Sept. 17, 1874] 



NA TURE 



405 



floweis, in long terminal racemes, and is much frequented by 

 insects ; E. pari/iflonim, on the contraiy, has small solitary 

 flowers, and is seldom visited by insects. Now, to the former 

 species the visits of insects are necessary, since the stamens ripen 

 before the pistil, and the flower has consequently lost the power 

 of self-fertilisation. In the latter, on the contrary, the stamens 

 and pistil come to maturity at the same time, and the flower 

 habitually fertilises itself. It is, however, no doubt sometimes 

 crossed by the agency of insects ; and indeed I am disposed to 

 believe that this is true of all flowers which are either coloured 

 or sweet scented. The degree in which flowers are dependent 

 on insects differs very much, and it seems to be a general rule 

 that in any genus where the flowers differ much in size, the 

 largest ones are specially dependent on insects. 



As already mentioned, the self-fertilisation of flowers is in 

 other cases still more effectually guarded against by the fact that 

 the stamens and pistils do not ripen at the same time. 



In some coses the pistil ripens before the stamens. Thus the 

 Aristolochia has a flower which consists of a long tube with a 

 narrow opening closed by stiff hairs which point backwards, so 

 that it much resembles an ordinary eel-trap. Small flies enter 

 the tube in search of honey, which from the direction of the 

 hairs they can do easily, though on the other hand, from the 

 same cause, it is impossible for them to return. Thus they are 

 imprisoned in the flower ; gradually, however, the pistil passes 



Fm. II 



maturity, the stamens ripen and shed their pollen, by which the 

 flies get thoroughly dusted. Then the hairs of the tube shrivel 

 up and release the prisoners, which carry the pollen to another 

 flower. 



Again, in the common Arum (Fig. 3), we find a somewhat 

 similar mode of fertilisation. The well-known green leaf encloses 

 a central pillar which supports a number of pistils near the base, 

 and of anthers somewhat higher. Now, in this case nothing 

 would at first sight seem easier or more natural than that the 

 poUen from the anthers sliould fall on and fertilise the pistils. 

 This, however, is not what occurs. The pislils (p) mature before 

 the anthers (<;), and by the time the pollen is shed have become 

 incapable of fertilisation. It is impossible, therefore, that the 

 plant should fertilise itself. Nor can the pollen be carried by 

 wind. When it is shed it drops to the bottom of the tube, 

 where it is so effectually sheltered that nothing short of a hurri- 

 cane could dislodge it ; and although Arum is common enough, 

 still the chances against any of the pollen so dislodged being 

 blown into the tube of another plant would be immense. 



As, however, in Aristolochia, so also in Arum, small insects 

 which, attracted by the showy central spadix, the prospect of 

 shelter or of honey, enter the tube while the stigmas are mature, 

 find themselves imprisoned, as the fringe of hairs, while 

 permitting their entrance, prevents them from returning. Afcer 

 a while, however, the period of maturity of the stigmas is over. 



and each secretes a drop of honey, thus repaying the insects 

 for their captivity. The anthers then ripen and shed their 

 pollen, which falls on and adheres to the insects. Then the 

 hairs gradually shrivel up and set the insects free, carrying 

 the pollen with them, so that those which then visit another 

 plant can hardly fail to deposit some of it on the stigmas. 

 Sometimes more than a hundred small flies will be found 

 in a single Arum. In these two cases there is obviously a great 

 advantage in the fact that the stigmas arrive at maturity before 

 the anthers. Generally, however, the advantage is the other 

 way, and the stamens ripen before the pistil. 



Of this we may take the thyme or the marjoram as an illus- 

 tration. The flowers are crowded together, and as the stigmas 

 do not come to maturity until all the anthers in the same head 

 have shed their pollen, it is obvious that bees creeping over the 



flowers must transfer the pollen from the anthers of one head to 

 the pistils of another. 



Fig. 4 represents a flower of the thyme (Thymus serpyllum), 

 and shows the four ripe stamens, and the short, as yet unde- 

 veloped pistil. Fig. 5, on the contrary, represents a somewhat 

 older flower, in which the stamens are past maturity, while the 

 pistil, on the other hand, is considerably elongated, and is ready 

 lor the reception of the pollen. 



Here it is at once obvious that insects alighting on the younger 

 (male) flowers would dust themselves with pollen, some of 

 which, if they subsequently alighted on an older flower, they 

 could not fail to deposit on the stigma. It should also be men- 

 tioned that in this genus there are likewise some small flowers 

 which contain no stamens. In some cases flowers which are first 

 male and then female, are male on the first day of opening, 

 female on the second. In others the period is longer. Thus 



NIgella, according to Sprengel, is male for si.-c days, after which 

 the stigma comes to maturity and lasts for three or four. * 



Fig. 6 represents a flower of Myosotisv,:rs!coIoy, a species often 

 known as the Forget-me-not, when just opened. It will be ob- 

 served that the pistil projects above the corolla and stamens, so 

 that it must be first touched by any insect aligliting on the 

 flower. Gradually, how ever, the corolla elongates, carrying up 

 the stamens with it, until at length they come opposite the 

 stigma, as shown in Fig. 7- Thus, if the flower has not already 

 been fertilised by insects, it is almost sure to fertihse itself. 



I will now call attention in more detail to some of our common 

 wild flowers, in order to show how beautifully they are adapted 

 to profit by the visits of insects, and how the various parts are 

 arranged so as to favour not only the transfer of pollen from one 

 flower to another, but also its deposition on that part of the 



" '* Da.s entdeckte Geheimniss der Natur," p. 287. 



