Sept. 17, 1874] 



NA TURE 



403 



We are now, however, more immediately concerned with bees 

 and flowers. 



Many flowers close their petals during rain, which is obviously 

 an advantage, since it prevents the honey and pollen from being 

 spoilt or washed away. Everybody, however, has observed that 

 even in fine weather certain flowers close at particular liours. 

 This habit of going to sleep is surely very curious. Why should 

 flowers do so ? 



In animals we can understand it ; they are tired and require 

 rest. But why should flowers sleep ? Why should some flowers 

 do so and not others ? Moreover, difieient flowers keep difl'erent 

 hours. The daisy opens at sunrise and closes at sunset, whence 

 its name "day's- eye. " The dandelion (Leonlodon taraxacuni) is 

 said to open at seven and close at five, .Irciiiina rubra to be open 

 from nine to three, Aymp/ura alba from about seven to four : The 

 common Mouse-ear Hawkweed (HUraiiuin pilosella] is said to 

 waken at eight and go to sleep at two ; the scarlet pimpernel (Aiia- 

 gallis aii>e>tsis) to wake at seven and close soon after two ; while 

 Tro^opoi^on pratfiisis opens at four in the morning, and closes 

 just before twelve, whence its Knglish name, "John go to bed 

 at noon." Farmers' boys in some parts are said to regulate their 

 dinner-time by it. Oiher flowers, on the contrary, open in the 

 evening. 



Now, it is obvious that flowers which are fertilised by night- 

 flying insects would derive no advantage from being open by day ; 

 and, on the other hand, that those which are fertilised by bees 

 would gain nothing by being open at night. Nay, it would 

 be a distinct disadvantage, because it would render them liable 

 to be robbed of their honey and pollen, by insects whiclr are 

 not capable of fertilising them. I \\'ould venture to suggest, 

 then, that the closing of flowers may have reference to the tiabits 

 of insects, and it may be observed also in support of this that 

 wind-fertilised flowers never sleep ; * and that some of those 

 flowers which attract insects by smell emit their scent at particular 

 hours : thus, Ilcsperis malronalisa.nA Lvc/iiiis vespcrlina smell in 

 the evening, and Orchis bifolia is particularly sweet at night. 



I now pass to the structure and modification of flowers. A 

 perfect flower consists of ( i ) an outer envelope or calyx, some- 

 times tubular, sometimes consisting of separate leaves, called 

 sepals ; (2) ail inner envelope or coroda, which is generally more 

 or less coloured, and which, like the caly.x, is sometimes tubular, 

 sometimes composed of separate leaves, called petals ; (3) of one 

 or more stamens, consisting of a stalk ox filament, and a head 

 or anthtr, in which the pollen is produced ; and (4) a pistil, 

 which is situated in the centre of the flower, and consists gene- 

 rally of three principal parts — one or more carpels at the base, 

 each containing one or more seeds ; the stalk or style ; and thirdly 

 the stigma, which in many familiar instances forms a small head 

 at the top of the style or ovary, to which the pollen must find its 

 way in order to fertilise the flower. In some cases the stigma is 

 sessile. Thus it will be seen that the pistil is normally sur- 

 rounded by a row of stamens, and it would seem at first sight a 

 very simple matter that the pollen of the latter should fall on the 

 former. 



This in fact does happen in many cases, and flowers which 

 thus fertihse themselves have evidently one great advantage- 

 few remain sterile for want of pollen. Everyone, however, who 

 has watclitd flowers and has observed how assiduously they are 

 visited by insects, will admit that these insects must often deposit 

 on the stigma, pollen brought from other plants, generally of the 

 same species. Fcir it is a remarkable fact that in most cases 

 bees confine themselves in each journey to a single species of 

 plant, though in the case of some very nearly allied forms this 

 is not so ; for instance, it is stated on good authority that Kanun- 

 cuius acris, R. repens, and R. bulbosus are not distinguished by 

 the bees, or at least are visited indifferently, as is also the case 

 with two of the species of clover, Trifi^hnm fragijernni and T. 

 repens. Now, it is clear, both from the structure of flowers and 

 also from direct experiment, that as a general rale it is an ad- 

 vantage to flowers to be fertilised by pollen from a different 

 plant. 



