis, 

 emerges from the keel on the right side of the bee. The 

 function of the brush is, as Mr. Farrer has shown 

 (Nature, vol. vi. p. 479, 1S72), to sweep the pollen out of 

 the keel, so that it may be transferred to the bees visiting 

 the flower, and may be in this way subservient to the 

 cross-fertilisation of the species. I believe that the twist- 

 ing of the pistil helps to ensure this end, since in conse- 

 quence of the brush being turned towards the left it tubs 

 against the bee and smears it with pollen. Thus the 

 mechanism for ensuring the crois-fertilisation of the plant 

 is made more complete. At present the supposition that 

 the asymmetrical character of the pistil is connected with 

 the above described peculiarities and in the growth of the 

 pod, is merely a conjecture. 



These facts have a certain '< earing on a peculiarity 

 in the structure of the staminal tube in Pliascolus inid- 

 tiflorus, the Scarlet-runner. This ilower, in common 

 with many Leguminosje, has a pair of nectar-holes at 

 the base of its staminal tube ; but the tenth stamen 

 differs, as far as I know, from that of any other Le- 

 guminous plar.t, in possessing a little flap which projects 

 from its upper surface just in front of the irectar-holes, 

 and which almost completely blocks up the tube of 

 the corolla. JMr. Farrer supposes (loc. at. p. 480) that by 

 pressing with its proboscis against this flap the bee levers 

 up the tenth stamen, and in this way passes its trunk into 

 the staminal tube. If this occurs at all, it must be like 

 gnawing holes in the corolla, an illegitimate way of 

 treating the flower, since it is impossible to believe that it 

 should have well developed, but totally useless, nectar- 

 holes. I believe the true function of this curious little flap 

 to be as follows : — In many Papilionacea:, Lathyrus for 

 instsnce, the insect visiting the flower rests on a platform 

 which is formed of the carina and the expanded alas, but 

 in the Scarlet-runner this platform is made up by the 

 alas alone, the carina being tightly coiled into a spinal 

 close up to the entrance to the tube to the corolla. The 

 aln; are attached, one on each side to the pro.ximal part 

 of the carina, so that when an insect rests on them, its 

 weight bears on the carina, and causes the pistil which is 

 contained in it as in a sheath to be forced out. The di- 

 rection of movement of the pistil is downward and to the 

 left, so that a bte resting on the expanded ate and push- 

 ing in its head to the left of the coiled-up carina would 

 come in contact with the pistil as it darted out of its 

 sheath ; but if the insect went to the right of the coil it 

 would escape the pistil altOj;cther. The end of the pistil 

 is covered with hairs, and performs the same function as 

 the brush in Lathyrus in smearing the bee with pollen. 

 It is, therefore, of great importance for the cross-fertilisa- 

 tion of the plant that the bees should go to the left of the 

 coil. As a matter of fact they all but invariably do go to 

 the left ; the very few bees that 1 h.ave seen going to the 

 right appear dissatisfied and unable to find their way into 

 the corolla. Now to reach the nectar-holes the insect's 

 proboscis has to pass down a tunnel formed above by the 

 tube of the vexillum, below by the upper surface of the 

 tenth stamen ; the entrance into this tunnel is a narrow 