I will not now enter on the large question why this confertili. 

 sation should be an advantage ; but that it is so has been clearly 

 proved. It has long been known that hybrids between different 

 varieties are often remarkably strong and vigorous ; Kolrenter 

 speaks with astonishment of the " staliira portentesa" oi home 

 plants thus raised by him ; indeed, says Mr. Darwin,* all experi- 

 menters have been struck with the wonderful vigour, height, size, 

 tenacity of life, precocity, and hardiness of their hybrid produc- 



* Spreneel, " Das entdeckte Geheimniss der Natur," p. J91. 

 t Animals and Plants andtr Domestiution, cb. xvii. 



tions. Mr. Darwin himself, however, was, I believe, the first to 

 show that if a flower is fertilised by pollen from a different plant, 

 the seedlings so produced are much stronger than if the plant is 

 fertilised by its own pollen. I have had the advantage of 

 seeing several of these experiments, and the difference is certainly 

 most strtiking. For instance, six crossed and six self-fertilised 

 seeds of Ipoiiuxa purpurea were grown in pairs on opposite sides 

 of the same pots ; the former reached a height of 7 ft., while the 

 others were on an average only 5 ft. 4.^ in. The first also 

 flowered more profusely. It is also remarkable that in some 

 cases plants are themselves more fertile if supplied with pollen 

 from a different flower, a different variety, and even as it would 

 appear in some cases, as in the Passion Flower, for instance, of a 

 different species. Nay, in some cases 'it would seem that pollen 

 has no effect whatever unless transferred to a different flower. 

 In Pulmonaria, for inst.ance, the pollen is said to be entirely 

 without effect on the stigma of the same plant. Fritz Muller his 

 made a variety of experiments on this interesting subject, whic'i 

 seem to show that in some cases, pollen, if placed on the stignii 

 of the same flower, has no more effect than so much inorgani : 

 dust ; while, which is perhaps even more extraordinary, in others 

 the pollen placed on the stigma of the same flower acted on ii 

 like a poison. This he observed in several species : the flower 

 faded and fell off ; the pollen masses themselves, and the stigm i 

 in contact with them, shrivelled up, turned brown, and decayed ; 

 while other flowers on the same branch, which were left un 

 fertilised, retained their freshness. 



We will now pass to the consideration of the means by which 

 self-fertilisation is checked, and cross-impregnation is effected, in 

 plants. In some cases the pollen is simply wind-borne, in others 

 it is carried by insects. These are attracted partly by the polle 1 

 itself, partly by the honey; while the bright colour and the scent 

 serve to indicate the spot where the pol en and honey can be 

 found. The calyx, whicli is not generally brightly coloureJ, 

 probably serves as a protection to the honey, »nd tends to pre- 

 vent bees and other insects from obtaining access to it by force. 



In many cases self-fertilisation is prevented by the separation 

 of the stamens and pistils, either in the place they occupy, or the 

 time of their maturity. They are frequently situated, either in 

 different flowers of the same plant, as in Euphorbia, or in 

 different plants, as in the Hop ; in other cases, although tli _ 

 stamens and pistils are situated in the same flower, they d ) 

 not mature at the same time, the anthers in some cases pro- 

 ducing their pollen before the pistil is ready to receive it, as 

 was first observed in Epilobium angustifolium by Sprengel, in 

 the year 1 790 ;* while in others the reverse is the case, and 

 the pistil, on the contrary, comes to maturity before the pollen 

 is formed. But even when the stamens and pistils are situa'ed 

 in the same flower and ripen at the same time, they are 

 sometimes so placed that it is difficult for the pollen to reach the 

 stigma. 



Moreover, it appears that if a supply of pollen from another 

 plant is secured, it is comparatively unimportant to exclude the 

 pollen of the plant itself, for in such cases the latter is neutral- 

 ised by the more powerful effect of the former. 



It is also interesting to notice that the contrivances by which 

 cross-fertilisation is favoured, or ensured, are probably of very 

 different geological antiquity. Thus, as MiiUer has pointed 

 out, t the special peculiarities of the Umbelliferae and Compositae 

 have been inherited respectively from the ancestral forms of those 

 orders ; those of Delphinium, Aquilegia, Linaria, and Pedicu- 

 laris, from the ancestral forms of the respective genera ; those of 

 Polygonum fagopyrum, P. bistorta, Lonicera caprifolium, &c., 

 from the ancestors of those species ; while in Lysitnachia vul- 

 garis, RliinantJius cristagalli, Veronica spicata, Euphrasia 

 odontites, and E. officinalis, we find that differences have arisen 

 even within the limits of one and the same species. 



The transference of the pollen from one flower to another, as 

 I have already mentioned, is elfected principally, either by the 

 wind or by insects. In the former case the tlower is rarely con- 

 spicuous ; indeed, Mr. Darwin finds it " an invariable rule that 

 when a flower is fertilised by the wind it never has a gaily- 

 coloured corolla." The conifers, grasses, birches, poplars, &c., 

 belong to this category. 



In such plants a much larger quantity of pollen is required 

 than where the fertilisation is effected by insects. Everyone has 

 observed the showers of yellow pollen produced by the Scotch 

 fir. Again, it is an advantage to these plants to flower before the 

 leaves are out, because the latter would greatly interfere with 



* " Das entdeckte Geheimniss der Nalur," 

 t MiUler, p. 44. 



